A three-dimensional numerical model of the sea breezes over South Florida
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A three-dimensional numerical model of the sea breezes over South Florida

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A three-dimensional numerical model of the sea breezes over South Florida

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    A three-dimensional, shallow, hydrostatic, primitive equation model is used to describe the initiation and evolution of the sea breeze convergence zones over south Florida as a function of the surface heat and momentum fluxes and of the large-scale synoptic forcing. The model has eight vertical levels with 1,188 grid points at each level. The horizontal resolution is 11 km, except near the lateral boundaries where coarser resolution is employed. The largescale synoptic velocity is determined from a balance of the eddy friction, pressure gradient, and Coriolis forces. The equations of motion are solved by a semi-implicit forwardupstream differencing technique. Vertical velocity is diagnosed by vertical integration. Pressure is calculated hydrostatically from a surface pressure tendency equation and integrated upward to a free surface. Model results are discussed for several • different initial conditions. It is shown that the curvature of the coastline and the prevailing synoptic flow have a profound influence on the locations of the sea breeze convergence zones. Under both southwesterly and southeasterly flow, convergence zones form along both coasts with the windward zone forming earlier and moving inland during the day, while the leeward zone remains essentially stationary along the lee coast. Under both flows, a subsidence region forms over Lake Okeechobee. Favorable asymmetries in the south Florida coastline, particularly over the southwestern corner of the peninsula, inland from the convex bulge along the west coast and east of Lake Okeechobee, are especially effective in generating intense convergence. This convergence is large enough to saturate volumes of air on the scale of several grid intervals in the model. It is shown that the sea breeze convergence concentrates synoptic-scale moisture and is relatively unaffected by evaporation from the ocean, at least for a period of110 hours or so. The predicted convergence patterns qualitatively agree with all existing observational studies of cumulus convection organization over south Florida on undisturbed days. It is demonstrated that the differential heating between land and water generates much larger convergences than does the differential surface roughness alone. The particular value of the surface roughness, however, is shown to be important in the intensity of the turbulent mixing of heat and momentum and thereby influences the magnitudes of convergence. The model was integrated, using synoptic data on a particular day as input, and the predictions compared against the observed cloud and radar patterns on that day. The agreement between the predicted convergence zones and the more persistent showers became obvious only after substantial areal saturation in the middle levels of the model. This result, along with the results of the southeasterly and southwesterly model runs, suggests that on days without organized synoptic disturbances the mesoscale moisture convergence is a primary control of the locations of the cumulonimbus complexes over south Florida at least for several hours after substantial cumulus convection has begun. The results of this report have an important implication for the south Florida cloud modification program of the Experimental Meteorology Laboratory (EML). In the past, it was felt that the Laboratory was attempting to modify isolated cloud groups. In reality, the results in this report suggest that the weather experiments at ERL are being conducted on a mesoscale system.
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