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Introduction

Coastal meteorology is the study of meteorological phenomena in the coastal zone caused, or significantly affected, by sharp changes in heat, moisture, and momentum transfers and elevation that occur between land and water. The coastal zone is defined as extending approximately 100 km to either side of the coastline. Examples of coastal meteorological phenomena include land and sea breezes, sea-breeze-related thunderstorms, coastal fronts, fog, haze, marine stratus clouds, and strong winds associated with coastal orography. In addition to their intrinsic importance to coastal weather, increased knowledge of these phenomena is important for understanding the physical, chemical, and biological oceanography of the coastal ocean. Practical application of this knowledge is vital for more accurate prediction of coastal weather and sea states, which affect transportation and commerce, pollutant dispersal, public safety, and military operations. This report reviews the state of the science of coastal meteorology. In addition, it recommends areas for scientific and technical progress.

The dynamical meteorology of the coastal zone may be considered in terms of the three subsidiary ideal problems illustrated in Figure 1.1; these three problems form the organizational basis for this report. The first problem is one where the coastal atmospheric circulation is primarily driven by the contrast in heating and is modulated by the contrast in surface friction between land and water. The second problem is one where the primary influence is due to steep coastal mountains, the presence of which may induce strong winds and other complex flow patterns. A third class of phenomena broadly consists of larger-scale meteorological systems that, by



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Coastal Meteorology: A Review of the State of the Science 1 Introduction Coastal meteorology is the study of meteorological phenomena in the coastal zone caused, or significantly affected, by sharp changes in heat, moisture, and momentum transfers and elevation that occur between land and water. The coastal zone is defined as extending approximately 100 km to either side of the coastline. Examples of coastal meteorological phenomena include land and sea breezes, sea-breeze-related thunderstorms, coastal fronts, fog, haze, marine stratus clouds, and strong winds associated with coastal orography. In addition to their intrinsic importance to coastal weather, increased knowledge of these phenomena is important for understanding the physical, chemical, and biological oceanography of the coastal ocean. Practical application of this knowledge is vital for more accurate prediction of coastal weather and sea states, which affect transportation and commerce, pollutant dispersal, public safety, and military operations. This report reviews the state of the science of coastal meteorology. In addition, it recommends areas for scientific and technical progress. The dynamical meteorology of the coastal zone may be considered in terms of the three subsidiary ideal problems illustrated in Figure 1.1; these three problems form the organizational basis for this report. The first problem is one where the coastal atmospheric circulation is primarily driven by the contrast in heating and is modulated by the contrast in surface friction between land and water. The second problem is one where the primary influence is due to steep coastal mountains, the presence of which may induce strong winds and other complex flow patterns. A third class of phenomena broadly consists of larger-scale meteorological systems that, by

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Coastal Meteorology: A Review of the State of the Science Figure 1.1 Three categories of problems in coastal meteorology.

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Coastal Meteorology: A Review of the State of the Science virtue of their passage across the coastline, produce distinct smaller-scale systems. Real coastal phenomena are always some combination of these idealized problems. Transfers of heat, momentum, and water vapor between the atmosphere and its underlying surface (be it land or water) are basic to these three ideal problems. This report therefore begins with Boundary Layer Processes (Chapter 2); this chapter contains an assessment of, and prospects for improvement in, our understanding of the approximately 1-km-deep layer of air adjacent to the surface, which is called the atmospheric boundary layer (ABL). Study of the ABL is intended to reveal how surface transfers are distributed upward. Over the ocean, those surface transfers are interactive, determined by the sea state, which in turn is determined by the atmospheric flow, which is influenced by the surface transfers, and so on. This fundamental coupling has been long recognized. However, there is another order of complexity over the coastal ocean because the sea state is significantly influenced by the ocean bottom. Over land, there is still significant uncertainty about the nature of surface transfer from terrain with variations in soil moisture, vegetation, and usage such as occur along the coast. These strong and frequently horizontal variations in surface transfers form a particularly formidable impediment to understanding of the ABL in coastal regions. But even in the absence of such horizontal variations, the marine boundary layer containing stratus clouds and drizzle is a complex problem involving the interplay of turbulence, cloud processes, and radiation. Problems in the first general category are discussed in Thermally Driven Effects (Chapter 3). Although recognition of the land-sea breeze dates back to antiquity, the understanding needed to make accurate forecasts is still lacking. In simple terms, the land-sea breeze is caused by the generally different temperatures of the land and sea, which produce an across-coast air temperature contrast. After this circulation begins, however, it modifies the conditions that produced it. Thus, the difficulty in making precise predictions lies in understanding more clearly the nature of this feedback. Uncertainties in our understanding of the ABL and nonlinear feedbacks between the sea breeze and resultant clouds are examples here. Issues associated with two special types of thermally driven phenomena (coastal fronts 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 encountered are discussed. In many situations, coastal mountains act as a barrier to the stably stratified marine air. Thus, air with an initial component of motion 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 mountain range. This type of motion is an important component of the meteorologi-

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Coastal Meteorology: A Review of the State of the Science cal problems in these regions. Local effects such as katabatic winds, gap winds, and eddies also are discussed in Chapter 4. As larger-scale meteorological systems move across the coast, they are affected by some combination of the effects discussed in the previous two paragraphs. In some situations, distinct subsystems, which would not exist without the coastal influence, are produced. These are discussed in Interactions with Larger-Scale Weather Systems (Chapter 5). Examples of these effects include cyclogenesis, which is enhanced at the east coast of the United States as upper-level disturbances cross the Appalachians and encounter the strong baroclinic zone at the coast; flow along the coast in winter with strong cooling of the air on the landward side, leading to the formation of fronts; and land-falling hurricanes whose low-level flows are so modified as to favor the formation of tornadoes. In general, the ocean affects, and is affected by, the atmosphere. In The Influence of the Atmospheric Boundary Layer on the Coastal Ocean (Chapter 6), aspects of this interaction that are particularly important for the ocean part of the coastal zone (shelf waters) are discussed. The processes governing air-sea fluxes of momentum, heat, mass, and gases are described in terms of local and remote forcings as well as wave state. Wind-driven coastal upwelling of colder water from below brings different chemical and biological compositions to the surface, produces an across-coast temperature difference unique to coastal regions, and influences atmospheric circulation. Interactions of this nature are important to understanding the coastal ocean and the chemical and biological processes occurring there. Another important application of coastal meteorology is the prediction of pollutant dispersal. In Air Quality (Chapter 7), the focus is on physical advective processes. The highly variable winds near the coast may sweep pollutants out to sea on a land breeze, but then bring them back with the sea breeze. More accurate estimates of the vertical motion fields associated with these wind systems are critical to determining the layers in which the pollutant resides (and the horizontal direction in which it will move). Capabilities and Opportunities (Chapter 8) discusses new observational and simulation tools that can be exploited to study the coastal environment. The critical issue is to have instruments in place to measure long time-scale variations (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 remote sensing as well as in situ devices is discussed in this context. Increases in the computer power of desktop machines will allow many investigators to explore, with numerical models, flow in the vicinity of many coastal environments. With regard to Educational and Human Resources (Chapter 9), the panel found that there is a relative lack of training of meteorology students in areas pertaining to coastal meteorology and a relative lack of cross-fertilization between the fields of coastal meteorology and coastal oceanography.