National Research Council-Coastal meteorology

National Research Council-Coastal meteorology

(Parte 1 de 2)

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Panel on Coastal MeteorologyCommittee on Meteorological Analysis, Prediction, and ResearchBoard on Atmospheric Sciences and ClimateCommission on Geosciences, Environment, and ResourcesNational Research CouncilNATIONAL ACADEMY PRESSWashington, D,C. 1992

NOTICE: The project that is the subject of this report was approved by the Governing Boardof the National Research Council, whose members are drawn from the councils of the NationalAcademy of Sciences, the National Academy of Engineering, and the Institute of Medicine.This report has been reviewed by a group other than the authors according to proceduresapproved by a Report Review Committee consisting of members of the National Academy ofSciences, the National Academy of Engineering, and the Institute of Medicine.The National Academy of Sciences is a private, nonprofit, self-perpetuating society of dis-tinguished scholars engaged in scientific and engineering research, dedicated to the furtheranceof science and technology and to their use for the general welfare. Upon the authority of thecharter granted to it by the Congress in 1863, the Academy has a mandate that requires it toadvise the federal government on scientific and technical matters. Dr. Frank Press is presidentof the National Academy of Sciences.The National Academy of Engineering was established in 1964, under the charter of theNational Academy of Sciences, as a parallel organization of outstanding engineers. It isautonomous in its administration and in the selection of its members, sharing with the NationalAcademy of Sciences the responsibility for advising the federal government. The NationalAcademy of Engineering also sponsors engineering programs aimed at meeting national needs,encourages education and research, and recognizes the superior achievements of engineers.Dr. Robert M. White is president of the National Academy of Engineering.The Institute of Medicine was established in 1970 by the National Academy of Sciences tosecure the services of eminent members of appropriate professions in the examination of policymatters pertaining to the health of the public. The Institute acts under the responsibility givento the National Academy of Sciences by its congressional charter to be an adviser to thefederal government and, upon its own initiative, to identify issues of medical care, research,and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.The National Research Council was organized by the National Academy of Sciences in 1916to associate the broad community of science and technology with the Academy's purposes offurthering knowledge and advising the federal government. Functioning in accordance withgeneral policies determined by the Academy, the Council has become the principal operatingagency of both the National Academy of Sciences and the National Academy of Engineering inproviding services to the government, the public, and the scientific and engineering communi-ties. The Council is administered jointly by both Academies and the Institute ofMedicine. Dr.Frank Press and Dr. Robert M. White are chairman and vice chairman, respectively, of theNational Research Council.Support for this project was provided jointly by the Department of the Interior, the NationalOceanic and Atmospheric Administration, the National Science Foundation, the U.S. ArmyResearch Office, the U.S. Army Atmospheric Sciences Laboratory, the U.S. Army Corps ofEngineers (Waterways Experiment Station), the U.S. Coast Guard (R&D Center), and the U.S.Navy Office of Naval Research under Grant No. N00014-90-J-4138.Library of Congress Catalog Card Number 91-68266International Standard Book Number 0-309-04687-4Copyright 1992 by the National Academy of SciencesS534Copies of this report are available from: NATIONAL ACADEMY PRESS, 2101 Constitu-tion Avenue, N.W., Washington, DC 20418Printed in the United States of AmericaCover photo: The United States at night: From 250 miles above the earth, lights of cities andtowns outline densely populated coasts. Complex weather created by adjoining water and landsurfaces affects over 100 million ueoole in the United States. Photo courtesv of the Air Force NOTICE: The project that is the subject of this report was approved by the Governing Boardof the National Research Council, whose members are drawn from the councils of the NationalAcademy of Sciences, the National Academy of Engineering, and the Institute of Medicine.This report has been reviewed by a group other than the authors according to proceduresapproved by a Report Review Committee consisting of members of the National Academy ofSciences, the National Academy of Engineering, and the Institute of Medicine.The National Academy of Sciences is a private, nonprofit, self-perpetuating society of dis-tinguished scholars engaged in scientific and engineering research, dedicated to the furtheranceof science and technology and to their use for the general welfare. Upon the authority of thecharter granted to it by the Congress in 1863, the Academy has a mandate that requires it toadvise the federal government on scientific and technical matters. Dr. Frank Press is presidentof the National Academy of Sciences.The National Academy of Engineering was established in 1964, under the charter of theNational Academy of Sciences, as a parallel organization of outstanding engineers. It isautonomous in its administration and in the selection of its members, sharing with the NationalAcademy of Sciences the responsibility for advising the federal government. The NationalAcademy of Engineering also sponsors engineering programs aimed at meeting national needs,encourages education and research, and recognizes the superior achievements of engineers.Dr. Robert M. White is president of the National Academy of Engineering.The Institute of Medicine was established in 1970 by the National Academy of Sciences tosecure the services of eminent members of appropriate professions in the examination of policymatters pertaining to the health of the public. The Institute acts under the responsibility givento the National Academy of Sciences by its congressional charter to be an adviser to thefederal government and, upon its own initiative, to identify issues of medical care, research,and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.The National Research Council was organized by the National Academy of Sciences in 1916to associate the broad community of science and technology with the Academy's purposes offurthering knowledge and advising the federal government. Functioning in accordance withgeneral policies determined by the Academy, the Council has become the principal operatingagency of both the National Academy of Sciences and the National Academy of Engineering inproviding services to the government, the public, and the scientific and engineering communi-ties. The Council is administered jointly by both Academies and the Institute ofMedicine. Dr.Frank Press and Dr. Robert M. White are chairman and vice chairman, respectively, of theNational Research Council.Support for this project was provided jointly by the Department of the Interior, the NationalOceanic and Atmospheric Administration, the National Science Foundation, the U.S. ArmyResearch Office, the U.S. Army Atmospheric Sciences Laboratory, the U.S. Army Corps ofEngineers (Waterways Experiment Station), the U.S. Coast Guard (R&D Center), and the U.S.Navy Office of Naval Research under Grant No. N00014-90-J-4138.Library of Congress Catalog Card Number 91-68266International Standard Book Number 0-309-04687-4Copyright 1992 by the National Academy of SciencesS534Copies of this report are available from: NATIONAL ACADEMY PRESS, 2101 Constitu-tion Avenue, N.W., Washington, DC 20418Printed in the United States of AmericaCover photo: The United States at night: From 250 miles above the earth, lights of cities andtowns outline densely populated coasts. Complex weather created by adjoining water and landsurfaces affects over 100 million ueoole in the United States. Photo courtesv of the Air Force

PANEL ON COASTAL METEOROLOGYRICHARD ROTUNNO (Chair), National Center for Atmospheric ResearchJUDITH A. CURRY, Pennsylvania State UniversityCHRISTOPHER W. FAIRALL, National Oceanographic and AtmosphericAdministrationCARL A. FRIEHE, University of California, IrvineWALTER A. LYONS, Colorado State UniversityJAMES E. OVERLAND, National Oceanic and AtmosphericAdministrationROGER A. PIELKE, Colorado State UniversityDAVID P. ROGERS, Scripps Institution of OceanographySTEVEN A. STAGE, Florida State UniversityPanel ConsultantsGARY L. GEERNAERT, Office of Naval ResearchJOHN W. NIELSEN, Texas A&M UniversityStaffWILLIAM A. SPRIGG, Staff Director IINIVFRSITY I

COMMITTEE ON METEOROLOGICAL ANALYSIS,PREDICTION, AND RESEARCHPETER V. HOBBS (Chair), University of WashingtonJAMES A. COAKLEY, Oregon State UniversityDENNIS G, DBAYEN, National Oceanographic and AtmosphericAdministrationFRANCO EINAUDI, National Aeronautics and Space AdministrationJ. MICHAEL FRITSCH, Pennsylvania State UniversityEARL E. GOSSARD, University of ColoradoVLTAY K. GUPTA, University of ColoradoDONALD R. JOHNSON, University of Wisconsin, MadisonTHOMAS W. SCHLATTER, National Oceanic and AtmosphericAdministrationROBERT J. SERAFIN, National Center for Atmospheric ResearchLEONARD SNELLMAN, retiredWARREN H. WHITE, Washington UniversityStaffWILLIAM A. SPRIGG, Staff Director COMMITTEE ON METEOROLOGICAL ANALYSIS,PREDICTION, AND RESEARCHPETER V. HOBBS (Chair), University of WashingtonJAMES A. COAKLEY, Oregon State UniversityDENNIS G, DBAYEN, National Oceanographic and AtmosphericAdministrationFRANCO EINAUDI, National Aeronautics and Space AdministrationJ. MICHAEL FRITSCH, Pennsylvania State UniversityEARL E. GOSSARD, University of ColoradoVLTAY K. GUPTA, University of ColoradoDONALD R. JOHNSON, University of Wisconsin, MadisonTHOMAS W. SCHLATTER, National Oceanic and AtmosphericAdministrationROBERT J. SERAFIN, National Center for Atmospheric ResearchLEONARD SNELLMAN, retiredWARREN H. WHITE, Washington UniversityStaffWILLIAM A. SPRIGG, Staff Director

BOARD ON ATMOSPHERIC SCIENCES AND CLIMATEJOHN A. DUTTON (Chair), Pennsylvania State UniversityJON F. BARTHOLIC, Michigan State UniversityE. ANN HERMAN, Tri-Space, Inc.RAFAEL L. BRAS, Massachusetts Institute of TechnologyMOUSTAFA T. CHAHINE, California Institute of TechnologyROBERT A. DUCE, Texas A&M UniversityTHOMAS E. GRAEDEL, AT&T Bell LaboratoriesDAVID D. HOUGHTON, University of Wisconsin, MadisonEUGENIA KALNAY, National Oceanic and Atmospheric AdministrationRICHARD S. LINDZEN, Massachusetts Institute of TechnologySYUKURO MANABE, National Oceanic and Atmospheric AdministrationGERALD R. NORTH, Texas A&M UniversityJAMES J. O'BRIEN, Florida State UniversityJOANNE SIMPSON, National Aeronautics and Space AdministrationEx Officio MembersERIC J. BARRON, Pennsylvania State UniversityPETER V. HOBBS, University of WashingtonCHARLES E. KOLB, Aerodyne Research, Inc.DONALD J. WILLIAMS, The Johns Hopkins UniversityStaffWILLIAM A. SPRIGG, Staff Director BOARD ON ATMOSPHERIC SCIENCES AND CLIMATEJOHN A. DUTTON (Chair), Pennsylvania State UniversityJON F. BARTHOLIC, Michigan State UniversityE. ANN HERMAN, Tri-Space, Inc.RAFAEL L. BRAS, Massachusetts Institute of TechnologyMOUSTAFA T. CHAHINE, California Institute of TechnologyROBERT A. DUCE, Texas A&M UniversityTHOMAS E. GRAEDEL, AT&T Bell LaboratoriesDAVID D. HOUGHTON, University of Wisconsin, MadisonEUGENIA KALNAY, National Oceanic and Atmospheric AdministrationRICHARD S. LINDZEN, Massachusetts Institute of TechnologySYUKURO MANABE, National Oceanic and Atmospheric AdministrationGERALD R. NORTH, Texas A&M UniversityJAMES J. O'BRIEN, Florida State UniversityJOANNE SIMPSON, National Aeronautics and Space AdministrationEx Officio MembersERIC J. BARRON, Pennsylvania State UniversityPETER V. HOBBS, University of WashingtonCHARLES E. KOLB, Aerodyne Research, Inc.DONALD J. WILLIAMS, The Johns Hopkins UniversityStaffWILLIAM A. SPRIGG, Staff Director

COMMISSION ON GEOSCIENCES,ENVIRONMENT, AND RESOURCESM. GORDON WOLMAN (Chair), The Johns Hopkins UniversityROBERT C. BEARDSLEY, Woods Hole Oceanographic InstitutionB. CLARK BURCHEDEL, Massachusetts Institute of TechnologyPETER S. EAGLESON, Massachusetts Institute of TechnologyHELEN M. INGRAM, University of ArizonaGENE E. LIKENS, New York Botanical GardenSYUKURO MANABE, National Oceanic and Atmospheric AdministrationJACK E. OLIVER, Cornell UniversityPHILIP A. PALMER, E. I. du Pont de Nemours & CompanyFRANK L. PARKER, Vanderbilt University/Clemson UniversityDUNCAN T. PATTEN, Arizona State UniversityMAXINE L. SAVITZ, Allied Signal Aerospace CompanyLARRY L. SMARR, University of Illinois, Urbana-ChampaignSTEVEN M. STANLEY, The Johns Hopkins UniversitySIR CRISPIN TICKELL, Radcliffe ObservatoryKARL K. TUREKIAN, Yale UniversityIRVIN L. WHITE, Battelle Pacific Northwest LaboratoriesStaffSTEPHEN RATTIEN, Executive DirectorSTEPHEN D. PARKER, Associate Executive DirectorJANICE E. MEHLER, Assistant Executive DirectorJEANETTE SPOON, Administrative OfficerCARLITA PERRY, Administrative AssistantROBIN LEWIS, Senior Project Assistant COMMISSION ON GEOSCIENCES,ENVIRONMENT, AND RESOURCESM. GORDON WOLMAN (Chair), The Johns Hopkins UniversityROBERT C. BEARDSLEY, Woods Hole Oceanographic InstitutionB. CLARK BURCHEDEL, Massachusetts Institute of TechnologyPETER S. EAGLESON, Massachusetts Institute of TechnologyHELEN M. INGRAM, University of ArizonaGENE E. LIKENS, New York Botanical GardenSYUKURO MANABE, National Oceanic and Atmospheric AdministrationJACK E. OLIVER, Cornell UniversityPHILIP A. PALMER, E. I. du Pont de Nemours & CompanyFRANK L. PARKER, Vanderbilt University/Clemson UniversityDUNCAN T. PATTEN, Arizona State UniversityMAXINE L. SAVITZ, Allied Signal Aerospace CompanyLARRY L. SMARR, University of Illinois, Urbana-ChampaignSTEVEN M. STANLEY, The Johns Hopkins UniversitySIR CRISPIN TICKELL, Radcliffe ObservatoryKARL K. TUREKIAN, Yale UniversityIRVIN L. WHITE, Battelle Pacific Northwest LaboratoriesStaffSTEPHEN RATTIEN, Executive DirectorSTEPHEN D. PARKER, Associate Executive DirectorJANICE E. MEHLER, Assistant Executive DirectorJEANETTE SPOON, Administrative OfficerCARLITA PERRY, Administrative AssistantROBIN LEWIS, Senior Project Assistant

Preface The unique weather and climate of the coastal zone, where the verydifferent properties of land and sea meet, strongly affect pollutant circula-tion, storm characteristics, air and sea current patterns, and local tempera-tures. Nearly half the U.S. population currently lives in coastal areas,1 andthis number is expected to grow in the next 20 years from about 1 10 millionto more than 127 million people. A better understanding of coastal meteo-rology would thus be of considerable benefit to the nation, since it affectsair pollution and disaster preparedness; ocean pollution and safeguardingnear-shore ecosystems; offshore oil exploration and drilling; military andmerchant ship operations; and a host of other activities affecting commerce,industry, transportation, health, safety, recreation, and national defense.As a result of progress in several areas of meteorological research, aswell as the development of new technologies, oportunities now exist forsignificant advances in both basic understanding and forecasting of a widevariety of important coastal meteorological phenomena. In recent yearsnew in situ and remote sensing measuring techniques have become avail-able that can be used to study and monitor coastal phenomena in consider-able detail. Numerical models are now available with sufficiently smallgrid spacings to resolve many coastal meteorological events. Widespreadavailability of small but powerful computer workstations will permit bothDepartment of Commerce (1990), Fifty Years of Population Change Along the Nation'sCoasts, 1960-2010, National Ocean Service, National Oceanic and Atmospheric Administra-tion, Washington, D.C., 41 p.

