Effects of Cumulus Convection on the Structure and Growth of the Mixed Layer over South Florida

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  • 1 National Hurricane and Experimental Meteorology Laboratory, NOAA, Coral Gables, Fla. 33124
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Abstract

Composites of rawinsonde observations of the subcloud layer over south central Florida obtained during the 1975 Florida Area Cumulus Experiment are presented. At a single location south of Lake Okeechobee three soundings per day were released, whenever possible, at approximately 1000, 1300 and 1600 LST during July and August. Soundings have been stratified according to the state of the weather in the vicinity of the release site and time of day. Weather conditions over the south Florida peninsula during the summer period normally ranged from undisturbed (no convective activity) in the morning to highly disturbed (intense convection with precipitation downdrafts) during the afternoon.

Analysis of 47 soundings that ascended into clear air shows rapid growth of the mixed layer beginning 2–3 h after sunrise until shortly before noon, the approximate time of the onset of convective precipitation downdrafts over the peninsula. From the application of simple mixed-layer theory, it is evident that significant subsidence within the cloud environment compensating the net cumulus mass flux occurs on the mesoscale (∼10–100 km). The maximum environmental sinking occurs several hours before the time of the heaviest peninsula-scale rainfall, indicating that the intensity of cumulus downdrafts relative to updrafts increases as the convective activity increases during the day. The subcloud layer structure for a number of soundings that ascended into the bases of nonprecipitating cumuli is also examined.

Abstract

Composites of rawinsonde observations of the subcloud layer over south central Florida obtained during the 1975 Florida Area Cumulus Experiment are presented. At a single location south of Lake Okeechobee three soundings per day were released, whenever possible, at approximately 1000, 1300 and 1600 LST during July and August. Soundings have been stratified according to the state of the weather in the vicinity of the release site and time of day. Weather conditions over the south Florida peninsula during the summer period normally ranged from undisturbed (no convective activity) in the morning to highly disturbed (intense convection with precipitation downdrafts) during the afternoon.

Analysis of 47 soundings that ascended into clear air shows rapid growth of the mixed layer beginning 2–3 h after sunrise until shortly before noon, the approximate time of the onset of convective precipitation downdrafts over the peninsula. From the application of simple mixed-layer theory, it is evident that significant subsidence within the cloud environment compensating the net cumulus mass flux occurs on the mesoscale (∼10–100 km). The maximum environmental sinking occurs several hours before the time of the heaviest peninsula-scale rainfall, indicating that the intensity of cumulus downdrafts relative to updrafts increases as the convective activity increases during the day. The subcloud layer structure for a number of soundings that ascended into the bases of nonprecipitating cumuli is also examined.

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