UPDRAFT EVOLUTION - A PERSPECTIVE FROM CLOUD-BASE

A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings f rom the Cooperative Convective Precipitation Experiment 1981 is used t o monitor the evolution of the updraft at cloud base of a large cumulu s congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes com pleted by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two wit h the south updraft rotating cyclonically and moving to the right of t he 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 u pdraft with the shear of the horizontal wind. Saturation-point analyse s 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 c loud ages, air from the upper subcloud and transition layers contribut es to the updraft. This air has little or no buoyancy, which leads to cloud collapse. Mass flux and saturation-point analyses predict the cl oud's demise adequately, in contrast to the trends of vertical velocit y, virtual potential temperature, or moisture at cloud base. A pressur e perturbation caused by updraft-shear interaction is an important mec hanism for cloud intensification, but it must act in concert with anot her forcing mechanism, typically a gust front, to tap the most unstabl e air found in the lower subcloud layer in the High Plains. The observ ations support the numerical simulations of cumulonimbi in the presenc e of strong shear, albeit for a much smaller cloud. Congestus clouds m erit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective clou d spectrum.