VIH CONTENTSresearch studies and operational forecasting of important weather phenome-na along coastlines, many of which depend on specific aspects of localgeography and topology.This report reviews the progress that has been made in recent years bythe small research community engaged in studies of coastal meteorology. Itis intended to guide researchers into those areas in which their efforts mightbe most productive. It should also alert policy makers, local and federalauthorities, and private organizations to the new tools that are available forimproving the safety and efficiency of operating in and managing coastalregions.Following a general introduction to the subject, this report reviews re-cent progress and current understanding of coastal meteorological phenom-ena, including land and sea breezes, coastal fronts, orographic effects, land-falling hurricanes, air quality, and coastal effects in the polar regions. Gapsin knowledge are identified, and recommendations for advancing basic un-derstanding and applications are given at the end of each chapter. Finalchapters address educational and human resource issues and highlight thenew observational and modeling tools that can be brought to bear on coastalmeteorological research and operations.On behalf of the Committee on Meteorological Analysis, Prediction,and Research, I wish to thank the panel members, particularly the panel'schairman, Richard Rotunno, for the outstanding job they have done in pro-ducing a report of value to both scientists and policy makers. Thanks areextended to Alan Weinstein of the Office of Naval Research for having theforesight to suggest this study and for recognizing the broad applicationsand interests in coastal meteorology among several federal agencies. Earlycollaboration with the Committee on the Coastal Ocean of the NationalResearch Council's Ocean Studies Board in helping to form the panel isgratefully acknowledged. Thanks are also extended to John S. Perry andKenneth Bergman for initial staff support of the study and to William A.Sprigg for helping to guide the report to its completion.Peter V. Hobbs, ChairmanCommittee on Meteorological Analysis,Prediction, and Research VIH CONTENTSresearch studies and operational forecasting of important weather phenome-na along coastlines, many of which depend on specific aspects of localgeography and topology.This report reviews the progress that has been made in recent years bythe small research community engaged in studies of coastal meteorology. Itis intended to guide researchers into those areas in which their efforts mightbe most productive. It should also alert policy makers, local and federalauthorities, and private organizations to the new tools that are available forimproving the safety and efficiency of operating in and managing coastalregions.Following a general introduction to the subject, this report reviews re-cent progress and current understanding of coastal meteorological phenom-ena, including land and sea breezes, coastal fronts, orographic effects, land-falling hurricanes, air quality, and coastal effects in the polar regions. Gapsin knowledge are identified, and recommendations for advancing basic un-derstanding and applications are given at the end of each chapter. Finalchapters address educational and human resource issues and highlight thenew observational and modeling tools that can be brought to bear on coastalmeteorological research and operations.On behalf of the Committee on Meteorological Analysis, Prediction,and Research, I wish to thank the panel members, particularly the panel'schairman, Richard Rotunno, for the outstanding job they have done in pro-ducing a report of value to both scientists and policy makers. Thanks areextended to Alan Weinstein of the Office of Naval Research for having theforesight to suggest this study and for recognizing the broad applicationsand interests in coastal meteorology among several federal agencies. Earlycollaboration with the Committee on the Coastal Ocean of the NationalResearch Council's Ocean Studies Board in helping to form the panel isgratefully acknowledged. Thanks are also extended to John S. Perry andKenneth Bergman for initial staff support of the study and to William A.Sprigg for helping to guide the report to its completion.Peter V. Hobbs, ChairmanCommittee on Meteorological Analysis,Prediction, and Research

Contents EXECUTIVE SUMMARY 1 INTRODUCTION 52 BOUNDARY LAYER PROCESSES 9Current Understanding and Challenges, 10The Generic Atmospheric Boundary Layer, 10Surface Interactions, 13Internal Boundary Layers, 15The Inhomogeneous Atmospheric Boundary Layer, 16Boundary Layer Clouds, 17Summary and Conclusions, 183 THERMALLY DRIVEN EFFECTS 19The Land Breeze and the Sea Breeze, 19Coastal Fronts, 26Ice-Edge Boundaries, 27Summary and Conclusions, 294 THE INFLUENCE OF OROGRAPHY 31Introduction and Basic Parameters, 31Low Froude Number Flow: Trapped Phenomena, 3Isolated Response: Kelvin Waveand Gravity Current, 3

Executive Summary Coastal meteorology is the study of meteorological phenomena in thecoastal zone that is, within about 100 km inland or offshore of a coastline.Weather in this region is caused, or significantly affected, by the sharpchanges that occur between land and sea in surface transfers and/or eleva-tion. With regard to surface transfers, study of the atmospheric boundarylayer (ABL) is fundamental. However, existing understanding of the ABLis applicable primarily to horizontally homogeneous conditions; it is there-fore poorly suited to the coastal zone, a region of strong horizontal inhomo-geneity. Hence, future observations and theories must focus more on thehorizontally inhomogeneous ABL.Differences in the vertical heat transfer across a coastline play a role ina number of coastal meteorological phenomena, such as the land-sea breezeand coastal fronts. Although these circulations are roughly understood, adeeper understanding is needed to make accurate predictions on the meso-scale. Further progress will come with high-density observational and high-resolution numerical modeling studies of situations with curved coastlines,heterogeneous surfaces, time-dependent large-scale flow, and clouds.Changes in elevation across a coastline significantly affect coastal me-teorology. In many situations the coastal mountains act as a barrier to thestably stratified marine air; the barrier may block air flowing toward it, or itmay act like a wall along which Kelvin waves may propagate. Furtherstudy is needed of the ageostrophic dynamics of these and other mesoscalewind features.Interactions of larger-scale weather systems with the coastal environ-

2 COASTAL METEOROLOGYment frequently involve a number of processes already mentioned, and theirnature is often difficult to distinguish in complex situations. The panelbelieves that focusing on specific interactions such as clouds interactingwith sea breezes, mountains interacting with coastal winds, and the effect ofcoastal fronts on extratropical cyclogenesis will lead to better understand-ing and hence to applications such as air quality and pollutant dispersalmodeling. Researchers may begin to concentrate on developing improvedtechniques for mesoscale data assimilation and to pave the way for site-specific forecasting of coastal weather and sea state.In the area of air-sea interactions in the coastal zone, the local process-es governing air-sea fluxes within an inhomogeneous boundary layer andvariable wave state need to be better understood, as does the role of meso-scale spatial inhomogeneities in controlling coastal dynamics.The panel found that existing buoy and coastal station networks areoutdated and inadequate in number (too often failing in accuracy and preci-sion of measurement), especially for obtaining data over water. The panelalso encourages collaboration between meteorologists and oceanographersthrough research programs and enhancements in college-level curricula thatfocus on problems of coastal meteorology and oceanography.The chapters that follow detail these findings and suggest a number ofspecific courses of action. The panel has drawn from these suggestions thefollowing general recommendations:With respect to boundary layer processes, WE RECOMMENDa complete reexamination of ABL processes in inhomogeneous con-ditions, including surface and boundary layer scaling theories, higher-order moment relationships throughout the ABL, and the relativeimportance of turbulent versus coherent motions.To enhance knowledge of thermally driven effects in coastalregions, WE RECOMMEND that high-density land and sea breezeobservational studies and high-resolution numerical modeling stud-ies in regional field experiments be extended to three dimensionsusing complex coastlines and topography, heterogeneous surfaces,and nonhomogeneous and time-dependent synoptic environmentsthat include cloud interactions. WE RECOMMEND also that de-tailed study be undertaken of other thermally driven circulationsand influences, nondiurnal in nature, including persistent coastalfronts and phenomena associated with ice sheet leads and polynyas.To develop greater understanding of orographic influences oncoastal meteorological phenomena, WE RECOMMEND numericaland observational investigations of ageostrophic dynamics for the 2 COASTAL METEOROLOGYment frequently involve a number of processes already mentioned, and theirnature is often difficult to distinguish in complex situations. The panelbelieves that focusing on specific interactions such as clouds interactingwith sea breezes, mountains interacting with coastal winds, and the effect ofcoastal fronts on extratropical cyclogenesis will lead to better understand-ing and hence to applications such as air quality and pollutant dispersalmodeling. Researchers may begin to concentrate on developing improvedtechniques for mesoscale data assimilation and to pave the way for site-specific forecasting of coastal weather and sea state.In the area of air-sea interactions in the coastal zone, the local process-es governing air-sea fluxes within an inhomogeneous boundary layer andvariable wave state need to be better understood, as does the role of meso-scale spatial inhomogeneities in controlling coastal dynamics.The panel found that existing buoy and coastal station networks areoutdated and inadequate in number (too often failing in accuracy and preci-sion of measurement), especially for obtaining data over water. The panelalso encourages collaboration between meteorologists and oceanographersthrough research programs and enhancements in college-level curricula thatfocus on problems of coastal meteorology and oceanography.The chapters that follow detail these findings and suggest a number ofspecific courses of action. The panel has drawn from these suggestions thefollowing general recommendations:With respect to boundary layer processes, WE RECOMMENDa complete reexamination of ABL processes in inhomogeneous con-ditions, including surface and boundary layer scaling theories, higher-order moment relationships throughout the ABL, and the relativeimportance of turbulent versus coherent motions.To enhance knowledge of thermally driven effects in coastalregions, WE RECOMMEND that high-density land and sea breezeobservational studies and high-resolution numerical modeling stud-ies in regional field experiments be extended to three dimensionsusing complex coastlines and topography, heterogeneous surfaces,and nonhomogeneous and time-dependent synoptic environmentsthat include cloud interactions. WE RECOMMEND also that de-tailed study be undertaken of other thermally driven circulationsand influences, nondiurnal in nature, including persistent coastalfronts and phenomena associated with ice sheet leads and polynyas.To develop greater understanding of orographic influences oncoastal meteorological phenomena, WE RECOMMEND numericaland observational investigations of ageostrophic dynamics for the

EXECUTIVE SUMMARYinitiation, intensification, and movement of coastal mesoscale windfeatures, such as coastal jets and eddies and gap winds, caused bythe interaction of the synoptic-scale flow with coastal orography.To understand the nature of the interactions of large-scale weathersystems with the coastal environment, WE RECOMMEND obser-vational, numerical, and theoretical studies that focus on specificinteractions for example, the effect of coastal fronts on extratrop-ical cyclogenesis in order to develop an understanding of the dy-namical processes involved.To improve understanding of the influence of the ABL on thecoastal ocean, WE RECOMMEND that a research program be un-dertaken to clarify (1) the local physical and chemical processesgoverning air-sea fluxes of momentum, heat, moisture, particulates,and gases within an inhomogeneous coastal boundary layer andvariable wave state and (2) the role of distant mesoscale spatialinhomogeneities in controlling atmosphere-ocean dynamics in a coastalenvironment.To address air quality issues in coastal regions, WE RECOM-MEND use of advanced modeling systems and tracer tests (for ver-ification) to determine the significant impacts of vertical motionsand shears in three-dimensional coherent mesoscale coastal circula-tions on the dispersion of gases, aerosols, and particulates, espe-cially in the range of 10 to 100 km.To apply advanced technology in coastal research, WE REC-OMMEND (1) the use of recently developed remote sensors toobtain detailed four-dimensional data sets along with the upgradingof buoy and surface station networks to obtain quality, long-dura-tion data sets describing coastal regions and (2) on-site use of high-performance workstations to provide decentralized computations duringstudy of local coastal phenomena, data assimilation methods, andreal-time forecasting.Finally, to focus attention on the subject of coastal meteorolo-gy, WE RECOMMEND increased use of interdisciplinary confer-ences and short courses, together with support of university train-ing programs, to encourage more scientists to explore the meteorologyof the coastal zone. EXECUTIVE SUMMARYinitiation, intensification, and movement of coastal mesoscale windfeatures, such as coastal jets and eddies and gap winds, caused bythe interaction of the synoptic-scale flow with coastal orography.To understand the nature of the interactions of large-scale weathersystems with the coastal environment, WE RECOMMEND obser-vational, numerical, and theoretical studies that focus on specificinteractions for example, the effect of coastal fronts on extratrop-ical cyclogenesis in order to develop an understanding of the dy-namical processes involved.To improve understanding of the influence of the ABL on thecoastal ocean, WE RECOMMEND that a research program be un-dertaken to clarify (1) the local physical and chemical processesgoverning air-sea fluxes of momentum, heat, moisture, particulates,and gases within an inhomogeneous coastal boundary layer andvariable wave state and (2) the role of distant mesoscale spatialinhomogeneities in controlling atmosphere-ocean dynamics in a coastalenvironment.To address air quality issues in coastal regions, WE RECOM-MEND use of advanced modeling systems and tracer tests (for ver-ification) to determine the significant impacts of vertical motionsand shears in three-dimensional coherent mesoscale coastal circula-tions on the dispersion of gases, aerosols, and particulates, espe-cially in the range of 10 to 100 km.To apply advanced technology in coastal research, WE REC-OMMEND (1) the use of recently developed remote sensors toobtain detailed four-dimensional data sets along with the upgradingof buoy and surface station networks to obtain quality, long-dura-tion data sets describing coastal regions and (2) on-site use of high-performance workstations to provide decentralized computations duringstudy of local coastal phenomena, data assimilation methods, andreal-time forecasting.Finally, to focus attention on the subject of coastal meteorolo-gy, WE RECOMMEND increased use of interdisciplinary confer-ences and short courses, together with support of university train-ing programs, to encourage more scientists to explore the meteorologyof the coastal zone.

Introduction Coastal meteorology is the study of meteorological phenomena in thecoastal zone caused, or significantly affected, by sharp changes in heat,moisture, and momentum transfers and elevation that occur between landand water. The coastal zone is defined as extending approximately 100 kmto either side of the coastline. Examples of coastal meteorological phenom-ena include land and sea breezes, sea-breeze-related thunderstorms, coastalfronts, fog, haze, marine stratus clouds, and strong winds associated withcoastal orography. In addition to their intrinsic importance to coastal weather,increased knowledge of these phenomena is important for understanding thephysical, chemical, and biological oceanography of the coastal ocean. Prac-tical application of this knowledge is vital for more accurate prediction ofcoastal weather and sea states, which affect transportation and commerce,pollutant dispersal, public safety, and military operations. This report re-views the state of the science of coastal meteorology. In addition, it recom-mends areas for scientific and technical progress.The dynamical meteorology of the coastal zone may be considered interms of the three subsidiary ideal problems illustrated in Figure 1.1; thesethree problems form the organizational basis for this report. The first prob-lem is one where the coastal atmospheric circulation is primarily driven bythe contrast in heating and is modulated by the contrast in surface frictionbetween land and water. The second problem is one where the primaryinfluence is due to steep coastal mountains, the presence of which mayinduce strong winds and other complex flow patterns. A third class ofphenomena broadly consists of larger-scale meteorological systems that, by

Thermally Driven Effects

Orographic Effects Tornado

Interaction with Larger-ScaleWeather SystemsFIGURE 1.1 Three categories of problems in coastal meteorology.

