Evolution and Structure of the Mesoscale Convection and Its Environment: A Case Study during the Early Onset of the Southeast Asian Summer Monsoon

Abstract

The evolution and structure of mesoscale convection in the South China Sea (SCS) region are documented for the first time mainly using the dual-Doppler radar dataset collected during the South China Sea Monsoon Experiment (SCSMEX) in 1998. In particular, this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast Asian monsoon (SEAM). For the case of 15 May 1998, interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection. In the prefrontal region, the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast-to-southwest-oriented convection. In the postfrontal region, the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east-to-west orientation. In addition, the convergence between the southerly monsoon flow and the northerly postfrontal flow generated deeper and stronger low-level convergence. The postfrontal convection was more intense and deeper than the prefrontal convection. The precipitation and kinematic structure of mesoscale convection were studied with special attention to significant departures from archetypal tropical oceanic convection. On 15 May, prefrontal convection showed a straight upward rainfall and updraft pattern with little tilt as a result of moderate vertical wind shear. The maximum low-level convergence and updraft were 20?30 km behind instead of within 1?2 km of the leading edge. Although the convection was intense with maximum reflectivity over 50 dBZ, both pre- and postfrontal convection had a very limited stratiform region as a result of a dry environmental upper layer. The observed mesoscale convection had a tendency to form stratiform rain ahead of the convective rain, and two different modes of leading stratiform structure were found separately in pre- and postfrontal convection.