3
Thermally Driven Effects

Differences in land and sea surface temperature and heat flux result in direct, thermally driven wind systems over a spectrum of temporal and spatial scales. The best known among these are the mesoscale land and sea (lake) breeze circulation systems (see, e.g., Defant, 1950), which are inherently diurnal in nature. Much less studied are the regimes induced by warm ocean waters adjacent to a large cold land mass. These circulations, which are not diurnal in nature, are termed coastal fronts. Even less well understood are systems present over the coastal ice-land boundary. As an example, in arctic regions, offshore katabatic winds are believed to play a key role in forming and altering polynyas and leads in coastal ice sheets.

THE LAND BREEZE AND THE SEA BREEZE

The land-sea breeze system (LSBS) typifies the class of mesoscale atmospheric systems induced by spatial inhomogeneities of surface heat flux into the boundary layer. The LSBS has been identified since the time of the classical Greeks (circa 350 B.C.). By the late 1960s, identifiable literature references exceeded 500 (Baralt and Brown, 1965; Jehn, 1973). Of all mesoscale phenomena, the LSBS over flat terrain has been among the most studied observationally, analytically, and numerically. This is undoubtedly a result of their geographically fixed nature, their frequent occurrence, their ease of recognition from conventional observations, the concentration of observers in coastal zones, and their importance to local weather and climate.



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Coastal Meteorology: A Review of the State of the Science 3 Thermally Driven Effects Differences in land and sea surface temperature and heat flux result in direct, thermally driven wind systems over a spectrum of temporal and spatial scales. The best known among these are the mesoscale land and sea (lake) breeze circulation systems (see, e.g., Defant, 1950), which are inherently diurnal in nature. Much less studied are the regimes induced by warm ocean waters adjacent to a large cold land mass. These circulations, which are not diurnal in nature, are termed coastal fronts. Even less well understood are systems present over the coastal ice-land boundary. As an example, in arctic regions, offshore katabatic winds are believed to play a key role in forming and altering polynyas and leads in coastal ice sheets. THE LAND BREEZE AND THE SEA BREEZE The land-sea breeze system (LSBS) typifies the class of mesoscale atmospheric systems induced by spatial inhomogeneities of surface heat flux into the boundary layer. The LSBS has been identified since the time of the classical Greeks (circa 350 B.C.). By the late 1960s, identifiable literature references exceeded 500 (Baralt and Brown, 1965; Jehn, 1973). Of all mesoscale phenomena, the LSBS over flat terrain has been among the most studied observationally, analytically, and numerically. This is undoubtedly a result of their geographically fixed nature, their frequent occurrence, their ease of recognition from conventional observations, the concentration of observers in coastal zones, and their importance to local weather and climate.

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Coastal Meteorology: A Review of the State of the Science By way of definition for this review, a land and sea breeze is a diurnal thermally driven circulation in which a definite surface convergence zone exists between air streams having over-water versus overland histories. These breezes are differentiated from the sustained onshore-offshore winds driven by the synoptic pressure field, which are termed sea-land winds. During lighter synoptic wind regimes, perturbations induced by the coastal discontinuity are often detectable but may not always result in a coherent recirculating wind system. The effects of the LSBS are many, including significantly altering the direction and speed of the ABL winds; influencing low-level stratiform and cumuliform clouds; initiating, suppressing, and modifying precipitating convective storms; recirculating and trapping pollutants released in or becoming entrained into the circulation; perturbing regional mixing depths; and creating strong near-shore temperature, moisture, and refractive index gradients. Improved understanding of LSBS should enhance applications to a wide variety of commercial, industrial, and defense activities (Raman, 1982). Sea-lake breeze inflow layers can vary from 100 m to over 1000 m in depth. Inland frontal penetration can vary from less than 1 km to over 100 km, with propagation speeds ranging from nearly stationary to >5 m/sec. The offshore extent of the inflow layer is less well known. Peak wind speeds are typically less than 10 m/sec. The overlying return flow layer depth is generally twice that of the inflow, but is often difficult to differentiate from the synoptic flow. Given the difficulty of measuring atmospheric mesoscale vertical motions, little is known about the detailed structure of updrafts associated with the sea breeze front. Some observational evidence from gliders, tetroons, Doppler lidar, etc., has suggested organized frontal zone motions of several m/sec. The coarse mesh size (often >5 km) used in most mesoscale simulations tends to portray peak vertical motions in the tens of centimeters per second range. More recent modeling studies (Lyons et al., 1991a, b) suggest that the sea breeze convergence zone is at times extremely narrow (perhaps <1000 m) and may produce regions of organized mesoscale ascent ranging from 1 to 4 m/sec. Even less is known about the broader and weaker subsidence regions offshore. Needed are improved measurements of vertical motions associated with the LSBS and companion modeling studies in which the mesh sizes used are adequate to resolve the observed features (Figure 3.1). Recent extremely fine-mesh two-dimensional simulations by Sha et al. (1991) resolved complex Kelvin-Helmholtz instabilities and other characteristics resembling a laboratory gravity current (Simpson, 1982). Circulations over large lakes are very similar to their oceanic counterparts. Smaller lakes, estuaries, and larger rivers also can significantly perturb the regional flow. The interactions between synoptic flow and water body size and orientation require additional study. Recent numerical simu-