INTRODUCTION 7virtue of their passage across the coastline, produce distinct smaller-scalesystems. Real coastal phenomena are always some combination of theseidealized problems.Transfers of heat, momentum, and water vapor between the atmosphereand its underlying surface (be it land or water) are basic to these three idealproblems. This report therefore begins with Boundary Layer Processes(Chapter 2); this chapter contains an assessment of, and prospects for im-provement in, our understanding of the approximately 1-km-deep layer ofair adjacent to the surface, which is called the atmospheric boundary layer(ABL). Study of the ABL is intended to reveal how surface transfers aredistributed upward. Over the ocean, those surface transfers are interactive,determined by the sea state, which in turn is determined by the atmosphericflow, which is influenced by the surface transfers, and so on. This funda-mental coupling has been long recognized. However, there is another orderof complexity over the coastal ocean because the sea state is significantlyinfluenced by the ocean bottom. Over land, there is still significant uncer-tainty about the nature of surface transfer from terrain with variations insoil moisture, vegetation, and usage such as occur along the coast. Thesestrong and frequently horizontal variations in surface transfers form a par-ticularly formidable impediment to understanding of the ABL in coastalregions. But even in the absence of such horizontal variations, the marineboundary layer containing stratus clouds and drizzle is a complex probleminvolving the interplay of turbulence, cloud processes, and radiation.Problems in the first general category are discussed in Thermally Driv-en Effects (Chapter 3). Although recognition of the land-sea breeze datesback to antiquity, the understanding needed to make accurate forecasts isstill lacking. In simple terms, the land-sea breeze is caused by the generallydifferent temperatures of the land and sea, which produce an across-coastair temperature contrast. After this circulation begins, however, it modifiesthe conditions that produced it. Thus, the difficulty in making precisepredictions lies in understanding more clearly the nature of this feedback.Uncertainties in our understanding of the ABL and nonlinear feedbacksbetween the sea breeze and resultant clouds are examples here. Issuesassociated with two special types of thermally driven phenomena (coastalfronts and ice-edge boundaries) also are discussed in Chapter 3.Coastal mountain ranges can significantly affect coastal meteorology.In The Influence of Orography (Chapter 4), the types of problems encoun-tered are discussed. In many situations, coastal mountains act as a barrierto the stably stratified marine air. Thus, air with an initial component ofmotion toward the barrier must eventually turn and flow along the barrier.Under the influence of the earth's rotation, waves known as Kelvin waves(see Figure 1.1) can propagate along a basin-wall-like coastal mountainrange. This type of motion is an important component of the meteorologi- INTRODUCTION 7virtue of their passage across the coastline, produce distinct smaller-scalesystems. Real coastal phenomena are always some combination of theseidealized problems.Transfers of heat, momentum, and water vapor between the atmosphereand its underlying surface (be it land or water) are basic to these three idealproblems. This report therefore begins with Boundary Layer Processes(Chapter 2); this chapter contains an assessment of, and prospects for im-provement in, our understanding of the approximately 1-km-deep layer ofair adjacent to the surface, which is called the atmospheric boundary layer(ABL). Study of the ABL is intended to reveal how surface transfers aredistributed upward. Over the ocean, those surface transfers are interactive,determined by the sea state, which in turn is determined by the atmosphericflow, which is influenced by the surface transfers, and so on. This funda-mental coupling has been long recognized. However, there is another orderof complexity over the coastal ocean because the sea state is significantlyinfluenced by the ocean bottom. Over land, there is still significant uncer-tainty about the nature of surface transfer from terrain with variations insoil moisture, vegetation, and usage such as occur along the coast. Thesestrong and frequently horizontal variations in surface transfers form a par-ticularly formidable impediment to understanding of the ABL in coastalregions. But even in the absence of such horizontal variations, the marineboundary layer containing stratus clouds and drizzle is a complex probleminvolving the interplay of turbulence, cloud processes, and radiation.Problems in the first general category are discussed in Thermally Driv-en Effects (Chapter 3). Although recognition of the land-sea breeze datesback to antiquity, the understanding needed to make accurate forecasts isstill lacking. In simple terms, the land-sea breeze is caused by the generallydifferent temperatures of the land and sea, which produce an across-coastair temperature contrast. After this circulation begins, however, it modifiesthe conditions that produced it. Thus, the difficulty in making precisepredictions lies in understanding more clearly the nature of this feedback.Uncertainties in our understanding of the ABL and nonlinear feedbacksbetween the sea breeze and resultant clouds are examples here. Issuesassociated with two special types of thermally driven phenomena (coastalfronts and ice-edge boundaries) also are discussed in Chapter 3.Coastal mountain ranges can significantly affect coastal meteorology.In The Influence of Orography (Chapter 4), the types of problems encoun-tered are discussed. In many situations, coastal mountains act as a barrierto the stably stratified marine air. Thus, air with an initial component ofmotion toward the barrier must eventually turn and flow along the barrier.Under the influence of the earth's rotation, waves known as Kelvin waves(see Figure 1.1) can propagate along a basin-wall-like coastal mountainrange. This type of motion is an important component of the meteorologi-

5 COASTAL METEOROLOGYcal problems in these regions. Local effects such as katabatic winds, gapwinds, and eddies also are discussed in Chapter 4.As larger-scale meteorological systems move across the coast, they areaffected by some combination of the effects discussed in the previous twoparagraphs. In some situations, distinct subsystems, which would not existwithout the coastal influence, are produced. These are discussed in Interac-tions with Larger-Scale Weather Systems (Chapter 5). Examples of theseeffects include cyclogenesis, which is enhanced at the east coast of theUnited States as upper-level disturbances cross the Appalachians and en-counter the strong baroclinic zone at the coast; flow along the coast inwinter with strong cooling of the air on the landward side, leading to theformation of fronts; and land-falling hurricanes whose low-level flows areso modified as to favor the formation of tornadoes.In general, the ocean affects, and is affected by, the atmosphere. In TheInfluence of the Atmospheric Boundary Layer on the Coastal Ocean (Chap-ter 6), aspects of this interaction that are particularly important for theocean part of the coastal zone (shelf waters) are discussed. The processesgoverning air-sea fluxes of momentum, heat, mass, and gases are describedin terms of local and remote forcings as well as wave state. Wind-drivencoastal upwelling of colder water from below brings different chemical andbiological compositions to the surface, produces an across-coast tempera-ture difference unique to coastal regions, and influences atmospheric circu-lation. Interactions of this nature are important to understanding the coastalocean and the chemical and biological processes occurring there.Another important application of coastal meteorology is the predictionof pollutant dispersal. In Air Quality (Chapter 7), the focus is on physicaladvective processes. The highly variable winds near the coast may sweeppollutants out to sea on a land breeze, but then bring them back with the seabreeze. More accurate estimates of the vertical motion fields associatedwith these wind systems are critical to determining the layers in which thepollutant resides (and the horizontal direction in which it will move).Capabilities and Opportunities (Chapter 8) discusses new observationaland simulation tools that can be exploited to study the coastal environment.The critical issue is to have instruments in place to measure long time-scalevariations (from 1 or 2 days to a season), as well as short-time fluctuations,so that relationships between the two can be discerned. The use of remotesensing as well as in situ devices is discussed in this context. Increases inthe computer power of desktop machines will allow many investigators toexplore, with numerical models, flow in the vicinity of many coastal envi-ronments. With regard to Educational and Human Resources (Chapter 9),the panel found that there is a relative lack of training of meteorologystudents in areas pertaining to coastal meteorology and a relative lack ofcross-fertilization between the fields of coastal meteorology and coastaloceanography. 5 COASTAL METEOROLOGYcal problems in these regions. Local effects such as katabatic winds, gapwinds, and eddies also are discussed in Chapter 4.As larger-scale meteorological systems move across the coast, they areaffected by some combination of the effects discussed in the previous twoparagraphs. In some situations, distinct subsystems, which would not existwithout the coastal influence, are produced. These are discussed in Interac-tions with Larger-Scale Weather Systems (Chapter 5). Examples of theseeffects include cyclogenesis, which is enhanced at the east coast of theUnited States as upper-level disturbances cross the Appalachians and en-counter the strong baroclinic zone at the coast; flow along the coast inwinter with strong cooling of the air on the landward side, leading to theformation of fronts; and land-falling hurricanes whose low-level flows areso modified as to favor the formation of tornadoes.In general, the ocean affects, and is affected by, the atmosphere. In TheInfluence of the Atmospheric Boundary Layer on the Coastal Ocean (Chap-ter 6), aspects of this interaction that are particularly important for theocean part of the coastal zone (shelf waters) are discussed. The processesgoverning air-sea fluxes of momentum, heat, mass, and gases are describedin terms of local and remote forcings as well as wave state. Wind-drivencoastal upwelling of colder water from below brings different chemical andbiological compositions to the surface, produces an across-coast tempera-ture difference unique to coastal regions, and influences atmospheric circu-lation. Interactions of this nature are important to understanding the coastalocean and the chemical and biological processes occurring there.Another important application of coastal meteorology is the predictionof pollutant dispersal. In Air Quality (Chapter 7), the focus is on physicaladvective processes. The highly variable winds near the coast may sweeppollutants out to sea on a land breeze, but then bring them back with the seabreeze. More accurate estimates of the vertical motion fields associatedwith these wind systems are critical to determining the layers in which thepollutant resides (and the horizontal direction in which it will move).Capabilities and Opportunities (Chapter 8) discusses new observationaland simulation tools that can be exploited to study the coastal environment.The critical issue is to have instruments in place to measure long time-scalevariations (from 1 or 2 days to a season), as well as short-time fluctuations,so that relationships between the two can be discerned. The use of remotesensing as well as in situ devices is discussed in this context. Increases inthe computer power of desktop machines will allow many investigators toexplore, with numerical models, flow in the vicinity of many coastal envi-ronments. With regard to Educational and Human Resources (Chapter 9),the panel found that there is a relative lack of training of meteorologystudents in areas pertaining to coastal meteorology and a relative lack ofcross-fertilization between the fields of coastal meteorology and coastaloceanography.

Boundary Layer Processes The atmospheric boundary layer (ABL) is the part of the lower tropo-sphere that interacts directly with the earth's surface through turbulent transportprocesses. A coast separates two drastically different surfaces, and a coast-al region has an inhomogeneous boundary layer. ABL processes are impor-tant in determining the evolution of atmospheric structure. The boundarylayer is also a buffer zone that interacts both with the "free" troposphericflow at its upper interface (through entrainment processes) and with thesurface (through surface exchange processes).Past studies of the ABL have emphasized certain idealized, near-equi-librium, and horizontally homogeneous boundary layer regimes (Wyngaard,1988). For example, Stull's (1988) comprehensive reference on boundarylayer meteorology devotes only about 5 percent of its discussion to geo-graphic effects. In fact, many common ABL conditions are still poorlyunderstood even for homogeneous surfaces. The horizontally inhomoge-neous and rapid temporal forcing conditions typical of coastal regions dic-tate consideration of problems that have rarely been investigated. Further-more, even those aspects of the physical processes that are.generally regardedas being well understood (e.g., bulk parameterization of surface fluxes)must be reevaluated before applying them to coastal environments.In this chapter, current understanding of boundary layer processes isexamined, and some important deficiencies are identified. Following intro-ductory material on boundary layers, generic problems in boundary layerprocesses are examined. Then, special coastal problems are considered thatinvolve surface fluxes, internal boundary layer growth, baroclinicity, and a

10 COASTAL METEOROLOGYvariety of phenomena that are either inherently inhomogeneous or are asso-ciated with inhomogeneous forcing.CURRENT UNDERSTANDING AND CHALLENGESThe ABL has been investigated extensively. Certain aspects are con-sidered to be well understood. This understanding has developed from acombination of sources: laboratory models (e.g., Deardorff and Willis,1982; Willis and Deardorff, 1974); three-dimensional primitive equationlarge-eddy simulations (e.g., Deardorff, 1974, 1980; Moeng, 1984a, b); andatmospheric measurements with aircraft and tethered balloons (e.g., Brost etal., 1982; Kaimal et al., 1976; Lenschow, 1973; Lenschow et al., 1980;Nicholls, 1984).A variety of ABL models are now available. Because a model repre-sents the reduction of a problem to its important components, it not only isa useful tool but also an expression of our understanding of the physics.Linear regression and crude parameterizations often imply little or no un-derstanding. Today, a hierarchy of complexity is available in atmosphericmodels. Similarity models are the simplest but typically are applicable onlyto idealized situations. One index of understanding is in simplified concep-tual models and useful scaling laws. By this measure, the cloud-free con-vective ABL is clearly the best-understood regime.Numerical solutions to systems of physical equations form the basis ofthe more sophisticated models commonly used in meteorology. Two ap-proaches are used, depending on whether solutions are sought for the en-semble average or for the volume average atmospheric budget and stateequations. Ensemble average models are often referred to as higher-orderclosure models. Grid-volume average models are usually referred to aslarge eddy simulation (LES) models. These approaches are fundamentallydifferent; an LES model produces an explicit simulation of a single realiza-tion of a three-dimensional, time-dependent atmospheric structure. An en-semble average model predicts or describes relationships between the mo-ments of the atmospheric variables (the first moments are averages of thevariables and the second moments are variances and fluxes). LES modelsare currently used strictly for research (such as developing parameteriza-tions); ensemble average models have a variety of practical as well as re-search applications.The Generic Atmospheric Boundary LayerScaling theories have their origins in dimensional analysis- importantvariables of the problem are selected, and other properties are calculatedfrom dimensionally consistent combinations of those variables. Modern 10 COASTAL METEOROLOGYvariety of phenomena that are either inherently inhomogeneous or are asso-ciated with inhomogeneous forcing.CURRENT UNDERSTANDING AND CHALLENGESThe ABL has been investigated extensively. Certain aspects are con-sidered to be well understood. This understanding has developed from acombination of sources: laboratory models (e.g., Deardorff and Willis,1982; Willis and Deardorff, 1974); three-dimensional primitive equationlarge-eddy simulations (e.g., Deardorff, 1974, 1980; Moeng, 1984a, b); andatmospheric measurements with aircraft and tethered balloons (e.g., Brost etal., 1982; Kaimal et al., 1976; Lenschow, 1973; Lenschow et al., 1980;Nicholls, 1984).A variety of ABL models are now available. Because a model repre-sents the reduction of a problem to its important components, it not only isa useful tool but also an expression of our understanding of the physics.Linear regression and crude parameterizations often imply little or no un-derstanding. Today, a hierarchy of complexity is available in atmosphericmodels. Similarity models are the simplest but typically are applicable onlyto idealized situations. One index of understanding is in simplified concep-tual models and useful scaling laws. By this measure, the cloud-free con-vective ABL is clearly the best-understood regime.Numerical solutions to systems of physical equations form the basis ofthe more sophisticated models commonly used in meteorology. Two ap-proaches are used, depending on whether solutions are sought for the en-semble average or for the volume average atmospheric budget and stateequations. Ensemble average models are often referred to as higher-orderclosure models. Grid-volume average models are usually referred to aslarge eddy simulation (LES) models. These approaches are fundamentallydifferent; an LES model produces an explicit simulation of a single realiza-tion of a three-dimensional, time-dependent atmospheric structure. An en-semble average model predicts or describes relationships between the mo-ments of the atmospheric variables (the first moments are averages of thevariables and the second moments are variances and fluxes). LES modelsare currently used strictly for research (such as developing parameteriza-tions); ensemble average models have a variety of practical as well as re-search applications.The Generic Atmospheric Boundary LayerScaling theories have their origins in dimensional analysis- importantvariables of the problem are selected, and other properties are calculatedfrom dimensionally consistent combinations of those variables. Modern

BOUNDARY LAYER PROCESSES 11ABL similarity theories are now based on arguments about the relativevariability and magnitude of the terms in the mean and turbulent budgetequations. In the ABL the similarity regimes are broken down by thevertical scale, assuming that the horizontal fields are statistically homoge-neous. Historically, this process has proceeded from the ground up. Figure2.1 depicts schematically the idealized mean structure of the ABL underconvective, well-mixed, cloud-free, horizontally homogeneous conditions ina synoptic regime that has sufficient subsidence to ensure the presence of acapping inversion. Compared to the ABL, the overlying free tropospherecan be considered essentially nonturbulent.Surface layer similarity theory is based on scaling parameters obtainedfrom the surface fluxes (Wyngaard, 1973). The theory is considered validin the region near the surface where various terms (particularly the gradi-ents) in the turbulent kinetic energy and scalar variance budget equationsare considerably more dependent on height than are the fluxes. Thus, theassumptions on which the theory is based are generally valid in the lowest10 percent of the ABL. For the convective ABL, we also have mixed layersimilarity (Moeng and Wyngaard, 1989) and inversion region similarity(Wyngaard and LeMone, 1980). For the stable ABL, no mixed layer exists.Instead, there is a gradual transition from the surface layer to the inversionlayer. A local similarity theory (Nieuwstadt, 1984) has been proposed for a i r Ree Ttepsphere Irwers! jjr,',Mixed 'Layer

SurfaofylayerWindSpeed PotentlaJTemperature Water VaporMixing RatioFIGURE 2.1 Idealized profiles of wind speed, potential temperature, and watervapor mixing ratio for the convectively mixed, cloud-free atmospheric boundarylayer (after Fairall et al., 1982).

12 COASTAL METEOROLOGYstable ABL in steady-state or slowly evolving conditions. This theory has,however, exhibited shortcomings in general application.In terms of simple models, the present state of understanding of thegeneric ABL can be described crudely as follows. It is natural to classifyconditions of the ABL by dynamical regimes of increasing complexity:cloud-free, convective; cloud-free and shear-driven; baroclinic; stable; stra-tocumulus; tradewind cumulus; and broken clouds. Coastal meteorologyencompasses all seven of these regimes. Cloud-modified and stable bound-ary layers remain essentially unsolved problems. A general similarity theo-ry that can handle all possible cloud regimes does not exist. Higher-orderclosure models have yet to demonstrate detailed agreement for even thesimplest cases (Holt and Raman, 1988), and recent LES studies (Moeng andWyngaard, 1989) have called into question the transport and dissipationclosures used in most second-order models. Transport and dispersion prop-erties of the ABL are strongly dependent on the characteristics of coherentstructures and the higher-order moments (Weil, 1990). These propertieshave only recently been studied for the homogeneous convective ABL (Moengand Rotunno, 1990; Moeng and Wyngaard, 1989). Although some under-standing of the idealized ABL (Figure 1.1) exists, actual coastal ABL struc-ture is often significantly different. For example, Figure 2.2 shows a stable 12 14 16 18 20 2 24 26 28 30 32Wind Speed(m/s) 294 291 302 306 1 2 3 4 5 6 7 8 9Potential Water VaporTemperature Mixing Ratio(K) (gTkg)FIGURE 2.2 Vertical profiles of wind speed, potential temperature, and watervapor mixing ratio along the north coast of California (after Winant et al., 1988).Note the strong low-level inversion in potential temperature at approximately 250 maltitude, coincident with the maximum in wind speed. Contrast these profiles withthe simple mixed layer structure depicted in Figure 2.1.

