3-DIMENSIONAL KINEMATIC AND MICROPHYSICAL EVOLUTION OF FLORIDA CUMULONIMBUS .3. VERTICAL MASS-TRANSPORT, MASS DIVERGENCE, AND SYNTHESIS

A statistical technique is employed to examine the evolving properties of the ensemble small-scale variability of high-resolution radar data collected in a multicellular Florida thunderstorm. This paper examine s vertical mass transport and mass divergence and synthesizes these ob servations with results from the first two parts of the study into a s elf-consistent conceptual model that describes the convective-to-strat iform transition of the storm. Vertical mass transport distributions i ndicate that the more numerous weak and moderate-strength upward and d ownward velocities, not the few strongest, accomplished most of the ve rtical mass transport in the storm. Hence, most of the mass of precipi tation is condensed outside the areas of intense upward motion. These data thus suggest a change in the way we think about convection. Altho ugh the few regions of strongest vertical motion play a part in the ov erall storm evolution by dispersing particles throughout the depth of the storm, it is the more prevalent weak and moderate-strength upward velocities that are the more important determinants of the precipitati on processes. An extension of bubble-based conceptual models of convec tion is proposed to account for the convective-to-stratiform transitio n. Bubbles of positively buoyant air produced at low levels are weaken ed by varying amounts of entrainment and slowed down by pressure gradi ent forces as they rise. Thus many bubbles are slowed and stopped at m id- and upper levels. The weakened parcels flatten, encompass more are a and, in the process, laterally spread their associated hydrometeors. As the weak updraft parcels congregate at mid- and upper levels of th e storm, they create the region of weak mean ascent that is characteri stic of stratiform mean vertical velocity profiles. Below the 0 degree s C level, precipitation-associated downdrafts dominate the ensemble o f smaller-scale drafts and create mean weak descent at low levels.