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A Synopsis of Coastal Meteorology: A Review of the State of the Science

Richard Rotunno

National Center for Atmospheric Research

INTRODUCTION

According to a recent study by the Department of Commerce, almost half the U.S. population lives in coastal areas and so is affected by the unique weather and climate of the coastal zone. Under the auspices of The National Academy of Sciences, the Panel on Coastal Meteorology has just completed a study of the state of the science of coastal meteorology. This presentation will cover the highlights of the study by concentrating on the perceived major scientific problems and the opportunities for progress.

Coastal meteorology is the study of meteorological phenomena in the coastal zone caused, or significantly affected, by the sharp changes that occur between land and sea in surface transfer or elevation. The coastal zone is subjectively defined as extending approximately 100 km to either side of the coastline. Examples of coastal meteorological phenomena include the sea breeze, sea-breeze-related thunderstorms, coastal fronts, marine stratus, fog and haze, enhanced winter snow storms, and strong winds associated with coastal orography. Increased knowledge of several or all of these is important for studies on the physical and chemical oceanography of the coastal ocean. The practical application of this knowledge is vital for more accurate prediction of the coastal weather and sea state, which affect defense, transportation and commerce, and pollutant dispersal.

The dynamical meteorology of the coastal zone may be thought of in terms of three subsidiary ideal problems; these three problems formed the organizational basis of our study. The first problem is one in which the coastal atmospheric circulation is primarily driven by the contrast in heating, and modulated by the contrast in surface friction, between land and sea. The second problem is one in which the primary influence is due to the steep coastal mountains whose presence may induce strong along-shore winds and other complex tow patterns. The third class of phenomena broadly consists of larger-scale meteorological systems that, by virtue of their passage across the coastline, produce distinct smaller-scale systems. Of course, reality is always some combination of these idealized problems.



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Environmental Science in the Coastal Zone: Issues for Further Research 2 A Synopsis of Coastal Meteorology: A Review of the State of the Science Richard Rotunno National Center for Atmospheric Research INTRODUCTION According to a recent study by the Department of Commerce, almost half the U.S. population lives in coastal areas and so is affected by the unique weather and climate of the coastal zone. Under the auspices of The National Academy of Sciences, the Panel on Coastal Meteorology has just completed a study of the state of the science of coastal meteorology. This presentation will cover the highlights of the study by concentrating on the perceived major scientific problems and the opportunities for progress. Coastal meteorology is the study of meteorological phenomena in the coastal zone caused, or significantly affected, by the sharp changes that occur between land and sea in surface transfer or elevation. The coastal zone is subjectively defined as extending approximately 100 km to either side of the coastline. Examples of coastal meteorological phenomena include the sea breeze, sea-breeze-related thunderstorms, coastal fronts, marine stratus, fog and haze, enhanced winter snow storms, and strong winds associated with coastal orography. Increased knowledge of several or all of these is important for studies on the physical and chemical oceanography of the coastal ocean. The practical application of this knowledge is vital for more accurate prediction of the coastal weather and sea state, which affect defense, transportation and commerce, and pollutant dispersal. The dynamical meteorology of the coastal zone may be thought of in terms of three subsidiary ideal problems; these three problems formed the organizational basis of our study. The first problem is one in which the coastal atmospheric circulation is primarily driven by the contrast in heating, and modulated by the contrast in surface friction, between land and sea. The second problem is one in which the primary influence is due to the steep coastal mountains whose presence may induce strong along-shore winds and other complex tow patterns. The third class of phenomena broadly consists of larger-scale meteorological systems that, by virtue of their passage across the coastline, produce distinct smaller-scale systems. Of course, reality is always some combination of these idealized problems.