BOUNDARY LAYER PROCESSES 13ABL with a baroclinically induced jet but a well-mixed humidity. Thus,there is a need both to improve our understanding of the idealized case andto study cases that exhibit stable boundary layers as well as those withstrong baroclinicity. Surface InteractionsSurface fluxes can be measured in homogeneous and moderately inho-mogeneous terrain using the eddy correlation technique (Businger, 1986;McMillen, 1988; Wyngaard, 1988), which is considered to be the measure-ment standard. In numerical models (or when the direct method is notavailable or applicable), bulk transfer coefficients and surface layer similar-ity parameterizations are used to relate the fluxes to the near-surface meanmeteorological variables and the surface properties. This approach is oftenquite successful over the open ocean but cannot be straightforwardly ap-plied over land; terrain, soil, and plant canopy interactions greatly compli-cate the physics (Priestly and Taylor, 1972; Sellers et al., 1986). This is thesituation for local coastal climatology where the intensity of the land-seabreeze circulation is relatively much stronger with dry, lightly vegetatedcoastal lands (Segal et al., 1988).Inhomogeneous surfaces cause special problems because it is difficultto relate the point measurements used to characterize the surface to thelarger-scale mean fluxes (Schuepp et al., 1990). Also, the bulk expressionsare intended to relate the average flux to the average bulk variables. SinceABL-scale eddy turnover time is about 15 minutes, it takes about an hour toaverage ABL variability to obtain a representative sample. In that hour aparcel of air in the ABL can easily travel horizontally about 20 km. Thiscalls into question whether one can rely on surface similarity expressionsthat are obtained from 1-hour averages of field measurements over homoge-neous terrain in models with 1-minute tune steps applied over 10 x 10 kmhorizontal grids. For example, Beljaars and Holtslag (1991) found thatcharacterization of momentum transfer on horizontal scales of a few kilo-meters required an "effective" roughness length considerably greater thanthe local value. The situation for moisture and sensible heat transfer is evenmore difficult. To quote Beljaars and Holtslag (1991), "More complicatedland surface schemes are certainly available to describe the physics in moredetail. . . . [H]owever, it is not clear whether all the parameters that specifythe land surface in such models can easily be determined." There is also adistinction between a patchy surface that is statistically homogeneous and a"nonstationary" situation where the average properties vary with positionand time. The applicability of surface layer similarity and the implicationsfor the bulk transfer coefficients for these conditions are virtually unex-plored. BOUNDARY LAYER PROCESSES 13ABL with a baroclinically induced jet but a well-mixed humidity. Thus,there is a need both to improve our understanding of the idealized case andto study cases that exhibit stable boundary layers as well as those withstrong baroclinicity. Surface InteractionsSurface fluxes can be measured in homogeneous and moderately inho-mogeneous terrain using the eddy correlation technique (Businger, 1986;McMillen, 1988; Wyngaard, 1988), which is considered to be the measure-ment standard. In numerical models (or when the direct method is notavailable or applicable), bulk transfer coefficients and surface layer similar-ity parameterizations are used to relate the fluxes to the near-surface meanmeteorological variables and the surface properties. This approach is oftenquite successful over the open ocean but cannot be straightforwardly ap-plied over land; terrain, soil, and plant canopy interactions greatly compli-cate the physics (Priestly and Taylor, 1972; Sellers et al., 1986). This is thesituation for local coastal climatology where the intensity of the land-seabreeze circulation is relatively much stronger with dry, lightly vegetatedcoastal lands (Segal et al., 1988).Inhomogeneous surfaces cause special problems because it is difficultto relate the point measurements used to characterize the surface to thelarger-scale mean fluxes (Schuepp et al., 1990). Also, the bulk expressionsare intended to relate the average flux to the average bulk variables. SinceABL-scale eddy turnover time is about 15 minutes, it takes about an hour toaverage ABL variability to obtain a representative sample. In that hour aparcel of air in the ABL can easily travel horizontally about 20 km. Thiscalls into question whether one can rely on surface similarity expressionsthat are obtained from 1-hour averages of field measurements over homoge-neous terrain in models with 1-minute tune steps applied over 10 x 10 kmhorizontal grids. For example, Beljaars and Holtslag (1991) found thatcharacterization of momentum transfer on horizontal scales of a few kilo-meters required an "effective" roughness length considerably greater thanthe local value. The situation for moisture and sensible heat transfer is evenmore difficult. To quote Beljaars and Holtslag (1991), "More complicatedland surface schemes are certainly available to describe the physics in moredetail. . . . [H]owever, it is not clear whether all the parameters that specifythe land surface in such models can easily be determined." There is also adistinction between a patchy surface that is statistically homogeneous and a"nonstationary" situation where the average properties vary with positionand time. The applicability of surface layer similarity and the implicationsfor the bulk transfer coefficients for these conditions are virtually unex-plored.

14 COASTAL METEOROLOGYOn the ocean side of a coastal region, special problems arise in interfa-cial transfer. For the open ocean, a reasonable set of bulk transfer coeffi-cients (e.g., Smith, 1988) is available that appears adequate for many appli-cations. These coefficients represent air-sea transfer processes for averagesurface wave conditions as a function of mean wind speed. Wind speed,wave spectrum, and bulk transfer coefficients have been related theoretical-ly (Geernaert et al., 1986; Huang et al., 1986), but field measurements incoastal regions (e.g., Geernaert et al., 1987; Smith et al., 1990a, b) havedemonstrated greatly increased drag coefficients in shallow water (Figure2.3).It is clear that fetch and shallow water effects upset the normal equilib-rium wind-wave relationships. Fetch is primarily an issue for offshore windconditions, but distortion of the open ocean directional wave spectrum incoastal shallow regions is important regardless of wind regime; it becomesincreasingly important as wind speeds increase. Sounds and estuaries addcomplexities, often with irregular coastlines, river deltas, and islands. Ef-fects on heat and gas fluxes also are unknown. (See Chapter 6 for furtherdiscussion and Geernaert (1990) for a comprehensive review of these con-

0.0 12 16 20 24 28Windspeed (m/sec)FIGURE 2.3 Distribution of neutral drag coefficient, CDN, with wind speed: (1)over deep open ocean, (2) over deep coastal ocean, (3) over deep water, (4) NorthSea depth of 30 m, (5) North Sea depth of 16 m, (6) Lough Neagh depth of 15 m (7)Lake Ontario depth of 10 m, (8) Lake Geneva depth of 3 m (from Geernaert 1990)

BOUNDARY LAYER PROCESSES 15cepts.) The evolving wave field is influenced by the wind stress, but thestress vector produced by a given wind field is dependent on the directionalwave field. Thus, predictions of wind and wave fields on the continentalshelves are strongly coupled. Significant and unexplained differences be-tween the mean wind direction and the mean stress direction have beenobserved in coastal regions (Geernaert, 1990). Substantial hydrostatic sta-bility modulation of stress and surface wind fields has also been observed inassociation with sea surface temperature variations typical of coastal re-gions. A comprehensive theoretical and experimental study of wind-wave-stress-scalar flux relationships on the continental shelves should be an im-portant component of a coastal meteorology research program.Internal Boundary LayersAir that is modified by flow over an abrupt change in surface propertiesis said to be confined to an internal boundary layer (IBL). Here we areconcerned with the effects of a boundary between two different but individ-ually homogeneous surfaces, as opposed to the boundary layer effects ofmore general forms of inhomogeneity discussed below. When the surfaceheat flux changes at such a boundary, a thermal IBL (Garratt, 1987; Lyons,1975) is formed on the downwind side. Growth in the depth of the IBL isusually parameterized in terms of the downwind distance from the interface.Typically, the form is that of a power law, but many different formulas areavailable. Convective conditions promote rapid growth; thus, the IBL depthquickly reaches the existing capping inversion. In this case IBL consider-ations are important only close to the transition. Stable, convection-sup-pressing conditions downwind of the transition result in slow IBL growth,and the tendency is to form a permanent surface-based inversion (Mulhearn,1981). The situation is similar to the afternoon-evening transition for theoverland convective ABL (Zeman and Lumley, 1979). In this case anyturbulence above the new surface-based inversion can be cut off from thesurface source of energy and, in the absence of other sources, begin todecay. In an LES study of the decay of convective turbulence, Nieuwstadtand Brest (1986) found that the ABL depth, divided by the convectivemixing velocity, formed a characteristic decay time scale, but the behaviorof various turbulent variables was not easily parameterized.Currently, the standard approach to describing near-surface meteoro-logical profiles is to use surface similarity expressions with one set ofscaling parameters for the IBL and a second set for the old ABL above andthe constraint that the profiles must match continuously at the IBL inter-face. This approach assumes that the dynamics above the IBL are unaffect-ed by its formation. This approximation can be valid only fairly close tothe transition region. More sophisticated model studies (e.g., Claussen, BOUNDARY LAYER PROCESSES 15cepts.) The evolving wave field is influenced by the wind stress, but thestress vector produced by a given wind field is dependent on the directionalwave field. Thus, predictions of wind and wave fields on the continentalshelves are strongly coupled. Significant and unexplained differences be-tween the mean wind direction and the mean stress direction have beenobserved in coastal regions (Geernaert, 1990). Substantial hydrostatic sta-bility modulation of stress and surface wind fields has also been observed inassociation with sea surface temperature variations typical of coastal re-gions. A comprehensive theoretical and experimental study of wind-wave-stress-scalar flux relationships on the continental shelves should be an im-portant component of a coastal meteorology research program.Internal Boundary LayersAir that is modified by flow over an abrupt change in surface propertiesis said to be confined to an internal boundary layer (IBL). Here we areconcerned with the effects of a boundary between two different but individ-ually homogeneous surfaces, as opposed to the boundary layer effects ofmore general forms of inhomogeneity discussed below. When the surfaceheat flux changes at such a boundary, a thermal IBL (Garratt, 1987; Lyons,1975) is formed on the downwind side. Growth in the depth of the IBL isusually parameterized in terms of the downwind distance from the interface.Typically, the form is that of a power law, but many different formulas areavailable. Convective conditions promote rapid growth; thus, the IBL depthquickly reaches the existing capping inversion. In this case IBL consider-ations are important only close to the transition. Stable, convection-sup-pressing conditions downwind of the transition result in slow IBL growth,and the tendency is to form a permanent surface-based inversion (Mulhearn,1981). The situation is similar to the afternoon-evening transition for theoverland convective ABL (Zeman and Lumley, 1979). In this case anyturbulence above the new surface-based inversion can be cut off from thesurface source of energy and, in the absence of other sources, begin todecay. In an LES study of the decay of convective turbulence, Nieuwstadtand Brest (1986) found that the ABL depth, divided by the convectivemixing velocity, formed a characteristic decay time scale, but the behaviorof various turbulent variables was not easily parameterized.Currently, the standard approach to describing near-surface meteoro-logical profiles is to use surface similarity expressions with one set ofscaling parameters for the IBL and a second set for the old ABL above andthe constraint that the profiles must match continuously at the IBL inter-face. This approach assumes that the dynamics above the IBL are unaffect-ed by its formation. This approximation can be valid only fairly close tothe transition region. More sophisticated model studies (e.g., Claussen,

16 COASTAL METEOROLOGY1987) have shown that substantial mean vertical motions are also inducedby the transition, even outside the IBL. Because the IBL exists for such ashort distance in convective conditions, the formation and growth of thestable IBL are more critical. Here a key issue is the physics of the entrain-ment processes at the top of the IBL and the associated induced verticalvelocity fields.The Inhomogeneous Atmospheric Boundary LayerUnderstanding ABL development and evolution in regions of abrupt orgradual changes of surface properties (coastal zones, ice-to-water surfacetransitions, ocean surface temperature fronts, etc.) involves consideration ofhorizontal advection, baroclinic forcing, nonequilibrium turbulence effects(for instance, the time derivative of the turbulent kinetic energy is not negli-gible), the special influence of local clouds, and fully three-dimensionaldynamical processes.A simple way to view this three-dimensional problem is to break itdown into a conceptual model: a surface grid with each surface pointoccupied by a mean and turbulence profile, governed by one-dimensionalturbulent mixing processes. Adjacent grid points are coupled in the normalmanner through horizontal advection and horizontal pressure gradients. Thisapproach has enjoyed some success with mixed layer models (Overland etal., 1983; Reynolds, 1984; Stage and Businger, 1980; Steyn and Oke, 1982)and higher-order closure models (Bennet and Hunkins, 1986; Tjernstrom,1990; Wai and Stage, 1990). However, decoupling of the turbulence dy-namics from the horizontal structure is a simplifying assumption that hasnever been tested. Clearly, similarity approaches (e.g., mixed layer models)that are tuned to quasi-equilibrium conditions are of limited applicability ininhomogeneous conditions. However, these limits have not been estab-lished. It may be that the techniques used in highly inhomogeneous urbanboundary layer models (e.g., Uno et al., 1989) are adaptable to coastalproblems.Both land-sea breeze cycles and cold air outbreaks have been examinedwith models, but a comprehensive and extensive program to compare modelresults with measurements has not been attempted. Baroclinic effects asso-ciated with a sloping inversion (Brost et al., 1982; Overland et al., 1983)are known to be substantial, especially in west coast regimes, but they tooare virtually unstudied.The relative lack of experimental studies of boundary layer physics incoastal zones is not the only reason to question present-day boundary layermodels. A recent LES study (Moeng and Wyngaard, 1989) of second-orderclosure parameterizations suggests that their rather modest successes in ho-mogeneous conditions (e.g., Holt and Raman, 1988) are not expected to 16 COASTAL METEOROLOGY1987) have shown that substantial mean vertical motions are also inducedby the transition, even outside the IBL. Because the IBL exists for such ashort distance in convective conditions, the formation and growth of thestable IBL are more critical. Here a key issue is the physics of the entrain-ment processes at the top of the IBL and the associated induced verticalvelocity fields.The Inhomogeneous Atmospheric Boundary LayerUnderstanding ABL development and evolution in regions of abrupt orgradual changes of surface properties (coastal zones, ice-to-water surfacetransitions, ocean surface temperature fronts, etc.) involves consideration ofhorizontal advection, baroclinic forcing, nonequilibrium turbulence effects(for instance, the time derivative of the turbulent kinetic energy is not negli-gible), the special influence of local clouds, and fully three-dimensionaldynamical processes.A simple way to view this three-dimensional problem is to break itdown into a conceptual model: a surface grid with each surface pointoccupied by a mean and turbulence profile, governed by one-dimensionalturbulent mixing processes. Adjacent grid points are coupled in the normalmanner through horizontal advection and horizontal pressure gradients. Thisapproach has enjoyed some success with mixed layer models (Overland etal., 1983; Reynolds, 1984; Stage and Businger, 1980; Steyn and Oke, 1982)and higher-order closure models (Bennet and Hunkins, 1986; Tjernstrom,1990; Wai and Stage, 1990). However, decoupling of the turbulence dy-namics from the horizontal structure is a simplifying assumption that hasnever been tested. Clearly, similarity approaches (e.g., mixed layer models)that are tuned to quasi-equilibrium conditions are of limited applicability ininhomogeneous conditions. However, these limits have not been estab-lished. It may be that the techniques used in highly inhomogeneous urbanboundary layer models (e.g., Uno et al., 1989) are adaptable to coastalproblems.Both land-sea breeze cycles and cold air outbreaks have been examinedwith models, but a comprehensive and extensive program to compare modelresults with measurements has not been attempted. Baroclinic effects asso-ciated with a sloping inversion (Brost et al., 1982; Overland et al., 1983)are known to be substantial, especially in west coast regimes, but they tooare virtually unstudied.The relative lack of experimental studies of boundary layer physics incoastal zones is not the only reason to question present-day boundary layermodels. A recent LES study (Moeng and Wyngaard, 1989) of second-orderclosure parameterizations suggests that their rather modest successes in ho-mogeneous conditions (e.g., Holt and Raman, 1988) are not expected to

