Executive Summary

Coastal meteorology is the study of meteorological phenomena in the coastal zone—that is, within about 100 km inland or offshore of a coastline. Weather in this region is caused, or significantly affected, by the sharp changes that occur between land and sea in surface transfers and/or elevation. With regard to surface transfers, study of the atmospheric boundary layer (ABL) is fundamental. However, existing understanding of the ABL is applicable primarily to horizontally homogeneous conditions; it is therefore poorly suited to the coastal zone, a region of strong horizontal inhomogeneity. Hence, future observations and theories must focus more on the horizontally inhomogeneous ABL.

Differences in the vertical heat transfer across a coastline play a role in a number of coastal meteorological phenomena, such as the land-sea breeze and coastal fronts. Although these circulations are roughly understood, a deeper understanding is needed to make accurate predictions on the mesoscale. 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 meteorology. In many situations the coastal mountains act as a barrier to the stably stratified marine air; the barrier may block air flowing toward it, or it may act like a wall along which Kelvin waves may propagate. Further study is needed of the ageostrophic dynamics of these and other mesoscale wind features.

Interactions of larger-scale weather systems with the coastal environ-



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Coastal Meteorology: A Review of the State of the Science Executive Summary Coastal meteorology is the study of meteorological phenomena in the coastal zone—that is, within about 100 km inland or offshore of a coastline. Weather in this region is caused, or significantly affected, by the sharp changes that occur between land and sea in surface transfers and/or elevation. With regard to surface transfers, study of the atmospheric boundary layer (ABL) is fundamental. However, existing understanding of the ABL is applicable primarily to horizontally homogeneous conditions; it is therefore poorly suited to the coastal zone, a region of strong horizontal inhomogeneity. Hence, future observations and theories must focus more on the horizontally inhomogeneous ABL. Differences in the vertical heat transfer across a coastline play a role in a number of coastal meteorological phenomena, such as the land-sea breeze and coastal fronts. Although these circulations are roughly understood, a deeper understanding is needed to make accurate predictions on the mesoscale. 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 meteorology. In many situations the coastal mountains act as a barrier to the stably stratified marine air; the barrier may block air flowing toward it, or it may act like a wall along which Kelvin waves may propagate. Further study is needed of the ageostrophic dynamics of these and other mesoscale wind features. Interactions of larger-scale weather systems with the coastal environ-

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Coastal Meteorology: A Review of the State of the Science ment frequently involve a number of processes already mentioned, and their nature is often difficult to distinguish in complex situations. The panel believes that focusing on specific interactions—such as clouds interacting with sea breezes, mountains interacting with coastal winds, and the effect of coastal fronts on extratropical cyclogenesis—will lead to better understanding and hence to applications such as air quality and pollutant dispersal modeling. Researchers may begin to concentrate on developing improved techniques 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 processes governing air-sea fluxes within an inhomogeneous boundary layer and variable wave state need to be better understood, as does the role of mesoscale spatial inhomogeneities in controlling coastal dynamics. The panel found that existing buoy and coastal station networks are outdated and inadequate in number (too often failing in accuracy and precision of measurement), especially for obtaining data over water. The panel also encourages collaboration between meteorologists and oceanographers through research programs and enhancements in college-level curricula that focus on problems of coastal meteorology and oceanography. The chapters that follow detail these findings and suggest a number of specific courses of action. The panel has drawn from these suggestions the following general recommendations: With respect to boundary layer processes, WE RECOMMEND a complete reexamination of ABL processes in inhomogeneous conditions, including surface and boundary layer scaling theories, higher-order moment relationships throughout the ABL, and the relative importance of turbulent versus coherent motions. To enhance knowledge of thermally driven effects in coastal regions, WE RECOMMEND that high-density land and sea breeze observational studies and high-resolution numerical modeling studies in regional field experiments be extended to three dimensions using complex coastlines and topography, heterogeneous surfaces, and nonhomogeneous and time-dependent synoptic environments that include cloud interactions. WE RECOMMEND also that detailed study be undertaken of other thermally driven circulations and influences, nondiurnal in nature, including persistent coastal fronts and phenomena associated with ice sheet leads and polynyas. To develop greater understanding of orographic influences on coastal meteorological phenomena, WE RECOMMEND numerical and observational investigations of ageostrophic dynamics for the

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Coastal Meteorology: A Review of the State of the Science initiation, intensification, and movement of coastal mesoscale wind features, such as coastal jets and eddies and gap winds, caused by the interaction of the synoptic-scale flow with coastal orography. To understand the nature of the interactions of large-scale weather systems with the coastal environment, WE RECOMMEND observational, numerical, and theoretical studies that focus on specific interactions—for example, the effect of coastal fronts on extratropical cyclogenesis—in order to develop an understanding of the dynamical processes involved. To improve understanding of the influence of the ABL on the coastal ocean, WE RECOMMEND that a research program be undertaken to clarify (1) the local physical and chemical processes governing air-sea fluxes of momentum, heat, moisture, particulates, and gases within an inhomogeneous coastal boundary layer and variable wave state and (2) the role of distant mesoscale spatial inhomogeneities in controlling atmosphere-ocean dynamics in a coastal environment. To address air quality issues in coastal regions, WE RECOMMEND use of advanced modeling systems and tracer tests (for verification) to determine the significant impacts of vertical motions and shears in three-dimensional coherent mesoscale coastal circulations on the dispersion of gases, aerosols, and particulates, especially in the range of 10 to 100 km. To apply advanced technology in coastal research, WE RECOMMEND (1) the use of recently developed remote sensors to obtain detailed four-dimensional data sets along with the upgrading of buoy and surface station networks to obtain quality, long-duration data sets describing coastal regions and (2) on-site use of high-performance workstations to provide decentralized computations during study of local coastal phenomena, data assimilation methods, and real-time forecasting. Finally, to focus attention on the subject of coastal meteorology, WE RECOMMEND increased use of interdisciplinary conferences and short courses, together with support of university training programs, to encourage more scientists to explore the meteorology of the coastal zone.

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