Evolution of the Kinematic Structure and Precipitation Characteristics of a Mesoscale Convective System on 20,May 1979

Abstract

The relationship of vertical motion to the occurrence of precipitation from the convective and stratiform regions of a mesoscale convective system (MCS) is presented. On 20?21 May 1979, an MCS developed over portions of Oklahoma, Texas, and Arkansas. The uniqueness of this system was its lack of squall-line characteristics and development of a large stratiform precipitation region. The evolution of the system is detailed by rawinsonde observations, radar cross sections, 15-min composite analyses of six NWS WSR-57 radars, and by raingages. The genesis stage of the MCS was described by strong convection along an east-west cold front that was reinforced by outflow generated by two mesoscale convective complexes (MCCS) that formed tile night before in Kansas and Missouri. The mature stage of the MCS was characterized by the development of a large stratiform precipitation region while convection was limited to the southern and eastern flanks of the system. Finally, in the dissipative stage, a moderate north-south squall line that developed over west Texas in the afternoon moved rapidly to the cast apparently associated with a short-wave aloft and appeared to sweep the entire system out of Oklahoma. A modified Cheng and Houze technique is applied to the radar composites to determine stratiform and convective regions utilizing temporal as well as areas considerations. For the system as a whole, the stratiform region generated 30?50% of the total precipitation. The vertical-motion profiles hold the key to the precipitation characteristics over the storm-scale network. The genesis period was characterized by a strongly convective profile. As the system matured, low-level upward motion cased, while middle-level upward motion was sustained. A large area of stratiform rain developed as the deep convection weakened. Water-budget considerations suggest that the stratiform region was maintained by a combination of mesoscale middle-level updraft and by horizontal transfer of convective debris.