BOUNDARY LAYER PROCESSES 1 7carry over to the coastal regime because homogeneous conditions do notseverely test the parameterizations. To quote Moeng and Wyngaard (1989,italics added for emphasis), "Most observational and model studies showthat in the absence of abrupt changes in boundary conditions, the heat fluxprofile is indeed essentially linear in the mixed layer. Thus, given theproper boundary conditions, second-order models will tend to have the cor-rect vertical profile of buoyant production rates within the mixed layer,regardless of the fidelity of their closure parameterizations" Moeng andWyngaard also point out that under homogeneous conditions the mean val-ue of the rate of dissipation of turbulent kinetic energy in the mixed layer isalso nearly independent of closure approximations.The importance of local coherent wind circulations generated by hetero-geneous surfaces also must be assessed. Existing work (e.g., Andre" et al.,1990; Hadfield et al., 1991, 1992; Pielke et al., 1991; Walko et al., 1992)suggests that, if the spatial scale of the inhomogeneity is sufficiently large,a well-defined atmospheric circulation can develop. Development of suchcoherent circulations is also dependent on the large-scale wind speed, withstronger winds inhibiting their development. Claussen (1991) has intro-duced the concept of a blending height that could be applied when coherentwind circulations are likely to occur. This height is correlated with thehorizontal scale of variability and the aerodynamic roughness of the land-scape. When this height nears the height of the boundary layer, adjacentboundary layers are not significantly homogenized, and the resultant hori-zontal gradient in boundary layer heating can produce a well-defined localflow. When this height is much less than the planetary boundary layerheight, however, the horizontal mixing of the boundary layer precludescoherent circulations. We need to understand the conditions under whichhorizontal inhomogeneities in surface heating and cooling generate coherentwind circulations. Boundary Layer CloudsClouds within the ABL greatly complicate the physical processes be-cause they represent a form of vertical and horizontal inhomogeneities,significantly affect the dynamics, and couple strongly with atmospheric ra-diation. For example, radiative heating effects of marine stratocumulusclouds can cause stress divergence to vary within the lower part of theABL. During the day, if the cloud layer warms faster than the subcloudlayer, the cloud may become decoupled, with a corresponding increase inwind stress and heat flux divergence between the top of the subcloud layerand the surface (e.g., Hignett, 1991; Rogers and Koracin, 1992).Stratiform clouds are persistent features of cool upwelling coastal re-gions, such as the west coast of the United States, and cool climate regions BOUNDARY LAYER PROCESSES 1 7carry over to the coastal regime because homogeneous conditions do notseverely test the parameterizations. To quote Moeng and Wyngaard (1989,italics added for emphasis), "Most observational and model studies showthat in the absence of abrupt changes in boundary conditions, the heat fluxprofile is indeed essentially linear in the mixed layer. Thus, given theproper boundary conditions, second-order models will tend to have the cor-rect vertical profile of buoyant production rates within the mixed layer,regardless of the fidelity of their closure parameterizations" Moeng andWyngaard also point out that under homogeneous conditions the mean val-ue of the rate of dissipation of turbulent kinetic energy in the mixed layer isalso nearly independent of closure approximations.The importance of local coherent wind circulations generated by hetero-geneous surfaces also must be assessed. Existing work (e.g., Andre" et al.,1990; Hadfield et al., 1991, 1992; Pielke et al., 1991; Walko et al., 1992)suggests that, if the spatial scale of the inhomogeneity is sufficiently large,a well-defined atmospheric circulation can develop. Development of suchcoherent circulations is also dependent on the large-scale wind speed, withstronger winds inhibiting their development. Claussen (1991) has intro-duced the concept of a blending height that could be applied when coherentwind circulations are likely to occur. This height is correlated with thehorizontal scale of variability and the aerodynamic roughness of the land-scape. When this height nears the height of the boundary layer, adjacentboundary layers are not significantly homogenized, and the resultant hori-zontal gradient in boundary layer heating can produce a well-defined localflow. When this height is much less than the planetary boundary layerheight, however, the horizontal mixing of the boundary layer precludescoherent circulations. We need to understand the conditions under whichhorizontal inhomogeneities in surface heating and cooling generate coherentwind circulations. Boundary Layer CloudsClouds within the ABL greatly complicate the physical processes be-cause they represent a form of vertical and horizontal inhomogeneities,significantly affect the dynamics, and couple strongly with atmospheric ra-diation. For example, radiative heating effects of marine stratocumulusclouds can cause stress divergence to vary within the lower part of theABL. During the day, if the cloud layer warms faster than the subcloudlayer, the cloud may become decoupled, with a corresponding increase inwind stress and heat flux divergence between the top of the subcloud layerand the surface (e.g., Hignett, 1991; Rogers and Koracin, 1992).Stratiform clouds are persistent features of cool upwelling coastal re-gions, such as the west coast of the United States, and cool climate regions

18 COASTAL METEOROLOGYsuch as the Arctic. Recent studies of the global radiation budget havehighlighted the possible role of stratiform clouds as an ameliorating influ-ence on the wanning of the atmosphere by high-altitude clouds (Ramanathanet al., 1989). Although a number of experiments provide considerable in-sight into the processes that control the development and dissipation ofstratiform clouds (Randall et al., 1984), the effect of variability of the coast-al ocean, topography, and the marine ABL on these clouds has receivedmuch less attention. The effect of coastally trapped waves on the depth ofthe marine layer may also play an important role in the persistence of theseclouds; the effect of the sea breeze on subsidence and the mesoscale pres-sure gradients may also be an important mesoscale process that controls thelife cycle and fractional coverage of coastal stratus. In turn, the stratusclouds can substantially modulate the sea breeze cycle by moving ashoreand reducing inland solar-induced convection. Skupniewicz et al. (1991)have presented the only measurement and model examination of the diurnalevolution of the coastal stratocumulus cloud edge and its dramatic effect onsea breeze front dynamics.SUMMARY AND CONCLUSIONSUnderstanding of coastal boundary layer processes will be improved bygeneral advances in boundary layer science. However, some problems spe-cific to the coastal regime require immediate study. Theoretical develop-ments, modeling studies, and field measurement programs are required toexplain key unknown properties and processes involving the ABL. Thepanel recommends the following:Studies should be conducted to determine the properties for inhomo-geneous and nonequilibrium conditions, including suitable surface flux andmixed layer similarity parameterizations, and the general relationships ofthe ensemble average first- and higher-order turbulence variables.Further research on the fundamental relationships among the ocean wavespectrum, surface fluxes, and bulk ABL properties should be conducted.Studies should be carried out to determine the physical process of thegrowth of the top of the stable IBL, including entrainment and inducedmean vertical velocity effects, and the decay processes of turbulence abovea newly formed IBL.Investigations to clarify coastal marine stratocumulus and overlandfair weather cumulus cloud regimes and their influence on land-sea breezecycles should be conducted.Studies should be undertaken to determine the spatial scale at whichhorizontal inhomogeneities in surface heating and cooling become largeenough to generate coherent wind circulations. 18 COASTAL METEOROLOGYsuch as the Arctic. Recent studies of the global radiation budget havehighlighted the possible role of stratiform clouds as an ameliorating influ-ence on the wanning of the atmosphere by high-altitude clouds (Ramanathanet al., 1989). Although a number of experiments provide considerable in-sight into the processes that control the development and dissipation ofstratiform clouds (Randall et al., 1984), the effect of variability of the coast-al ocean, topography, and the marine ABL on these clouds has receivedmuch less attention. The effect of coastally trapped waves on the depth ofthe marine layer may also play an important role in the persistence of theseclouds; the effect of the sea breeze on subsidence and the mesoscale pres-sure gradients may also be an important mesoscale process that controls thelife cycle and fractional coverage of coastal stratus. In turn, the stratusclouds can substantially modulate the sea breeze cycle by moving ashoreand reducing inland solar-induced convection. Skupniewicz et al. (1991)have presented the only measurement and model examination of the diurnalevolution of the coastal stratocumulus cloud edge and its dramatic effect onsea breeze front dynamics.SUMMARY AND CONCLUSIONSUnderstanding of coastal boundary layer processes will be improved bygeneral advances in boundary layer science. However, some problems spe-cific to the coastal regime require immediate study. Theoretical develop-ments, modeling studies, and field measurement programs are required toexplain key unknown properties and processes involving the ABL. Thepanel recommends the following:Studies should be conducted to determine the properties for inhomo-geneous and nonequilibrium conditions, including suitable surface flux andmixed layer similarity parameterizations, and the general relationships ofthe ensemble average first- and higher-order turbulence variables.Further research on the fundamental relationships among the ocean wavespectrum, surface fluxes, and bulk ABL properties should be conducted.Studies should be carried out to determine the physical process of thegrowth of the top of the stable IBL, including entrainment and inducedmean vertical velocity effects, and the decay processes of turbulence abovea newly formed IBL.Investigations to clarify coastal marine stratocumulus and overlandfair weather cumulus cloud regimes and their influence on land-sea breezecycles should be conducted.Studies should be undertaken to determine the spatial scale at whichhorizontal inhomogeneities in surface heating and cooling become largeenough to generate coherent wind circulations.

Thermally Driven Effects Differences in land and sea surface temperature and heat flux result indirect, thermally driven wind systems over a spectrum of temporal andspatial scales. The best known among these are the mesoscale land and sea(lake) breeze circulation systems (see, e.g., Defant, 1950), which are inher-ently diurnal in nature. Much less studied are the regimes induced by warmocean waters adjacent to a large cold land mass. These circulations, whichare not diurnal in nature, are termed coastal fronts. Even less well under-stood are systems present over the coastal ice-land boundary. As an exam-ple, in arctic regions, offshore katabatic winds are believed to play a keyrole in forming and altering polynyas and leads in coastal ice sheets.THE LAND BREEZE AND THE SEA BREEZEThe land-sea breeze system (LSBS) typifies the class of mesoscale at-mospheric systems induced by spatial inhomogeneities of surface heat fluxinto the boundary layer. The LSBS has been identified since the time of theclassical Greeks (circa 350 B.C.). By the late 1960s, identifiable literaturereferences exceeded 500 (Baralt and Brown, 1965; Jehn, 1973). Of allmesoscale phenomena, the LSBS over flat terrain has been among the moststudied observationally, analytically, and numerically. This is undoubtedlya result of their geographically fixed nature, their frequent occurrence, theirease of recognition from conventional observations, the concentration ofobservers in coastal zones, and their importance to local weather and cli-mate.

20 COASTAL METEOROLOGYBy way of definition for this review, a land and sea breeze is a diurnalthermally driven circulation in which a definite surface convergence zoneexists between air streams having over-water versus overland histories. Thesebreezes are differentiated from the sustained onshore-offshore winds drivenby the synoptic pressure field, which are termed sea-land winds. Duringlighter synoptic wind regimes, perturbations induced by the coastal discon-tinuity are often detectable but may not always result in a coherent recircu-lating wind system. The effects of the LSBS are many, including signifi-cantly altering the direction and speed of the ABL winds; influencing low-levelstratiform and cumuliform clouds; initiating, suppressing, and modifyingprecipitating convective storms; recirculating and trapping pollutants re-leased in or becoming entrained into the circulation; perturbing regionalmixing depths; and creating strong near-shore temperature, moisture, andrefractive index gradients. Improved understanding of LSBS should en-hance applications to a wide variety of commercial, industrial, and defenseactivities (Raman, 1982).Sea-lake breeze inflow layers can vary from 100 m to over 1000 m indepth. Inland frontal penetration can vary from less than 1 km to over 100km, with propagation speeds ranging from nearly stationary to >5 m/sec.The offshore extent of the inflow layer is less well known. Peak windspeeds are typically less than 10 m/sec. The overlying return flow layerdepth is generally twice that of the inflow, but is often difficult to differen-tiate from the synoptic flow. Given the difficulty of measuring atmosphericmesoscale vertical motions, little is known about the detailed structure ofupdrafts associated with the sea breeze front. Some observational evidencefrom gliders, tetroons, Doppler lidar, etc., has suggested organized frontalzone motions of several m/sec. The coarse mesh size (often >5 km) used inmost mesoscale simulations tends to portray peak vertical motions in thetens of centimeters per second range. More recenUnodeling studies (Lyonset al., 199la, b) suggest that the sea breeze convergence zone is at timesextremely narrow (perhaps <1000 m) and may produce regions of organizedmesoscale ascent ranging from 1 to 4 m/sec. Even less is known about thebroader and weaker subsidence regions offshore. Needed are improvedmeasurements of vertical motions associated with the LSBS and companionmodeling studies in which the mesh sizes used are adequate to resolve theobserved features (Figure 3.1). Recent extremely fine-mesh two-dimen-sional simulations by Sha et al. (1991) resolved complex Kelvin-Helmholtzinstabilities and other characteristics resembling a laboratory gravity cur-rent (Simpson, 1982).Circulations over large lakes are very similar to their oceanic counter-parts. Smaller lakes, estuaries, and larger rivers also can significantly per-turb the regional flow. The interactions between synoptic flow and waterbody size and orientation require additional study. Recent numerical simu- 20 COASTAL METEOROLOGYBy way of definition for this review, a land and sea breeze is a diurnalthermally driven circulation in which a definite surface convergence zoneexists between air streams having over-water versus overland histories. Thesebreezes are differentiated from the sustained onshore-offshore winds drivenby the synoptic pressure field, which are termed sea-land winds. Duringlighter synoptic wind regimes, perturbations induced by the coastal discon-tinuity are often detectable but may not always result in a coherent recircu-lating wind system. The effects of the LSBS are many, including signifi-cantly altering the direction and speed of the ABL winds; influencing low-levelstratiform and cumuliform clouds; initiating, suppressing, and modifyingprecipitating convective storms; recirculating and trapping pollutants re-leased in or becoming entrained into the circulation; perturbing regionalmixing depths; and creating strong near-shore temperature, moisture, andrefractive index gradients. Improved understanding of LSBS should en-hance applications to a wide variety of commercial, industrial, and defenseactivities (Raman, 1982).Sea-lake breeze inflow layers can vary from 100 m to over 1000 m indepth. Inland frontal penetration can vary from less than 1 km to over 100km, with propagation speeds ranging from nearly stationary to >5 m/sec.The offshore extent of the inflow layer is less well known. Peak windspeeds are typically less than 10 m/sec. The overlying return flow layerdepth is generally twice that of the inflow, but is often difficult to differen-tiate from the synoptic flow. Given the difficulty of measuring atmosphericmesoscale vertical motions, little is known about the detailed structure ofupdrafts associated with the sea breeze front. Some observational evidencefrom gliders, tetroons, Doppler lidar, etc., has suggested organized frontalzone motions of several m/sec. The coarse mesh size (often >5 km) used inmost mesoscale simulations tends to portray peak vertical motions in thetens of centimeters per second range. More recenUnodeling studies (Lyonset al., 199la, b) suggest that the sea breeze convergence zone is at timesextremely narrow (perhaps <1000 m) and may produce regions of organizedmesoscale ascent ranging from 1 to 4 m/sec. Even less is known about thebroader and weaker subsidence regions offshore. Needed are improvedmeasurements of vertical motions associated with the LSBS and companionmodeling studies in which the mesh sizes used are adequate to resolve theobserved features (Figure 3.1). Recent extremely fine-mesh two-dimen-sional simulations by Sha et al. (1991) resolved complex Kelvin-Helmholtzinstabilities and other characteristics resembling a laboratory gravity cur-rent (Simpson, 1982).Circulations over large lakes are very similar to their oceanic counter-parts. Smaller lakes, estuaries, and larger rivers also can significantly per-turb the regional flow. The interactions between synoptic flow and waterbody size and orientation require additional study. Recent numerical simu-

