UPSCALE EVOLUTION OF MCSS - DOPPLER RADAR ANALYSIS AND ANALYTICAL INVESTIGATION

The development of two small mesoscale convective systems (MCSs) in no rtheastern Colorado is investigated via dual-Doppler radar analysis. T he first system developed from several initially isolated cumulonimbi, which gradually coalesced into a minimal MCS with relatively little s tratiform precipitation. The second system developed more rapidly alon g an axis of convection and generated a more extensive and persistent stratiform echo and MCS cloud shield. In both systems, the volumetric precipitation rate exhibited an early meso-beta-scale convective cycle (a maximum and subsequent minimum), followed by reintensification int o a modest mature stage. This sequence is similar to that noted previo usly in the developing stage of larger MCSs by McAnelly and Cotton. Th ey speculated that the early meso-beta convective cycle is a character istic feature of development in many MCSs that is dynamically linked t o a rather abrupt transition toward mature stage structure. This study presents kinematic evidence in support of this hypothesis for these c ases, as derived from dual-Doppler radar analyses over several-hour pe riods. Mature stage MCS characteristics such as deepened low- to midle vel convergence and mesoscale descent developed fairly rapidly, about 1 h after the early meso-beta convective maximum. The dynamic linkage between the meso-beta convective cycle and evolution toward mature str ucture is examined with a simple analytical model of the linearized at mospheric response to prescribed heating. Heating functions that appro ximate the temporal and spatial characteristics of the meso-beta conve ctive cycle are prescribed. The solutions show that the cycle forces a response within and near the thermally forced region that is consiste nt with the observed kinematic evolution in the MCSs. The initial resp onse to an intensifying convective ensemble is a self-suppressing mech anism that partially explains the weakening after a meso-beta convecti ve maximum. A lagged response then favors reintensification and areal growth of the weakened ensemble. A conceptual model of MCS development is proposed whereby the early meso-beta convective cycle and the resp onse to it are hypothesized to act as a generalized forcing-feedback m echanism that helps explain the upscale growth of a convective ensembl e into an organized MCS.