Characteristic Structure and Growth of the Nonprecipitating Cumulus Layer over South Florida

Abstract

A study has been carried out of the structure of the planetary boundary layer over south central Florida under locally undisturbed conditions using data from the 1975 Florida Area Cumulus Experiment (FACE). From a series of rawinsonde, observations (129 total) during July and August at approximately 1000, 1300 and 1600 LST, 24 soundings have been identified as revealing both mixed-layer and cumulus-layer structures. The soundings have been composited to provide a representation of the average structure of the two layers. The inversion that is found to exist at the top of the cumulus layer is much less pronounced than the trade wind inversion as might be expected considering the relatively rapid growth and short lifetime of this phenomenon. A model for the growth of the mixed and cumulus layers is presented that includes effects of updraft mass flux, detrainment and evaporation. A closure assumption is proposed that relates the buoyancy flux in the cloud environment at the base of the cumulus inversion to the buoyancy flux within cumulus updrafts at that level. Application of the model to the 1975 FACE data has provided lower bound estimates to the downward buoyancy flux at the cumulus inversion.