THERMALLY DRIVEN EFFECTS 21lations have suggested that surface heat flux differences of 100 W/m2 ormore over several tens of kilometers can generate sea-breeze-like circula-tions (Segal et al., 1988). These physiographic mesoscale circulations canresult from differences in soil type, land use, soil moisture (from irrigationand precipitation), snow cover, smoke/haze, and cloudiness. Additionalfield programs investigating mesoscale processes in both LSBS and physio-graphic systems are warranted as a test of the validity of available models.The LSBSs are not exclusively "clear weather" phenomena; they sub-stantially affect and interact with various cloud types. The modifications offog and stratus in the LSBSs over the Great Lakes, Gulf, and Atlantic coastshave received less attention than those along the Pacific coastline. Advec-tion through the coastal domain of middle- and high-level cloud decks,large smoke plumes, and urban- and regional-scale photochemical and sul-fate hazes can materially affect the energetics of the LSBS, although theseimpacts have not been quantified. The LSBS profoundly affects the forma-tion and fate of shallow convective clouds. Cumulus suppression in subsid-ing regions of the sea breeze cell has long been noted in satellite imagery.Under very light wind conditions, cumulus growth is enhanced within thesea breeze frontal zone updrafts. When the prevailing regional flow advectssmall convective clouds seaward across the frontal zone, the responses aremore complex. Dissipation often occurs, but the relative roles played bysubsidence versus disruption of cloud-capped thermals rooted in the surfacesuperadiabatic layer are uncertain. Studies of the interactions of large ed-dies or convective thermals with the sea breeze frontal zone are warranted.The impact of the sea breeze front on deeper (precipitating) convectiveclouds is more complex. At midlatitudes the sea breeze can either enhanceor weaken convective storms (Chandik and Lyons, 1971). On a scale oftens of kilometers, the Florida sea breeze has been found to trigger thegeneral development of deep convection (Burpee and Lahiff, 1984). Thun-derstorm development is intermittent along such frontal zones, and it isuncertain whether perturbations in the frontal convergence zone or localizedresponses to inhomogeneities in the surface energy budgets (or both) causeindividual storms to form. Fine-mesh numerical modeling studies suggestthat different spatial scales of topography and shoreline geometry produce aspectrum of convective-scale responses. Sea breeze thunderstorms affect-ing the Kennedy Space Center (Lyons et al., 1992) are initiated by both theeast and west coast sea breezes. In addition, however, strong local conver-gence onto features such as Merritt Island trigger smaller-scale thunder-storms embedded within the larger sea breeze circulation (Figure 3.2). Thecomplex feedbacks between the precipitating clouds and the LSBS are onlypartially understood. Also, convective responses to sea breezes over moun-tainous islands such as those found in the "marine continent" of southeastAsia require further study. Sea breeze thunderstorms contribute approxi- THERMALLY DRIVEN EFFECTS 21lations have suggested that surface heat flux differences of 100 W/m2 ormore over several tens of kilometers can generate sea-breeze-like circula-tions (Segal et al., 1988). These physiographic mesoscale circulations canresult from differences in soil type, land use, soil moisture (from irrigationand precipitation), snow cover, smoke/haze, and cloudiness. Additionalfield programs investigating mesoscale processes in both LSBS and physio-graphic systems are warranted as a test of the validity of available models.The LSBSs are not exclusively "clear weather" phenomena; they sub-stantially affect and interact with various cloud types. The modifications offog and stratus in the LSBSs over the Great Lakes, Gulf, and Atlantic coastshave received less attention than those along the Pacific coastline. Advec-tion through the coastal domain of middle- and high-level cloud decks,large smoke plumes, and urban- and regional-scale photochemical and sul-fate hazes can materially affect the energetics of the LSBS, although theseimpacts have not been quantified. The LSBS profoundly affects the forma-tion and fate of shallow convective clouds. Cumulus suppression in subsid-ing regions of the sea breeze cell has long been noted in satellite imagery.Under very light wind conditions, cumulus growth is enhanced within thesea breeze frontal zone updrafts. When the prevailing regional flow advectssmall convective clouds seaward across the frontal zone, the responses aremore complex. Dissipation often occurs, but the relative roles played bysubsidence versus disruption of cloud-capped thermals rooted in the surfacesuperadiabatic layer are uncertain. Studies of the interactions of large ed-dies or convective thermals with the sea breeze frontal zone are warranted.The impact of the sea breeze front on deeper (precipitating) convectiveclouds is more complex. At midlatitudes the sea breeze can either enhanceor weaken convective storms (Chandik and Lyons, 1971). On a scale oftens of kilometers, the Florida sea breeze has been found to trigger thegeneral development of deep convection (Burpee and Lahiff, 1984). Thun-derstorm development is intermittent along such frontal zones, and it isuncertain whether perturbations in the frontal convergence zone or localizedresponses to inhomogeneities in the surface energy budgets (or both) causeindividual storms to form. Fine-mesh numerical modeling studies suggestthat different spatial scales of topography and shoreline geometry produce aspectrum of convective-scale responses. Sea breeze thunderstorms affect-ing the Kennedy Space Center (Lyons et al., 1992) are initiated by both theeast and west coast sea breezes. In addition, however, strong local conver-gence onto features such as Merritt Island trigger smaller-scale thunder-storms embedded within the larger sea breeze circulation (Figure 3.2). Thecomplex feedbacks between the precipitating clouds and the LSBS are onlypartially understood. Also, convective responses to sea breezes over moun-tainous islands such as those found in the "marine continent" of southeastAsia require further study. Sea breeze thunderstorms contribute approxi-

2 COASTAL METEOROLOGY 2 COASTAL METEOROLOGY

THERMALLYDRIVEN EFFECTS 23

24 COASTAL METEOROLOGYMALLLAKEMAINLAND FLORIDA TIX RIVER FIGURE 3.2 Response of deep convective clouds to coastal circulations in thevicinity of the Kennedy Space Center. In addition to the primary east and westcoast sea breezes formed by the contrast between the Florida peninsula and sur-rounding ocean, numerous lakes, surface land use inhomogeneities, estuaries andislands perturb the mesoscale flow. The convective response is more complex thansuggested by earlier sea breeze thunderstorm studies. As an example, while thegeneI Atlantic sea breeze (ASB) develops, enhanced convergence onto MerrittIsland triggers rapid growth of a small thundershower by late morning. Widespreadconvection along the ASB does not develop until the late-afternoon approach of animpulse associated with the west coast sea breeze (graphics courtesy of Cecil S

THERMALLY DRIVEN EFFECTS 25mately 40 percent of Florida's rainfall and probably more in many othercoastal regions. Sea breeze convective storm development is sensitive tosmall variations in middle-tropospheric temperature lapse rates and mois-ture, such as those postulated in global greenhouse warming scenarios. Thus,middle tropospheric changes that affect this significant source of precipita-tion in tropical coastal areas is potentially important.Except for a few recent studies (Ohara et al., 1989), the land breeze isunderstood even less. Wind speeds are typically <5 m/sec, and the offshoreflowing layer is often <100 m deep. The land breeze can often be commin-gled with stronger and deeper terrain-induced katabatic flows. The forma-tion of thunderstorms associated with the offshore boundary of the landbreeze over Gulf and Atlantic coastal waters has been addressed only invery cursory ways. Intense Great Lakes snow squalls also interact in com-plex ways with the land breeze (Passarelli and Braham, 1981).The transition of the land breeze into a sea breeze, occurring offshore,is largely undocumented. The breakdown of the land-lake breeze front is notwell understood. Sometimes it retreats offshore as a distinct front, while atother times it simply pushes inland and dissipates after sunset. On otheroccasions, strong onshore flow may continue over coastal regions until pastmidnight local time (the "fossil" sea breeze). Comprehensive studies of theLSBS through consecutive diurnal cycles are desired, with emphasis on theland breeze and the morning and evening transition periods.The LSBS and many other similar mesoscale circulations are poorlyresolved in conventional weather-observing network systems, creating seri-ous problems in operational forecasting. Local forecasters employ simpleforecasting techniques using synoptic observational data (Lyons, 1972) topredict potential sea breeze occurrences. The character of the LSBS iscontrolled by a variety of factors, including land-sea surface temperaturedifferences; latitude and day of the year; the synoptic wind and its orienta-tion to the shoreline; the thermal stability of the lowest 200 to 300 mb ofthe atmosphere; patterns of land use and soil moisture; surface solar radia-tion as affected by haze, smoke, stratiform, and convective cloudiness; andthe geometry of the shoreline and complexity of the surrounding terrain.Many of these factors are considered within mesoscale numerical modelingsystems that are well suited to land-sea breeze simulation.Our understanding of the LSBS is not comprehensive, being largelyconfined to idealized conditions. When large-scale winds are virtually non-existent over an infinitely long, two-dimensional flat coastline, it is compar-atively easy to describe the basic dynamics of the LSBS (Defant, 1951).Numerous analytical studies of sea breeze phenomena have been conducted(see, for example, Haurwitz, 1947, and Rotunno, 1983). Nonlinear numeri-cal modeling studies using two-dimensional models have been summarizedby Pielke (1984). Newer nonhydrostatic, fine-mesh, nested-grid numerical THERMALLY DRIVEN EFFECTS 25mately 40 percent of Florida's rainfall and probably more in many othercoastal regions. Sea breeze convective storm development is sensitive tosmall variations in middle-tropospheric temperature lapse rates and mois-ture, such as those postulated in global greenhouse warming scenarios. Thus,middle tropospheric changes that affect this significant source of precipita-tion in tropical coastal areas is potentially important.Except for a few recent studies (Ohara et al., 1989), the land breeze isunderstood even less. Wind speeds are typically <5 m/sec, and the offshoreflowing layer is often <100 m deep. The land breeze can often be commin-gled with stronger and deeper terrain-induced katabatic flows. The forma-tion of thunderstorms associated with the offshore boundary of the landbreeze over Gulf and Atlantic coastal waters has been addressed only invery cursory ways. Intense Great Lakes snow squalls also interact in com-plex ways with the land breeze (Passarelli and Braham, 1981).The transition of the land breeze into a sea breeze, occurring offshore,is largely undocumented. The breakdown of the land-lake breeze front is notwell understood. Sometimes it retreats offshore as a distinct front, while atother times it simply pushes inland and dissipates after sunset. On otheroccasions, strong onshore flow may continue over coastal regions until pastmidnight local time (the "fossil" sea breeze). Comprehensive studies of theLSBS through consecutive diurnal cycles are desired, with emphasis on theland breeze and the morning and evening transition periods.The LSBS and many other similar mesoscale circulations are poorlyresolved in conventional weather-observing network systems, creating seri-ous problems in operational forecasting. Local forecasters employ simpleforecasting techniques using synoptic observational data (Lyons, 1972) topredict potential sea breeze occurrences. The character of the LSBS iscontrolled by a variety of factors, including land-sea surface temperaturedifferences; latitude and day of the year; the synoptic wind and its orienta-tion to the shoreline; the thermal stability of the lowest 200 to 300 mb ofthe atmosphere; patterns of land use and soil moisture; surface solar radia-tion as affected by haze, smoke, stratiform, and convective cloudiness; andthe geometry of the shoreline and complexity of the surrounding terrain.Many of these factors are considered within mesoscale numerical modelingsystems that are well suited to land-sea breeze simulation.Our understanding of the LSBS is not comprehensive, being largelyconfined to idealized conditions. When large-scale winds are virtually non-existent over an infinitely long, two-dimensional flat coastline, it is compar-atively easy to describe the basic dynamics of the LSBS (Defant, 1951).Numerous analytical studies of sea breeze phenomena have been conducted(see, for example, Haurwitz, 1947, and Rotunno, 1983). Nonlinear numeri-cal modeling studies using two-dimensional models have been summarizedby Pielke (1984). Newer nonhydrostatic, fine-mesh, nested-grid numerical

26 COASTAL METEOROLOGYmodels will be applied profitably to further studies of these types. Manyprevious modeling efforts emphasized coastal circulations in which the forcingfunctions were spatially homogeneous and temporally steady-state or diur-nally varying. Observational and modeling studies need to be extended tocoastal circulations occurring with nonhomogeneous and nonstationary syn-optic environments, irregular shorelines and complex topography, and het-erogeneous land use or land characteristics and soil moisture. Additionalmodeling challenges include accounting for the advection of middle- andupper-level clouds through the domain; changes in soil moisture; dynamicfeedback between the LSBS and deep convective storms; and turbidity dueto regional smoke, pollution, fog, and haze. There is evidence that gravitywaves are excited by regional air flowing over the sea breeze, which isdynamically equivalent to a mountain. These features are worthy of contin-ued investigation.While hemispheric and synoptic-scale numerical forecasting became wellestablished in the 1950s, it was not until the mid-1980s that more powerfulcomputers allowed organized attempts at operational mesoscale forecastingof coastal circulations and their effects (Lyons et al., 1987). Affordablehigh-speed computing and increasingly sophisticated numerical modelingtechniques now allow extended experiments in operational coastal zone windforecasting to be undertaken, yielding excellent opportunities to test thebreadth and depth of our understanding of the LSBS.Interaction of the LSBS with the urban heat island and greatly enhancedroughness lengths in large cities has been studied in New York, Tokyo,Toronto, and elsewhere. Studies of interacting sea breeze and topographi-cally forced flows (such as the Catalina eddy) are yielding improved under-standing of the complex interactions between mesoscale systems. Evenwith the large number of studies in coastal Southern California, the devel-opment and morphology of sea and land breeze circulations in mountainouscoastal terrain warrant much additional attention.Coastal thermally driven mesoscale circulations interact with smaller-scale surface-atmosphere energy exchange processes, cloud systems on avariety of scales, and the larger-scale synoptic patterns in which the mesos-cale circulations are embedded. The LSBS represents a challenging prob-lem for future observational and modeling programs, since it embodies manyof the complex issues involved in atmospheric-scale interactions.COASTAL FRONTSWhen air over land is colder than air over the sea for extended periods,the direct circulation that develops across a coast does not normally exhibittypical diurnal characteristics. A longer-lived cousin of the land and seabreeze front, called a coastal front (Bosart et al., 1972), can form and re- 26 COASTAL METEOROLOGYmodels will be applied profitably to further studies of these types. Manyprevious modeling efforts emphasized coastal circulations in which the forcingfunctions were spatially homogeneous and temporally steady-state or diur-nally varying. Observational and modeling studies need to be extended tocoastal circulations occurring with nonhomogeneous and nonstationary syn-optic environments, irregular shorelines and complex topography, and het-erogeneous land use or land characteristics and soil moisture. Additionalmodeling challenges include accounting for the advection of middle- andupper-level clouds through the domain; changes in soil moisture; dynamicfeedback between the LSBS and deep convective storms; and turbidity dueto regional smoke, pollution, fog, and haze. There is evidence that gravitywaves are excited by regional air flowing over the sea breeze, which isdynamically equivalent to a mountain. These features are worthy of contin-ued investigation.While hemispheric and synoptic-scale numerical forecasting became wellestablished in the 1950s, it was not until the mid-1980s that more powerfulcomputers allowed organized attempts at operational mesoscale forecastingof coastal circulations and their effects (Lyons et al., 1987). Affordablehigh-speed computing and increasingly sophisticated numerical modelingtechniques now allow extended experiments in operational coastal zone windforecasting to be undertaken, yielding excellent opportunities to test thebreadth and depth of our understanding of the LSBS.Interaction of the LSBS with the urban heat island and greatly enhancedroughness lengths in large cities has been studied in New York, Tokyo,Toronto, and elsewhere. Studies of interacting sea breeze and topographi-cally forced flows (such as the Catalina eddy) are yielding improved under-standing of the complex interactions between mesoscale systems. Evenwith the large number of studies in coastal Southern California, the devel-opment and morphology of sea and land breeze circulations in mountainouscoastal terrain warrant much additional attention.Coastal thermally driven mesoscale circulations interact with smaller-scale surface-atmosphere energy exchange processes, cloud systems on avariety of scales, and the larger-scale synoptic patterns in which the mesos-cale circulations are embedded. The LSBS represents a challenging prob-lem for future observational and modeling programs, since it embodies manyof the complex issues involved in atmospheric-scale interactions.COASTAL FRONTSWhen air over land is colder than air over the sea for extended periods,the direct circulation that develops across a coast does not normally exhibittypical diurnal characteristics. A longer-lived cousin of the land and seabreeze front, called a coastal front (Bosart et al., 1972), can form and re-