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Coastal Meteorology: A Review of the State of the Science lations have suggested that surface heat flux differences of 100 W/m2 or more over several tens of kilometers can generate sea-breeze-like circulations (Segal et al., 1988). These physiographic mesoscale circulations can result from differences in soil type, land use, soil moisture (from irrigation and precipitation), snow cover, smoke/haze, and cloudiness. Additional field programs investigating mesoscale processes in both LSBS and physiographic systems are warranted as a test of the validity of available models. The LSBSs are not exclusively ''clear weather” phenomena; they substantially affect and interact with various cloud types. The modifications of fog and stratus in the LSBSs over the Great Lakes, Gulf, and Atlantic coasts have received less attention than those along the Pacific coastline. Advection through the coastal domain of middle-and high-level cloud decks, large smoke plumes, and urban-and regional-scale photochemical and sulfate hazes can materially affect the energetics of the LSBS, although these impacts have not been quantified. The LSBS profoundly affects the formation and fate of shallow convective clouds. Cumulus suppression in subsiding regions of the sea breeze cell has long been noted in satellite imagery. Under very light wind conditions, cumulus growth is enhanced within the sea breeze frontal zone updrafts. When the prevailing regional flow advects small convective clouds seaward across the frontal zone, the responses are more complex. Dissipation often occurs, but the relative roles played by subsidence versus disruption of cloud-capped thermals rooted in the surface superadiabatic layer are uncertain. Studies of the interactions of large eddies or convective thermals with the sea breeze frontal zone are warranted. The impact of the sea breeze front on deeper (precipitating) convective clouds is more complex. At midlatitudes the sea breeze can either enhance or weaken convective storms (Chandik and Lyons, 1971). On a scale of tens of kilometers, the Florida sea breeze has been found to trigger the general development of deep convection (Burpee and Lahiff, 1984). Thunderstorm development is intermittent along such frontal zones, and it is uncertain whether perturbations in the frontal convergence zone or localized responses to inhomogeneities in the surface energy budgets (or both) cause individual storms to form. Fine-mesh numerical modeling studies suggest that different spatial scales of topography and shoreline geometry produce a spectrum of convective-scale responses. Sea breeze thunderstorms affecting the Kennedy Space Center (Lyons et al., 1992) are initiated by both the east and west coast sea breezes. In addition, however, strong local convergence onto features such as Merritt Island trigger smaller-scale thunderstorms embedded within the larger sea breeze circulation (Figure 3.2). The complex feedbacks between the precipitating clouds and the LSBS are only partially understood. Also, convective responses to sea breezes over mountainous islands such as those found in the “marine continent” of southeast Asia require further study. Sea breeze thunderstorms contribute approxi-

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Coastal Meteorology: A Review of the State of the Science Figure 3.1 (caption on next page)

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Coastal Meteorology: A Review of the State of the Science Figure 3.1 Four two-dimensional mesoscale model realizations of a lake breeze in a 3000-meter deep east-west plane across southern Lake Michigan using 27-, 9-, 1-, and 0.33-km horizontal mesh sizes. All frames are at 1500 LT. Shown are U, the wind component, 1 m/sec isotachs (left); and W, the vertical motion field (right). Peak W values increase from 23 cm/sec (at 27 km mesh size) to 212 cm/sec (at 0.33 km mesh size). Negative values are shown as dashed lines.

