Drivers of a sudden mesoscale rainstorm in arid and semi-arid regions at the edge of the western Pacific subtropical high

摘要

An extreme rainstorm occurred in southeastern Hami in Xinjiang, China, on July 31, 2018. The region experienced maximum accumulated precipitation of 110 mm over 12 h, causing a flood that killed 20 people and left eight missing. The present study uses multiple data sources in order to conduct an in-depth analysis of this extreme rainfall event. Results show that the western Pacific subtropical high (WPSH) could directly impact the arid and semi-arid areas under the certain circulation patters. During the heavy rainfall period, the 500 hPa WPSH was anomalously northerly; several mesoscale cloud clusters generated in front of the 700 hPa Hexi corridor jet stream and then merged and developed along the 500 hPa steering flow. The atmosphere was conditionally unstable over the region, and unstable convective energy was triggered by warm frontogenesis at the low Troposphere. Warm frontal frontogenesis was determined by the horizontal divergence and the tilt term during convective initiation, and deformation and tilt terms as convective cloud at matured. Additionally, the intensification of the 700 hPa Hexi corridor southeasterly jet and topographic uplifting were responsible for the long-term persistence of the mesoscale convective system, which should be paid more attention by forecasters in the future forecasting of this region.