THERMALLY DRIVEN EFFECTS 27main quasi-stationary, parallel to the coast, for several days. Coastal frontshave been reported and studied in many parts of the world, including theEast and Gulf coasts of the United States (Bosart, 1975, 1981, 1984), theBlack Sea (Draghici, 1984), Norway (0kland, 1990), the Netherlands (Roeloffzenet al., 1986), and Japan (Fujibe, 1990). The importance of coastal fronts forfreezing rain and coastal cyclogenesis is discussed in Chapter 5.Along the Carolinas, coastal fronts are typically 1000 km long, withtemperature contrasts as large as 20C. Carolina coastal fronts tend to formwithin the boundary layer temperature gradient produced by differentialheating of air across the margin of the Gulf Stream. This process has beenstudied by Riordan (1990), who used radar, ship, buoy, and aircraft datafrom the Genesis of Atlantic Lows Experiment (GALE). Frontal formationwas found to be a discontinuous process, with the front forming in segmentsaligned with bands of shallow convection (Figure 3.2). The inland motionof the front was also discontinuous, for reasons that are not understood.This behavior contrasts with that of New England coastal fronts, whichhave been found to form along the coast and retain their identity as theymove inland (Nielsen, 1989).A wide variety of triggering mechanisms have been shown to providethe sustained differential heating or confluence necessary for coastal fronto-genesis. The most common occurs when a cold anticyclone approaches acoastline and winds become parallel to the coast. Air over land remainscold, while air just offshore continuously receives large heat fluxes fromthe sea surface. For example, over the Gulf Stream convective rainbandsoften form which may be accompanied by considerable lightning activity(Hobbs, 1987; Biswas and Hobbs, 1990). Other processes that are favor-able to coastal frontogenesis are frictional retardation and turning of thewind, upstream blocking of cold air, or lee convergence.ICE-EDGE BOUNDARIESAn understanding of the meteorology of coastal regions where sea ice ispresent is important for navigation, exploring for mineral resources, coastalbiological activity, and modeling sea ice and climate. In the present discus-sion, only ice-land boundary regions will be considered. This includesalmost all the Antarctic ice cover and the ice cover overlying the high-latitude continental shelves in the northern hemisphere. While many char-acteristics of the marginal ice zone (the boundary between sea ice and openocean; see Johannessen et al., 1988) are similar to those of coastal ice-edgeboundaries, the marginal ice zone will not be addressed here.A summary of meteorological processes occurring at ice-edge bound-aries is given by Barry (1986). Barry concluded that our basic knowledgeof meteorological conditions over ice-edge boundaries is very limited. The THERMALLY DRIVEN EFFECTS 27main quasi-stationary, parallel to the coast, for several days. Coastal frontshave been reported and studied in many parts of the world, including theEast and Gulf coasts of the United States (Bosart, 1975, 1981, 1984), theBlack Sea (Draghici, 1984), Norway (0kland, 1990), the Netherlands (Roeloffzenet al., 1986), and Japan (Fujibe, 1990). The importance of coastal fronts forfreezing rain and coastal cyclogenesis is discussed in Chapter 5.Along the Carolinas, coastal fronts are typically 1000 km long, withtemperature contrasts as large as 20C. Carolina coastal fronts tend to formwithin the boundary layer temperature gradient produced by differentialheating of air across the margin of the Gulf Stream. This process has beenstudied by Riordan (1990), who used radar, ship, buoy, and aircraft datafrom the Genesis of Atlantic Lows Experiment (GALE). Frontal formationwas found to be a discontinuous process, with the front forming in segmentsaligned with bands of shallow convection (Figure 3.2). The inland motionof the front was also discontinuous, for reasons that are not understood.This behavior contrasts with that of New England coastal fronts, whichhave been found to form along the coast and retain their identity as theymove inland (Nielsen, 1989).A wide variety of triggering mechanisms have been shown to providethe sustained differential heating or confluence necessary for coastal fronto-genesis. The most common occurs when a cold anticyclone approaches acoastline and winds become parallel to the coast. Air over land remainscold, while air just offshore continuously receives large heat fluxes fromthe sea surface. For example, over the Gulf Stream convective rainbandsoften form which may be accompanied by considerable lightning activity(Hobbs, 1987; Biswas and Hobbs, 1990). Other processes that are favor-able to coastal frontogenesis are frictional retardation and turning of thewind, upstream blocking of cold air, or lee convergence.ICE-EDGE BOUNDARIESAn understanding of the meteorology of coastal regions where sea ice ispresent is important for navigation, exploring for mineral resources, coastalbiological activity, and modeling sea ice and climate. In the present discus-sion, only ice-land boundary regions will be considered. This includesalmost all the Antarctic ice cover and the ice cover overlying the high-latitude continental shelves in the northern hemisphere. While many char-acteristics of the marginal ice zone (the boundary between sea ice and openocean; see Johannessen et al., 1988) are similar to those of coastal ice-edgeboundaries, the marginal ice zone will not be addressed here.A summary of meteorological processes occurring at ice-edge bound-aries is given by Barry (1986). Barry concluded that our basic knowledgeof meteorological conditions over ice-edge boundaries is very limited. The

28 COASTAL METEOROLOGYrecent Marginal Ice Zone Experiment (MIZEX) off the East Greenland andBering seas has provided insight into processes occurring at ice-ocean boundaries,but no such program has been undertaken for land-ice boundaries. With theexception of Antarctic katabatic winds (see Chapter 4), relatively little re-search has been conducted on mesoscale and small-scale coastal processesat the land-ice boundary. Many of the same physical processes and phe-nomena, which are described elsewhere in this report, such as land and seabreezes, occur in ice-edge coastal regions. However, there are unique pro-cesses occurring at the ice edge, particularly during the cold seasons of theyear, that are associated with the characteristics of sea ice. A summary ofthe observed features of coastal sea ice is provided by Wadhams (1986) inthe context of the seasonal sea ice zone.Even during winter, regions of open water occur between sea ice aroundcoasts. An overview is given by Smith et al. (1990b) of polynyas and leads,which are openings in pack ice due to ice drift divergence and local melt-ing. Offshore katabatic winds (see Chapter 4) are believed to play animportant role in the formation and maintenance of polynyas and leads.Particularly during winter, leads and polynyas are a major source of ex-change of heat, moisture, and gases between the ocean and atmosphere.Polynyas and leads are sites of active brine formation, affecting the localwater density structure and current field and cumulatively affecting thestructure of the halocline. Leads and polynyas serve as corridors for migra-tion of marine mammals. During spring localized plankton blooms occur,which are important biologically and are also possibly important as a localsource of cloud condensation nuclei (see Chapter 6).A feature that occurs along the Antarctic coast is the presence of iceshelves, over which glacier ice streams into the sea. The largest ice shelvesin the Antarctic are the Ross and Ronne-Filchner. Ice shelves determine thecapability of the fast-flowing internal ice streams associated with marineice sheets to disperse the glacier ice rapidly into the surrounding ocean.Marine ice sheets are characterized by being grounded on beds well belowsea level. If the backstress exerted on the ice stream by the ice shelf isinsufficient, accelerated discharge of land ice through ice streams to the seamay result in the collapse of the marine ice sheet (Binschadler, 1991).Leads and polynyas affect the atmosphere in the ice-edge coastal re-gions in the following ways. Extreme sea-air temperature differences (20to 40C) are commonly associated with leads and polynyas during winter,and heat fluxes of several hundred watts per square meter are typical (Smithet al., 1990b). The sensible heat flux in air is several times larger than thelatent heat flux because of the relatively low value of saturation-specifichumidity at the freezing point. Schnell et al. (1989) found that wide leadsand polynyas release enough energy to create buoyant plumes that penetratethe boundary layer; in one case a hydrometer plume reached a height of 4 28 COASTAL METEOROLOGYrecent Marginal Ice Zone Experiment (MIZEX) off the East Greenland andBering seas has provided insight into processes occurring at ice-ocean boundaries,but no such program has been undertaken for land-ice boundaries. With theexception of Antarctic katabatic winds (see Chapter 4), relatively little re-search has been conducted on mesoscale and small-scale coastal processesat the land-ice boundary. Many of the same physical processes and phe-nomena, which are described elsewhere in this report, such as land and seabreezes, occur in ice-edge coastal regions. However, there are unique pro-cesses occurring at the ice edge, particularly during the cold seasons of theyear, that are associated with the characteristics of sea ice. A summary ofthe observed features of coastal sea ice is provided by Wadhams (1986) inthe context of the seasonal sea ice zone.Even during winter, regions of open water occur between sea ice aroundcoasts. An overview is given by Smith et al. (1990b) of polynyas and leads,which are openings in pack ice due to ice drift divergence and local melt-ing. Offshore katabatic winds (see Chapter 4) are believed to play animportant role in the formation and maintenance of polynyas and leads.Particularly during winter, leads and polynyas are a major source of ex-change of heat, moisture, and gases between the ocean and atmosphere.Polynyas and leads are sites of active brine formation, affecting the localwater density structure and current field and cumulatively affecting thestructure of the halocline. Leads and polynyas serve as corridors for migra-tion of marine mammals. During spring localized plankton blooms occur,which are important biologically and are also possibly important as a localsource of cloud condensation nuclei (see Chapter 6).A feature that occurs along the Antarctic coast is the presence of iceshelves, over which glacier ice streams into the sea. The largest ice shelvesin the Antarctic are the Ross and Ronne-Filchner. Ice shelves determine thecapability of the fast-flowing internal ice streams associated with marineice sheets to disperse the glacier ice rapidly into the surrounding ocean.Marine ice sheets are characterized by being grounded on beds well belowsea level. If the backstress exerted on the ice stream by the ice shelf isinsufficient, accelerated discharge of land ice through ice streams to the seamay result in the collapse of the marine ice sheet (Binschadler, 1991).Leads and polynyas affect the atmosphere in the ice-edge coastal re-gions in the following ways. Extreme sea-air temperature differences (20to 40C) are commonly associated with leads and polynyas during winter,and heat fluxes of several hundred watts per square meter are typical (Smithet al., 1990b). The sensible heat flux in air is several times larger than thelatent heat flux because of the relatively low value of saturation-specifichumidity at the freezing point. Schnell et al. (1989) found that wide leadsand polynyas release enough energy to create buoyant plumes that penetratethe boundary layer; in one case a hydrometer plume reached a height of 4

THERMALLY DRIVEN EFFECTS 29km. Aircraft lidar measurements indicated the presence of small ice crys-tals 'in the plume, which are believed to modify substantially the local radi-ative balance. In particular, the larger leads and polynyas are likely toprovide substantial amounts of water vapor, especially to the wintertimepolar atmosphere, contributing to the presence of widespread low-level stra-tus clouds in these regions. Convection, associated with substantial precip-itation, from larger polynyas seems likely; this precipitation has the poten-tial to significantly influence the accumulation of snow both on glaciers andon the sea ice itself. The panel notes that the forthcoming Lead Experiment(LEADEX) in the Beaufort Sea, although not occurring in the coastal zone,will address some of these issues and improve our general knowledge aboutleads. At the same time, the state of the sea ice, including the presence ofleads and polynyas, is strongly dependent on atmospheric processes (see,e.g., Hibler, 1979). The atmosphere influences the state of the sea ice boththermodynamically (e.g., via radiative heat, sensible heat, and latent heatfluxes) and dynamically (e.g., via surface wind stress).SUMMARY AND CONCLUSIONSSome existing gaps in scientific understanding associated with thermal-ly driven effects may be addressed through modeling studies and field pro-grams. To spur progress we recommend the following:Observational and modeling studies of the LSBS should be extendedto cover the entire diurnal cycle, with emphasis on improving knowledge ofoffshore regions, the morphology and dynamics of the land breeze, and theformation and breakdown of the sea breeze front.Remote sensing techniques and fine-mesh mesoscale numerical mod-els should be applied to better understand the finer-scale, three-dimensionalstructure of the sea breeze front, its associated mesoscale vertical motions,and the development of internal boundary layers above complex coastlinesand heterogeneous surfaces.Research should be directed to understand the three-dimensional LSBSinteractions with inhomogeneous and time-dependent synoptic flows, non-uniform land and water surfaces, irregular coastlines, and complex terrain,as well as the dynamic feedbacks between the LSBS and stratiform cloudsand precipitating and nonprecipitating convective cloud systems.The geographical distribution of coastal front occurrences, their spa-tial coverage, and their modes of propagation should be documented andtheir variability assessed.A combination of case studies and model simulations should be con-ducted to determine the site-specific, large-scale conditions leading to coastalfront formation, which is often difficult to observe directly in real time THERMALLY DRIVEN EFFECTS 29km. Aircraft lidar measurements indicated the presence of small ice crys-tals 'in the plume, which are believed to modify substantially the local radi-ative balance. In particular, the larger leads and polynyas are likely toprovide substantial amounts of water vapor, especially to the wintertimepolar atmosphere, contributing to the presence of widespread low-level stra-tus clouds in these regions. Convection, associated with substantial precip-itation, from larger polynyas seems likely; this precipitation has the poten-tial to significantly influence the accumulation of snow both on glaciers andon the sea ice itself. The panel notes that the forthcoming Lead Experiment(LEADEX) in the Beaufort Sea, although not occurring in the coastal zone,will address some of these issues and improve our general knowledge aboutleads. At the same time, the state of the sea ice, including the presence ofleads and polynyas, is strongly dependent on atmospheric processes (see,e.g., Hibler, 1979). The atmosphere influences the state of the sea ice boththermodynamically (e.g., via radiative heat, sensible heat, and latent heatfluxes) and dynamically (e.g., via surface wind stress).SUMMARY AND CONCLUSIONSSome existing gaps in scientific understanding associated with thermal-ly driven effects may be addressed through modeling studies and field pro-grams. To spur progress we recommend the following:Observational and modeling studies of the LSBS should be extendedto cover the entire diurnal cycle, with emphasis on improving knowledge ofoffshore regions, the morphology and dynamics of the land breeze, and theformation and breakdown of the sea breeze front.Remote sensing techniques and fine-mesh mesoscale numerical mod-els should be applied to better understand the finer-scale, three-dimensionalstructure of the sea breeze front, its associated mesoscale vertical motions,and the development of internal boundary layers above complex coastlinesand heterogeneous surfaces.Research should be directed to understand the three-dimensional LSBSinteractions with inhomogeneous and time-dependent synoptic flows, non-uniform land and water surfaces, irregular coastlines, and complex terrain,as well as the dynamic feedbacks between the LSBS and stratiform cloudsand precipitating and nonprecipitating convective cloud systems.The geographical distribution of coastal front occurrences, their spa-tial coverage, and their modes of propagation should be documented andtheir variability assessed.A combination of case studies and model simulations should be con-ducted to determine the site-specific, large-scale conditions leading to coastalfront formation, which is often difficult to observe directly in real time

30 COASTAL METEOROLOGYbecause coastal fronts tend to form offshore and sometimes remain nearlystationary.Studies should be supported to elucidate processes of heat and mois-ture fluxes into the atmosphere from leads and polynyas, particularly in thepresence of extreme horizontal thermal discontinuity.Interactions between the atmosphere and sea ice on the mesoscale inthe coastal zone should be examined. 30 COASTAL METEOROLOGYbecause coastal fronts tend to form offshore and sometimes remain nearlystationary.Studies should be supported to elucidate processes of heat and mois-ture fluxes into the atmosphere from leads and polynyas, particularly in thepresence of extreme horizontal thermal discontinuity.Interactions between the atmosphere and sea ice on the mesoscale inthe coastal zone should be examined.