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Coastal Meteorology: A Review of the State of the Science Figure 3.2 Response of deep convective clouds to coastal circulations in the vicinity of the Kennedy Space Center. In addition to the primary east and west coast sea breezes formed by the contrast between the Florida peninsula and surrounding ocean, numerous lakes, surface land use inhomogeneities, estuaries, and islands perturb the mesoscale flow. The convective response is more complex than suggested by earlier sea breeze thunderstorm studies. As an example, while the general Atlantic sea breeze (ASB) develops, enhanced convergence onto Merritt Island triggers rapid growth of a small thundershower by late morning. Widespread convection along the ASB does not develop until the late-afternoon approach of an impulse associated with the west coast sea breeze (graphics courtesy of Cecil S. Keen).

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Coastal Meteorology: A Review of the State of the Science mately 40 percent of Florida's rainfall and probably more in many other coastal regions. Sea breeze convective storm development is sensitive to small variations in middle tropospheric temperature lapse rates and moisture, such as those postulated in global greenhouse warming scenarios. Thus, middle tropospheric changes that affect this significant source of precipitation in tropical coastal areas is potentially important. Except for a few recent studies (Ohara et al., 1989), the land breeze is understood even less. Wind speeds are typically <5 m/sec, and the offshore flowing layer is often <100 m deep. The land breeze can often be commingled with stronger and deeper terrain-induced katabatic flows. The formation of thunderstorms associated with the offshore boundary of the land breeze over Gulf and Atlantic coastal waters has been addressed only in very cursory ways. Intense Great Lakes snow squalls also interact in complex 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 not well understood. Sometimes it retreats offshore as a distinct front, while at other times it simply pushes inland and dissipates after sunset. On other occasions, strong onshore flow may continue over coastal regions until past midnight local time (the ''fossil" sea breeze). Comprehensive studies of the LSBS through consecutive diurnal cycles are desired, with emphasis on the land breeze and the morning and evening transition periods. The LSBS and many other similar mesoscale circulations are poorly resolved in conventional weather-observing network systems, creating serious problems in operational forecasting. Local forecasters employ simple forecasting techniques using synoptic observational data (Lyons, 1972) to predict potential sea breeze occurrences. The character of the LSBS is controlled by a variety of factors, including land-sea surface temperature differences; latitude and day of the year; the synoptic wind and its orientation to the shoreline; the thermal stability of the lowest 200 to 300 mb of the atmosphere; patterns of land use and soil moisture; surface solar radiation as affected by haze, smoke, stratiform, and convective cloudiness; and the geometry of the shoreline and complexity of the surrounding terrain. Many of these factors are considered within mesoscale numerical modeling systems that are well suited to land-sea breeze simulation. Our understanding of the LSBS is not comprehensive, being largely confined to idealized conditions. When large-scale winds are virtually nonexistent over an infinitely long, two-dimensional flat coastline, it is comparatively 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 numerical modeling studies using two-dimensional models have been summarized by Pielke (1984). Newer nonhydrostatic, fine-mesh, nested-grid numerical

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Coastal Meteorology: A Review of the State of the Science models will be applied profitably to further studies of these types. Many previous modeling efforts emphasized coastal circulations in which the forcing functions were spatially homogeneous and temporally steady-state or diurnally varying. Observational and modeling studies need to be extended to coastal circulations occurring with nonhomogeneous and nonstationary synoptic environments, irregular shorelines and complex topography, and heterogeneous land use or land characteristics and soil moisture. Additional modeling challenges include accounting for the advection of middle-and upper-level clouds through the domain; changes in soil moisture; dynamic feedback between the LSBS and deep convective storms; and turbidity due to regional smoke, pollution, fog, and haze. There is evidence that gravity waves are excited by regional air flowing over the sea breeze, which is dynamically equivalent to a mountain. These features are worthy of continued investigation. While hemispheric and synoptic-scale numerical forecasting became well established in the 1950s, it was not until the mid-1980s that more powerful computers allowed organized attempts at operational mesoscale forecasting of coastal circulations and their effects (Lyons et al., 1987). Affordable high-speed computing and increasingly sophisticated numerical modeling techniques now allow extended experiments in operational coastal zone wind forecasting to be undertaken, yielding excellent opportunities to test the breadth and depth of our understanding of the LSBS. Interaction of the LSBS with the urban heat island and greatly enhanced roughness lengths in large cities has been studied in New York, Tokyo, Toronto, and elsewhere. Studies of interacting sea breeze and topographically forced flows (such as the Catalina eddy) are yielding improved understanding of the complex interactions between mesoscale systems. Even with the large number of studies in coastal Southern California, the development and morphology of sea and land breeze circulations in mountainous coastal terrain warrant much additional attention. Coastal thermally driven mesoscale circulations interact with smaller-scale surface-atmosphere energy exchange processes, cloud systems on a variety of scales, and the larger-scale synoptic patterns in which the mesoscale circulations are embedded. The LSBS represents a challenging problem for future observational and modeling programs, since it embodies many of the complex issues involved in atmospheric-scale interactions. COASTAL FRONTS When air over land is colder than air over the sea for extended periods, the direct circulation that develops across a coast does not normally exhibit typical diurnal characteristics. A longer-lived cousin of the land and sea breeze front, called a coastal front (Bosart et al., 1972), can form and re-

