Three-Dimensional Kinematic and Microphysical Evolution of Florida Cumulonimbus. Part III: Vertical Mass Transport, Mass Divergence, and Synthesis

Abstract

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 examines vertical mass transport and mass divergence and synthesizes these observations with results from the first two parts of the study into a self-consistent conceptual model that describes the convective-to-stratiform transition of the storm. Vertical mass transport distributions indicate that the more numerous weak and moderate-strength upward and downward velocities, not the few strongest, accomplished most of the vertical mass transport in the storm. Hence, most of the mass of precipitation is condensed outside the areas of intense upward motion. These data thus suggest a change in the way we think about convection. Although the few regions of strongest vertical motion play a part in the overall 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 precipitation processes. An extension of bubble-based conceptual models of convection is proposed to account for the convective-to-stratiform transition. Bubbles of positively buoyant air produced at low levels are weakened by varying amounts of entrainment and slowed down by pressure gradient forces as they rise. Thus many bubbles are slowed and stopped at mid- and upper levels. The weakened parcels flatten, encompass more area and, in the process, laterally spread their associated hydrometeors. As the weak updraft parcels congregate at mid- and upper levels of the storm, they create the region of weak mean ascent that is characteristic of stratiform mean vertical velocity profiles. Below the 0°C level, precipitation-associated downdrafts dominate the ensemble of smaller-scale drafts and create mean weak descent at low levels.