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Environmental Science in the Coastal Zone: Issues for Further Research THE ATMOSPHERIC BOUNDARY LEVEL The transfer of heat, momentum, and water vapor between the atmosphere and the lower surface (be it land or sea) is basic to these three ideal problems. As such, our study begins with an assessment of, and prospects for improvement in, our understanding of the approximately 1 km-deep layer of air adjacent to the surface called the atmospheric boundary layer (ABL). Study of the ABL is intended to reveal how the effects of surface transfers are distributed upward. The model of the ABL is best understood when it is cloud-free, convective, and horizontally homogeneous. However, near the coast, the ABL is anything but. Stratus, fog, and drizzle complicate the situation, as they depend on a complex interplay between cloud physics, radiation, and turbulence. Perhaps the most severe scientific problem is how to treat boundary layers that are not horizontally homogeneous. Over land, there is still significant uncertainty concerning the nature of surface transfer from terrain with variation in vegetation and usage, such as occurs along the coast. Over the ocean, those surface transfers are determined by the sea state, which in turn is determined by the atmospheric flow, which is influenced by the surface transfers, etc. This fundamental coupling has long been recognized. However, there is another order of complexity over the coastal ocean, because there the sea state is significantly influenced by the ocean shelf. Areas in need of research are ABL processes in inhomogeneous and nonequilibrium conditions (better understanding of these may lead to better surface-flux and mixed-layer scaling theories); fundamental relationships between the ocean wave spectrum, the surface fluxes, and bulk ABL properties; and coastal marine stratocumulus. THERMALLY DRIVEN EFFECTS Although the recognition of the land-sea breeze dates back to antiquity, the deeper understanding needed to make accurate forecasts is still lacking. The land-sea breeze is produced by the generally different temperatures of the land and sea, which produce an across-coast, air-temperature (density) difference. After this circulation begins, however, it modifies the conditions that produced it; the difficulty in making precise predictions lies in the difficulty with understanding more precisely the nature of this feedback. The aforementioned uncertainties in our understanding of the ABL are certainly central problems here. Beyond the simple two-dimensional picture, coastline curvature, nearshore islands, and different synoptic-scale wind orientation present important scientific problems. Perhaps the most challenging problem is the interaction of the land-sea breeze with cumulus convection. Issues associated with two special types of thermally driven phenomena (coastal fronts and ice-edge boundaries) are also discussed in the study. Areas identified for further study are

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Environmental Science in the Coastal Zone: Issues for Further Research observational and modeling studies of the land-sea breeze to cover the entire diurnal cycle, with emphasis on improving knowledge of offshore regions; the fine-scale structure of the sea-breeze front, including the associated vertical motions, and internal boundary layers above complex coastlines and heterogeneous surfaces; three-dimensional interactions of the land-sea breeze with variable synoptic-scale flow, nonuniform land and water surfaces, irregular coastlines, and complex terrain; dynamical interactions of the land-sea breeze with stratus clouds and with precipitating and nonprecipitating cumulus convection; geographical distribution, spatial coverage, and modes of propagation of coastal fronts; and processes of heat and moisture flux from leads and polynyas. THE INFLUENCE OF OROGRAPHY Coastal mountain ranges can significantly affect coastal meteorology. In many situations, the coastal mountains act as a barrier to the stably stratified marine air. Thus air with a component of motion toward the barrier must eventually turn and flow along the barrier. Also, the coastal mountains may act like the side of a basin within which the marine air is contained; under the influence of the earth's rotation, waves known as Kelvin waves may propagate along the basin-wall-like coastal mountain. Special bounder-layer flows are also observed to be under the influence of the coastal mountains. For example, during the Coastal Ocean Dynamics Experiment, a strong along-shore jet was documented. It had a strong diurnal component as evidenced by the depression on the marine inversion near the coastal mountains during the day. The boundary-layer structure showed interesting complexity inasmuch as the potential temperature was well-mixed to the inversion but the wind speed increased strongly through the same layer. Phenomena that appear similar to flow separation in classical fluid dynamics also occur in the lee of capes and other coastline salients. These types of motion are important components of the meteorological problem in these coastal regions. Areas identified for further study are case studies of structure and path of storm systems modified by coastal orography; climatology of synoptic regimes conducive to coastally trapped phenomena; methods to include coastal phenomena in numerical forecast models. INTERACTIONS WITH LARGER-SCALE SYSTEMS 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. Examples of these effects include cyclogenesis 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