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Coastal Meteorology: A Review of the State of the Science main quasi-stationary, parallel to the coast, for several days. Coastal fronts have been reported and studied in many parts of the world, including the East and Gulf coasts of the United States (Bosart, 1975, 1981, 1984), the Black Sea (Draghici, 1984), Norway (Økland, 1990), the Netherlands (Roeloffzen et al., 1986), and Japan (Fujibe, 1990). The importance of coastal fronts for freezing rain and coastal cyclogenesis is discussed in Chapter 5. Along the Carolinas, coastal fronts are typically 1000 km long, with temperature contrasts as large as 20°C. Carolina coastal fronts tend to form within the boundary layer temperature gradient produced by differential heating of air across the margin of the Gulf Stream. This process has been studied by Riordan (1990), who used radar, ship, buoy, and aircraft data from the Genesis of Atlantic Lows Experiment (GALE). Frontal formation was found to be a discontinuous process, with the front forming in segments aligned with bands of shallow convection (Figure 3.2). The inland motion of the front was also discontinuous, for reasons that are not understood. This behavior contrasts with that of New England coastal fronts, which have been found to form along the coast and retain their identity as they move inland (Nielsen, 1989). A wide variety of triggering mechanisms have been shown to provide the sustained differential heating or confluence necessary for coastal frontogenesis. The most common occurs when a cold anticyclone approaches a coastline and winds become parallel to the coast. Air over land remains cold, while air just offshore continuously receives large heat fluxes from the sea surface. For example, over the Gulf Stream convective rainbands often form which may be accompanied by considerable lightning activity (Hobbs, 1987; Biswas and Hobbs, 1990). Other processes that are favorable to coastal frontogenesis are frictional retardation and turning of the wind, upstream blocking of cold air, or lee convergence. ICE-EDGE BOUNDARIES An understanding of the meteorology of coastal regions where sea ice is present is important for navigation, exploring for mineral resources, coastal biological activity, and modeling sea ice and climate. In the present discussion, only ice-land boundary regions will be considered. This includes almost all the Antarctic ice cover and the ice cover overlying the high-latitude continental shelves in the northern hemisphere. While many characteristics of the marginal ice zone (the boundary between sea ice and open ocean; see Johannessen et al., 1988) are similar to those of coastal ice-edge boundaries, the marginal ice zone will not be addressed here. A summary of meteorological processes occurring at ice-edge boundaries is given by Barry (1986). Barry concluded that our basic knowledge of meteorological conditions over ice-edge boundaries is very limited. The

