Evolution of the Dynamics and Thermodynamics of a Mesoscale Convective System: A Case Study

Abstract

A well-organized severe storm complex on 30 May 1976 developed in the southeastern part of the mesonetwork of the National Severe Storms Laboratory (NSSL) in central Oklahoma and merged, in its mature stage, with a band of thunderstorms along a stationary front. Rawinsonde observations at nine stations within the mesonetwork, supplemented by satellite time lapse images, have been analyzed to depict the kinematic and thermodynamic structure of the storm and its near environment. An objective analysis technique is used to describe the evolving storm structure. Among the significant features the analysis revealed, the low-level horizontal convergence was confined to the southeast prior to the appearance of the first cloud in the satellite images, and a well-defined mixed layer, capped by a strong stable layer, developed with northwest to southeast horizontal temperature and moisture, gradients. The height of the mixed layer increased with time in the dry northwest side, while it decreased in the southeast part accompanied by a developing low-level southerly jet. Then, the differential mixed-layer growth became accelerated. The low-level southerly jet pushed the moist air into the strong stable layer above a shallow mixed layer, and brought about the destruction of the stable layer. The mesoscale convergence and the heat flux from the superadiabatic layer in the low levels in the southeast region caused a rapid growth of the mixed layer to the lifting condensation level, releasing potential instability, and storm development ensued. The southerly air transported further northwest along the sloping mixed-layer top lost its momentum in the strong stable layer and descended with the air coming from the northwest at middle levels. In the cloud-free region, the strength of the inversion layer was maintained by the growth of the mixed layer and the differential mesoscale sinking enhanced by nearby active convection.