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Environmental Science in the Coastal Zone: Issues for Further Research 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 modified so as to favor the formation of tornadoes. Areas identified for further study are dynamics of the local intensification of cyclone winds by coastal topography and the resulting modification of storm intensity and motion; the cause of tornadoes associated with land-falling hurricanes; the influence of the coastal-heating discontinuity in the along-shore propagation and local intensification of coastal fronts; and the influence of coastal fronts on midlatitude coastal cyclogenesis. INFLUENCES ON THE COAST OCEAN In general, the ocean affects, and is affected by, the atmosphere. We discuss, next, aspects of this interaction that are particularly important for the coastal zone (shelf waters). In the northern hemisphere, an along-coast wind with the coast on the left brings the sea into motion in the along-coast direction. Due to the Coriolis effect, the water motion is deflected away from the coast necessitating its replacement by water from below—this phenomenon is know as coastal upwelling. The water from below is colder and, in general, is of different chemical and biological composition. The details of the cross-shelf transport (necessary to feed the upwelling) are poorly understood, since the ocean is responding to atmospheric influences over a large range of time and space. This wind-stress data from the Coastal Ocean Dynamics Experiment shows the mean and a considerable standard deviation. Also, the along-shore ocean currents may be highly irregular. There is evidence that some of the irregularity is due to wind-stress variations along and across the coastal zone. The colder water now along the coast means there is yet another across-coast temperature difference that can produce changes in the atmospheric circulation, which can affect the ocean, etc. Interactions of this nature are important to the understanding of the coastal ocean and the chemical and biological processes occurring there. Areas identified for further study are the coupled ocean-atmosphere processes that control the interactions between the wind field, ABL structure, and upper ocean; the local physical and chemical processes governing air-sea fluxes of momentum, heat, moisture, particulate, and gas within an inhomogeneous coastal ABL and variable wave state; and the role of remote mesoscale spatial inhomogeneities in controlling atmosphere-ocean dynamics in a coastal environment.

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Environmental Science in the Coastal Zone: Issues for Further Research AIR QUALITY Another important application of coastal meteorology is the prediction of pollutant dispersal. Our study covered issues relevant to the coastal environment. 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 for determining the layers at which the pollutant will ultimately reside (and the horizontal direction in which it will move). Further progress here would be helped by comprehensive tracer studies conducted at increasingly more complicated coastal sites (allowing for evaluation, validation, and eventual widespread use of improved dispersion models), and improved coordination between air pollution and boundary-layer field observation programs conducted on both sides of the littoral. CAPABILITIES AND OPPORTUNITIES Observations The present observational network of routine in situ data is not adequate for most applications. The coastal rawinsondes, especially over the West Coast, are sparse. The buoy network is also sparse and only measures conditions near the surface. There are transient ship reports that supplement the buoy reports. The National Oceanic and Atmospheric Administration's observational-equipment modernization will offer some improvements and some degradation. The first network Next Generation Weather Radar (NEXRAD) will provide an increase in over-water coverage: Doppler winds out to 150 km, reflectivity out to 400 km. Returns from the moving sea surface may possibly be interpreted to measure surface winds. No new rawinsondes are planned, and some coastal sondes may be moved inland. But efforts continue to use passive and active satellite techniques to infer the atmospheric and sea state. And surface-based remote sensors can give highly detailed spatial and temporal detail in the boundary layer. Models The emergence of high-performance workstations having substantial fractions of the calculation-speed performance and superior throughput of present-day mainframe supercomputers will allow researchers to run regional models with high resolution and to conduct numerous sensitivity studies.

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Environmental Science in the Coastal Zone: Issues for Further Research Human Resources It is the experience of the panel members that few universities have courses in the meteorology of coastal zones. Related areas of meteorological instrumentation and observational techniques are also underrepresented. To improve our capabilities and opportunities, the Panel on Coastal Meteorology recommends: the use of recently developed remote sensors to obtain detailed, four-dimensional data sets to describe coastal regions and the upgrade of buoy and surface station networks to obtain quality, long-duration data sets; the on-site use of affordable, high-performance work stations that can provide decentralized computations during study of local phenomena, be used to determine the sensitivity of coastal processes to various influences, and be used to study techniques for assimilating data into real-time forecasts; and the increased use of conferences, short courses, and university training programs to encourage more scientists to explore the meteorology of the coastal zone.