Mechanism of a torrential rainstorm that occurred to the west of a Mei-yu frontal low

Heavy rainfall usually occurs to the southeast of a Mei-yu frontal low. The rainstorms appeared to the west of Mei-yu frontal lows are difficult to forecast because their formation mechanisms are not well understood. An extreme rainfall event occurred to the west of a Mei-yu frontal low in Anqing City, Anhui Province, China, on 13 July 2010, which was not well predicted. Based on observation data, NCEP/NCAR reanalysis data and high-resolution numerical simulation output, the mechanisms of this severe rainstorm are analyzed in this paper. The results indicate that the eastward moving meso-α-scale low was a key synoptic system that resulted in the formation of the mesoscale convective system causing the rainstorm. As the low moved eastward and strengthened, the increasing northerly flow and the southerly warm/moist flow formed an eastwest elongated convergence line (frontal zone) at the west part of the low. While the low moved to the coastal region, its intensity was enhanced and its moving speed was reduced. As a result, the convergence line and the frontal zone remained quasi-stationary near Anqing. Furthermore, Anqing is located in a valley between the Dabie and Wannan Mountains; the northerly winds flowed around Dabie Mountains and entered the valley, leading to an intense local convergence and frontogenesis near Anqing. Under the unstable environments with sufficient water vapor, the local intense frontogenesis along with intense convergence triggered and maintained the quasi-stationary mesoscale convective system that resulted in the record-breaking rainfall in Anqing.