Development of a Torrential-Rain-Producing Convective System along a Meiyu Front in East China

The development of a quasi-linear torrential-rain-producing mesoscale convective system (MCS) along a Meiyu front in East China during the early morning hours of 8 July 2007 is studied using high-resolution surface data, radar reflectivity and a 24-h cloud-permitting simulation with the finest grid spacing of 1.11 km. Results show the presence of a low-level jet, and precipitable water exceeding 60 mm in the pre-storm environment, but with little favorable quasi-geostrophic forcing. Results also show that the evolution of the regional-scale rainfall intensity is highly correlated with that of the vertically integrated southwesterly moisture fluxes in a vertical cross section located 100-200 km upstream from the linear MCS. It is found from both the observations and simulation that the quasi-stationary linear MCS forms through the continuous convective backbuilding, organization into west-east-oriented rainbands with life spans of 4-10 hours, and their subsequent southeastward propagations. New convection could be continuously initiated at the western end of the MCS as moist southwesterly monsoonal flows ascend convectively generated cold domes. Two scales of convective organization are found: One is the eastward to northeastward “echo training” of convective cells along individual rainbands, and the other is the southeastward “band training” of the rainbands along the quasi-linear MCS. We conclude that the repeated backbuilding and convective initiation and the subsequent echo training along the same path account for the torrential rainfall production in the present case, whereas the band training is responsible for the longevity of the rainbands and the formation of the quasi-linear MCS.