Quasi-stationary extreme rain produced by mesoscale convective system on the Mei-Yu front

Abstract

A mesoscale convective system (MCS) occurred on the Mei-Yu front in the Jiang-Huai River Basin of China on 7–8 July 2007, which caused extreme rainfall. The MCS formed in an environment of moderate convective available potential energy, high precipitable water, and an almost unidirectional southwesterly low-level jet. The favorable environment for MCS initiation and development featured a low-level convergence between northeasterly wind north of the Mei-Yu front and warm-moist southwesterly airflow. The evaporative cooling generated cold outflow which continuously promoted new convection at the leading edge of the MCS. WRF model simulations reproduced the observed back-building initiation and upscale organization. The flow-parallel MCS was affected by the low-level jet, vertical wind shear, and near-surface cold outflow in a stable nocturnal planetary boundary layer. Stratiform precipitation was reinforced by the downstream propagation of cumulonimbus and advection of ice-phase hydrometeors by the mid-upper level wind. The quasi-stationarity was a product of subtle dynamical balance between the near-surface cold outflow and the background low-level southerly flow. Sensitivity experiments addressed the role of near-surface outflow and diurnal forcing in MCS organization.