Updraft Evolution: A Perspective from Cloud Base

Abstract

A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft?shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains. The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.