The Influence of Orography INTRODUCTION AND BASIC PARAMETERSThe transition from a nearly flat ocean to land in the coastal zone isoften accompanied by major changes in elevation. Flow over and aroundsuch changes in orography in a rotating stratified fluid represents one of theclassic problems in meteorology and oceanography. For a far-field windperpendicular to a barrier, the horizontal extent and magnitude of upstreammodification of the flow pattern in response to the barrier should be deter-mined. There is also a downwind modification of the flow, which is not, ingeneral, symmetric with the upwind influence.Low-level air flow is generally blocked by a mountain when the param-eter known as the Froude number is less than unity (Smith, 1979, 1989).The Froude number provides a measure of the relative importance of poten-tial and kinetic energy in flow around and over obstacles. It is defined by:, (1)where N is the static stability and is equal to (-g3z0/0 )I/2, U is the speed ofthe free air stream, hm is the height of the ridge, is the constant meanpotential temperature, and g is gravity. For typical atmospheric stratifica-tion of N = IQ~1 to 10~2, an elevation of only 100 m is often sufficient tocause "blocking" of the onshore flow at low levels. Such blocking occursalong the west and east coasts of the United States; along the east coast, itis often referred to as cold air damming. Leeside effects can be importantfor flow directed offshore; for island wakes; in regions where the coastline

32 COASTAL METEOROLOGYcurves, such as Southern California; or where there are inland marine re-gions, such as Puget Sound, San Francisco Bay, and southeast Alaska.In the coastal zone it is not necessary to have an upstream velocitydirected toward the mountains for the orography to influence coastal winds.If a localized region of high or low pressure is generated in the coastalzone, it will, under certain conditions, be trapped and propagate along thecoastline within the coastal zone. This is a common phenomenon, for ex-ample, along the coasts of California and Australia.The Froude number considers the relative importance of vertical dis-placement of isentropic surfaces in flow around and over obstacles. Asecond factor is the influence of the earth's rotation on upstream flow de-celeration (Queney, 1948). One can consider the influence of rotation througha Rossby number: Rm = U/flm, (2)where U is the upstream velocity, / the Coriolis force, and /m is the half-width of the ridge; little flow deceleration is found when Rm is less thanunity. Numerical simulations by Pierrehumbert and Wyman (1985) andtrajectory analyses by Chen and Smith (1987) suggest that in the region ofsteep topography the deceleration zone will grow upstream to a width of:lNhJf, (3)This parameter JR is known as the radius of deformation. Steep topographyis defined by the nondimensional slope, (hm/lmXN/f), being greater than 1.For the coastal case, /,, is often on the order of 50 to 150 km and / > / ;K Km'this contrasts with broad mountain ranges such as the Rockies with / on mthe order of 500 km. In the broad mountain. case, /m > /R, the flow staysquasi-geostrophic with Rm < 1 (i.e., wind blows perpendicular to the pres-sure gradient as it flows over the topography, with little upstream influ-ence). The coastal region, however, is often in the knife-edge mountaincase, /R > /m, where Rm > 1. Here one expects the coastal mountains torepresent a wall, and the momentum balance in the along-shore directionnear the wall is not expected to be geostrophic. The smoothed topographiesin current-generation numerical weather prediction (NWP) models do noteven qualitatively represent knife-edge slopes and thus do not correctlyinclude coastal phenomena.To further delineate the influence of orography on coastal meteorology,let L be the scale for motion in the along-coast (y) direction, and / be thescale in the cross-shore direction (-*), wherelm <KL. (4)We can nondimensionalize the equations of motion in the following man-ner. The cross-shore wind component, w, and along-shore wind component,v, are scaled by UL/l, time by l/U, vertical distances by D = fl/N. and 32 COASTAL METEOROLOGYcurves, such as Southern California; or where there are inland marine re-gions, such as Puget Sound, San Francisco Bay, and southeast Alaska.In the coastal zone it is not necessary to have an upstream velocitydirected toward the mountains for the orography to influence coastal winds.If a localized region of high or low pressure is generated in the coastalzone, it will, under certain conditions, be trapped and propagate along thecoastline within the coastal zone. This is a common phenomenon, for ex-ample, along the coasts of California and Australia.The Froude number considers the relative importance of vertical dis-placement of isentropic surfaces in flow around and over obstacles. Asecond factor is the influence of the earth's rotation on upstream flow de-celeration (Queney, 1948). One can consider the influence of rotation througha Rossby number: Rm = U/flm, (2)where U is the upstream velocity, / the Coriolis force, and /m is the half-width of the ridge; little flow deceleration is found when Rm is less thanunity. Numerical simulations by Pierrehumbert and Wyman (1985) andtrajectory analyses by Chen and Smith (1987) suggest that in the region ofsteep topography the deceleration zone will grow upstream to a width of:lNhJf, (3)This parameter JR is known as the radius of deformation. Steep topographyis defined by the nondimensional slope, (hm/lmXN/f), being greater than 1.For the coastal case, /,, is often on the order of 50 to 150 km and / > / ;K Km'this contrasts with broad mountain ranges such as the Rockies with / on mthe order of 500 km. In the broad mountain. case, /m > /R, the flow staysquasi-geostrophic with Rm < 1 (i.e., wind blows perpendicular to the pres-sure gradient as it flows over the topography, with little upstream influ-ence). The coastal region, however, is often in the knife-edge mountaincase, /R > /m, where Rm > 1. Here one expects the coastal mountains torepresent a wall, and the momentum balance in the along-shore directionnear the wall is not expected to be geostrophic. The smoothed topographiesin current-generation numerical weather prediction (NWP) models do noteven qualitatively represent knife-edge slopes and thus do not correctlyinclude coastal phenomena.To further delineate the influence of orography on coastal meteorology,let L be the scale for motion in the along-coast (y) direction, and / be thescale in the cross-shore direction (-*), wherelm <KL. (4)We can nondimensionalize the equations of motion in the following man-ner. The cross-shore wind component, w, and along-shore wind component,v, are scaled by UL/l, time by l/U, vertical distances by D = fl/N. and

THE INFLUENCE OF OROGRAPHY 33pressure p by PJIU. The equations of motion for such a shallow systembecome (Overland, 1984):j L +C/3 "I =V ~ J"~L (cross - shore)and (6)where R{ = U/fl = V/fL is a coastal Rossby number, CD' = CD(7 + V)/D is acoastal drag coefficient that indicates the relative importance of surfacefriction, and P is a synoptic-scale pressure imposed on the coastal layer.The term dPJdy on the right-hand side of Eq. (6) nondimensionally equalsunity and represents the along-shore pressure gradient associated with theincident geostrophic wind, U. For many coastal problems, the left-handside of Eq. (5) is small, even though Rt is on the order of Eq. (2); the flowin the along-shore direction is in geostrophic balance. The left-hand side ofEq. (6), however, is on the order of Eq. (2), and v can exhibit accelerationsin response to the imposed along-shore pressure gradient. Small l/L andRf ~ 1 are the heart of the coastal zone semigeostrophic approximation. Theonly remaining free parameter is the nondimensional mountain height= (Nlf)(hjl). (7)This may also be written as RJF Thus, the coastal mountain problem canbe specified in terms of R{ and Fr, a Rossby number and a Froude number.Note that for hJD = 1 (i.e., steep topography), the offshore length scale isdefined by / = /R, the Rossby radius of deformation, which scales coastalinfluence offshore of mountainous coasts to be 10 to 100 km.The foregoing discussion suggests that mesoscale meteorological features(10 to 100 km) are generated in the vicinity of coastal orography, and thatageostrophic motions are anticipated in the along-shore direction. However,the upstream flow is seldom stationary and uniform; vertical stratification isseldom constant While theoretical considerations define the scales and pro-cesses important to the coastal zone, they are less successful in fully explainingparticular case studies (Walter and Overland, 1982; Mass and Ferber, 1990).LOW FROUDE NUMBER FLOW: TRAPPED PHENOMENAIsolated Response: Kelvin Wave and Gravity CurrentAlong subtropical mountainous coastlines such as California and Aus-tralia, subsidence in the subtropical high pressure often develops a strongmarine inversion structure below the height of the coastal topography. ThisCflRft is thp mnst ctiiHiiH nf nmcrranbir. rrtactal nhp>.nnmAna In tkio naoa +tiA THE INFLUENCE OF OROGRAPHY 33pressure p by PJIU. The equations of motion for such a shallow systembecome (Overland, 1984):j L +C/3 "I =V ~ J"~L (cross - shore)and (6)where R{ = U/fl = V/fL is a coastal Rossby number, CD' = CD(7 + V)/D is acoastal drag coefficient that indicates the relative importance of surfacefriction, and P is a synoptic-scale pressure imposed on the coastal layer.The term dPJdy on the right-hand side of Eq. (6) nondimensionally equalsunity and represents the along-shore pressure gradient associated with theincident geostrophic wind, U. For many coastal problems, the left-handside of Eq. (5) is small, even though Rt is on the order of Eq. (2); the flowin the along-shore direction is in geostrophic balance. The left-hand side ofEq. (6), however, is on the order of Eq. (2), and v can exhibit accelerationsin response to the imposed along-shore pressure gradient. Small l/L andRf ~ 1 are the heart of the coastal zone semigeostrophic approximation. Theonly remaining free parameter is the nondimensional mountain height= (Nlf)(hjl). (7)This may also be written as RJF Thus, the coastal mountain problem canbe specified in terms of R{ and Fr, a Rossby number and a Froude number.Note that for hJD = 1 (i.e., steep topography), the offshore length scale isdefined by / = /R, the Rossby radius of deformation, which scales coastalinfluence offshore of mountainous coasts to be 10 to 100 km.The foregoing discussion suggests that mesoscale meteorological features(10 to 100 km) are generated in the vicinity of coastal orography, and thatageostrophic motions are anticipated in the along-shore direction. However,the upstream flow is seldom stationary and uniform; vertical stratification isseldom constant While theoretical considerations define the scales and pro-cesses important to the coastal zone, they are less successful in fully explainingparticular case studies (Walter and Overland, 1982; Mass and Ferber, 1990).LOW FROUDE NUMBER FLOW: TRAPPED PHENOMENAIsolated Response: Kelvin Wave and Gravity CurrentAlong subtropical mountainous coastlines such as California and Aus-tralia, subsidence in the subtropical high pressure often develops a strongmarine inversion structure below the height of the coastal topography. ThisCflRft is thp mnst ctiiHiiH nf nmcrranbir. rrtactal nhp>.nnmAna In tkio naoa +tiA

34 COASTAL METEOROLOGYstability scale can be replaced by the difference in temperature, 2 - 0across the inversion occurring at height hv The coastal zone semigeo-strophic equations, (5) and (6), admit Kelvin wave solutionsh{lD = e* G(y - (8)where G is an arbitrary function. The solution is trapped to a unit distancefrom the coast (i.e., a Rossby radius) and propagates at a unit speed that indimensional terms has the phase speed c - flR. On the other hand, if theequations are initialized with a density front, a nonlinear gravity current,with some Kelvin-like aspects, can form. Along-shore disturbances canalso be forced by an along-shore pressure gradient dPJdy imposed by thesynoptic-scale pressure field above the coastal layer. The existence of thisclass ofmesoscale coastal features has been documented in California (Beardsleyet al.f 1987; Dorman, 1985, 1987; Mass and Albright, 1987; Winant et al.,1988; Zemba and Friehe, 1987) and Australia (Holland and Leslie, 1986).Figure 4.1 shows a typical summer trapped feature along the west coast ofthe United States. The GOES visible imagery (Figure 4. la) shows a wedge-like feature that propagates northward from central California to BritishColumbia in about 2 days. Note that in the sea-level pressure analysis(Figure 4.1b) the wind shifts from northerly to southerly with the passage of FIGURE 4.1 Typical summertime northward surge of marine air trapped to thewest coast: (a) GOES visible image 1800 GMT, May 27, 1983; (b) observed windand subjective sea-level pressure analysis for the same time based on satellite imag-es and available surface synoptic data. Note that there is wind shift in the coastalzone as the surge propagates northward (after Mass et al.. 1986}.

THE INFLUENCE OF OROGRAPHY 35the feature. The case for South Africa (Bannon, 1981; Gill, 1977; Reasonand Jury, 1990) is quite distinct from the Australia and California cases(Reason and Steyn, 1990) (see Table 4.1). Calculation of the Rossby num-ber based on half the mountain width yields 0.1, so dynamics lie within thequasi-geostrophic regime (i.e., small mountain slope) and blocking of anincipient flow will not persist (Pierrehumbert and Wyman, 1985).While these isolated wave/frontal features are perhaps the most obviousof coastal phenomena, understanding their source mechanisms and compos-ite nature (density flow versus propagating wave) is uncertain in any partic-ular realization. These isolated trapped phenomena are generally initiatedby changes in the synoptic-scale flow. The climatology of such changes isnot well documented and is an area for further research. There is also aneed to understand all factors that contribute to the depth and spatial vari-ability of coastal marine stratus and fog as a result of interactions amongwave dynamics, radiation, and cloud processes.DammingThe case of damming refers to blocked winds on the windward side of amountain for uniform onshore flow or modification of a frontal feature bycoastal orography. This phenomenon is less well documented for coastalregions than for other mountain regions. The balance of the wind andpressure (mass) fields within the storm is disrupted at the coast. As aresult, the path of the storm can change abruptly, and, in certain instances,barrier jets and enhanced surface winds can develop in the coastal zone.Mass and Ferber (1990) show the development of ridging along the coast ofwestern Washington state with the approach of a cold frontal system (Fig-ure 4.2). When this orographically induced pressure field is added to thesynoptic-scale pressure field, it produces large along-shore pressure gradi-ents, which the momentum field responds to by producing an along-shorewind jet that is stronger than the winds in the weather system farther off-shore. Similar super-geostrophic winds have been observed at coastal sta-tions along Alaska (Businger and Walter, 1988; Reynolds, 1983). What isnot known for these cases is how the storm system itself is modified by thepresence of a coastline and orography. This interaction of storms and orog-raphy to produce coastal jets is a major area for research.A related phenomenon along the east coast of the United States is Ap-palachian cold air damming (Bell and Bosart, 1988; Xu, 1990). Theseepisodes arise when there is high pressure over New England and onshoreflow toward the Appalachians with an estimated Froude number of 0.3 to0.4. A semigeostrophic system is set up on the eastern slopes with a low-level wind maximum parallel to the ridge. This wind maximum is fed by apool of cold air from the north, which creates a cold dome along the easternTn ths Rp.1 anrl Rnsart f1988 studv. over-water winds wfire not THE INFLUENCE OF OROGRAPHY 35the feature. The case for South Africa (Bannon, 1981; Gill, 1977; Reasonand Jury, 1990) is quite distinct from the Australia and California cases(Reason and Steyn, 1990) (see Table 4.1). Calculation of the Rossby num-ber based on half the mountain width yields 0.1, so dynamics lie within thequasi-geostrophic regime (i.e., small mountain slope) and blocking of anincipient flow will not persist (Pierrehumbert and Wyman, 1985).While these isolated wave/frontal features are perhaps the most obviousof coastal phenomena, understanding their source mechanisms and compos-ite nature (density flow versus propagating wave) is uncertain in any partic-ular realization. These isolated trapped phenomena are generally initiatedby changes in the synoptic-scale flow. The climatology of such changes isnot well documented and is an area for further research. There is also aneed to understand all factors that contribute to the depth and spatial vari-ability of coastal marine stratus and fog as a result of interactions amongwave dynamics, radiation, and cloud processes.DammingThe case of damming refers to blocked winds on the windward side of amountain for uniform onshore flow or modification of a frontal feature bycoastal orography. This phenomenon is less well documented for coastalregions than for other mountain regions. The balance of the wind andpressure (mass) fields within the storm is disrupted at the coast. As aresult, the path of the storm can change abruptly, and, in certain instances,barrier jets and enhanced surface winds can develop in the coastal zone.Mass and Ferber (1990) show the development of ridging along the coast ofwestern Washington state with the approach of a cold frontal system (Fig-ure 4.2). When this orographically induced pressure field is added to thesynoptic-scale pressure field, it produces large along-shore pressure gradi-ents, which the momentum field responds to by producing an along-shorewind jet that is stronger than the winds in the weather system farther off-shore. Similar super-geostrophic winds have been observed at coastal sta-tions along Alaska (Businger and Walter, 1988; Reynolds, 1983). What isnot known for these cases is how the storm system itself is modified by thepresence of a coastline and orography. This interaction of storms and orog-raphy to produce coastal jets is a major area for research.A related phenomenon along the east coast of the United States is Ap-palachian cold air damming (Bell and Bosart, 1988; Xu, 1990). Theseepisodes arise when there is high pressure over New England and onshoreflow toward the Appalachians with an estimated Froude number of 0.3 to0.4. A semigeostrophic system is set up on the eastern slopes with a low-level wind maximum parallel to the ridge. This wind maximum is fed by apool of cold air from the north, which creates a cold dome along the easternTn ths Rp.1 anrl Rnsart f1988 studv. over-water winds wfire not

36 COASTAL METEOROLOGY FIGURE 4.2 Sea-level pressureanalysis and surface winds forWashington state, 2100 GMT, March5, 1988. Note the coastal windjet, which developed as an ageo-strophic response to the increasedalong-shore pressure gradient (af-ter Mass and Ferber, 1991).directly considered; however, it is clear from their analyses that the cold airdamming region extended to coastal weather stations.Results from research on inland mountain systems may be relevant todamming, particularly the Alpine experiment (Chen and Smith, 1987; Daviesand Pichler, 1990; Schuman, 1987, for example), which considers blockingand synoptic weather system/orographic interaction in the Alps. Anothercase is along the Sierra Nevada Mountains (Parish, 1982). One difficulty isthat these cases may have more gentle slopes than those at coastlines andthus more of a quasi-geostrophic than semigeostrophic response. A summa-ry of Rm and Fr for several cases is presented in Table 4.1. Poor verifica-tion of coastal weather forecasts is often attributed to the formation ofmesoscale systems by the interaction of storms with coastal orography andto the feedback of these features on storm intensity within the coastal zone(100 km), yet even basic documentation of this interaction and feedback islacking (Bane et al., 1990). Gap WindsA special case of trapped phenomena is a sea-level channel between twomountainous coastlines where the width of the strait is on the order of theRossby radius or less. This establishes a semigeostrophic system in the straitwith winds accelerating down the strait in response to the synoptic-scale along-strait pressure gradient (Overland and Walter, 1981) (Figure 4.3). In a case

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