There is generally a large thermal contrast between the ocean and the land that drives the well-known sea-breeze circulation, which results in the confluence of air originating over the ocean with air originating over the land. The sea-breeze is associated with many processes that contribute to the recirculation and trapping of pollution, the evolution of precipitating convective storms, the creation of strong nearshore thermal, moisture and aerosol gradients, and the formation and transport of fog and low cloud in the coastal zone.
Along the southern California coast pollutants circulate between the cool, shallow marine boundary layer, often less than 100 m deep, and heated coastal regions [ Hanna et al., 1991]. Major ozone episodes in the vicinity of Santa Barbara, California are often associated with the storage of ozone precursors in the shallow marine layer over the Santa Barbara Channel [ Moore et al., 1991] and the onshore flow of marine air as a miniature cold front [ McElroy and Smith, 1991]. A coherent marine layer, with an imbedded thermal internal boundary layer that forms at the shoreline, can propagate inland for distances of 20 to 50 km. The boundary layer depth can vary from 100--200 m at the shore to several kilometers inland [ McElroy and Smith, 1991]. Wilczak et al. [1991] used dual-Doppler radar, Doppler sodar, aircraft, a surface mesonet, and rawinsonde data to study boundary layer flows within the Santa Barbara Channel. They observed a wide variety of flow features, including mesoscale wind vortices, sea- and land-breezes, and thermally-driven upslope and downslope winds. A significant feature of this area is an eddy that forms over the Channel. They suggested that its formation is dependent on the initial stratification of the atmosphere and the interaction of drainage and large scale-lows. A second feature, known as the Gaviota eddy, occurs as a result of surface heating that generates a sea-breeze flow that opposes the large-scale ambient circulation.
The coastal front is characteristic of east and gulf coasts of
the United States [ Nielsen and Neilley, 1990;
Riordan, 1990; Doyle and Warner, 1993b]. Coastal fronts are shallow
mesoscale boundaries that separate moist marine air from cold, dry
continental air. Along the Carolinas, coastal fronts are
typically 1000 km long, with temperature contrasts as large as
20
C, and may persist for several days. The mechanisms for
the initiation of these features include differential friction,
differential diabatic heating [e.g., Riordan, 1990;
Doyle and Warner, 1993b], and land-sea circulations [ Nielsen, 1989].
For example, these fronts can form in response to the temperature
gradient produced by differential heating across the margin of
the Gulf Stream [ Riordan, 1990; Holt and Raman, 1992;
Doyle and Warner, 1993a]. These fronts are particularly important
because they are often associated with cyclogenesis and heavy
precipitation [ Doyle and Warner, 1993b]. Huang and
Raman [1992] used a three-dimensional numerical model to show that
differential boundary layer modification may be the main
mechanism for the formation of coastal fronts along the North
Carolina coast. Quasi-stationary rainbands, which are produced
by cumulus convection along the front, encroach inland with the
warm air advection. The characteristics of these rainbands were
investigated by Dodge and Burpee [1993], who compared
formation over the Gulf Stream and over the continental shelf.
The sea-breeze convergence over the Florida peninsula produces
the highest number of days with thunderstorms in the United
States. Boybeyi and Raman [1992] found that the spatial
and temporal variation of sea-breeze convergence zones and the
associated convective activity depend to a large extent on the
direction and magnitude of the ambient wind. Southeasterly winds
result in a strong convergence zone and heavy rainfall along the
west coast of the Florida peninsula. This convergence zone
shifts to the east coast in southwesterly winds, and occurs
simultaneously on both coasts when the ambient winds are weak.
The convergence of the east and west coast sea-breezes is the
primary control on the timing and location of rapid convective
development [ Nicholls et al., 1991].