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Coastal Meteorology: A Review of the State of the Science recent Marginal Ice Zone Experiment (MIZEX) off the East Greenland and Bering seas has provided insight into processes occurring at ice-ocean boundaries, but no such program has been undertaken for land-ice boundaries. With the exception of Antarctic katabatic winds (see Chapter 4), relatively little research has been conducted on mesoscale and small-scale coastal processes at the land-ice boundary. Many of the same physical processes and phenomena, which are described elsewhere in this report, such as land and sea breezes, occur in ice-edge coastal regions. However, there are unique processes occurring at the ice edge, particularly during the cold seasons of the year, that are associated with the characteristics of sea ice. A summary of the observed features of coastal sea ice is provided by Wadhams (1986) in the context of the seasonal sea ice zone. Even during winter, regions of open water occur between sea ice around coasts. 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 melting. Offshore katabatic winds (see Chapter 4) are believed to play an important role in the formation and maintenance of polynyas and leads. Particularly during winter, leads and polynyas are a major source of exchange of heat, moisture, and gases between the ocean and atmosphere. Polynyas and leads are sites of active brine formation, affecting the local water density structure and current field and cumulatively affecting the structure of the halocline. Leads and polynyas serve as corridors for migration of marine mammals. During spring localized plankton blooms occur, which are important biologically and are also possibly important as a local source of cloud condensation nuclei (see Chapter 6). A feature that occurs along the Antarctic coast is the presence of ice shelves, over which glacier ice streams into the sea. The largest ice shelves in the Antarctic are the Ross and Ronne-Filchner. Ice shelves determine the capability of the fast-flowing internal ice streams associated with marine ice sheets to disperse the glacier ice rapidly into the surrounding ocean. Marine ice sheets are characterized by being grounded on beds well below sea level. If the backstress exerted on the ice stream by the ice shelf is insufficient, accelerated discharge of land ice through ice streams to the sea may result in the collapse of the marine ice sheet (Binschadler, 1991). Leads and polynyas affect the atmosphere in the ice-edge coastal regions in the following ways. Extreme sea-air temperature differences (20° to 40°C) are commonly associated with leads and polynyas during winter, and heat fluxes of several hundred watts per square meter are typical (Smith et al., 1990b). The sensible heat flux in air is several times larger than the latent heat flux because of the relatively low value of saturation-specific humidity at the freezing point. Schnell et al. (1989) found that wide leads and polynyas release enough energy to create buoyant plumes that penetrate the boundary layer; in one case a hydrometer plume reached a height of 4

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Coastal Meteorology: A Review of the State of the Science km. Aircraft lidar measurements indicated the presence of small ice crystals in the plume, which are believed to modify substantially the local radiative balance. In particular, the larger leads and polynyas are likely to provide substantial amounts of water vapor, especially to the wintertime polar atmosphere, contributing to the presence of widespread low-level stratus clouds in these regions. Convection, associated with substantial precipitation, from larger polynyas seems likely; this precipitation has the potential to significantly influence the accumulation of snow both on glaciers and on 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 about leads. At the same time, the state of the sea ice, including the presence of leads and polynyas, is strongly dependent on atmospheric processes (see, e.g., Hibler, 1979). The atmosphere influences the state of the sea ice both thermodynamically (e.g., via radiative heat, sensible heat, and latent heat fluxes) and dynamically (e.g., via surface wind stress). SUMMARY AND CONCLUSIONS Some existing gaps in scientific understanding associated with thermally driven effects may be addressed through modeling studies and field programs. To spur progress we recommend the following: Observational and modeling studies of the LSBS should be extended to cover the entire diurnal cycle, with emphasis on improving knowledge of offshore regions, the morphology and dynamics of the land breeze, and the formation and breakdown of the sea breeze front. Remote sensing techniques and fine-mesh mesoscale numerical models should be applied to better understand the finer-scale, three-dimensional structure of the sea breeze front, its associated mesoscale vertical motions, and the development of internal boundary layers above complex coastlines and heterogeneous surfaces. Research should be directed to understand the three-dimensional LSBS interactions with inhomogeneous and time-dependent synoptic flows, nonuniform land and water surfaces, irregular coastlines, and complex terrain, as well as the dynamic feedbacks between the LSBS and stratiform clouds and precipitating and nonprecipitating convective cloud systems. The geographical distribution of coastal front occurrences, their spatial coverage, and their modes of propagation should be documented and their variability assessed. A combination of case studies and model simulations should be conducted to determine the site-specific, large-scale conditions leading to coastal front formation, which is often difficult to observe directly in real time

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Coastal Meteorology: A Review of the State of the Science because coastal fronts tend to form offshore and sometimes remain nearly stationary. Studies should be supported to elucidate processes of heat and moisture fluxes into the atmosphere from leads and polynyas, particularly in the presence of extreme horizontal thermal discontinuity. Interactions between the atmosphere and sea ice on the mesoscale in the coastal zone should be examined.