Causation Analysis of the "21st May" Torrential Rain in the West of Southern Xinjiang in 2018

Abstract: Xinjiang belongs to arid and semi-arid region, where the precipitation over 24mm reaches the torrential rain level. An extreme torrential rain occurred in the West of Southern Xinjiang from May 21 to 22, 2018. During the precipitation, there were heavy rains at the six stations, five of which exceeded the extreme value. Especially the hourly precipitation of one station exceeded 50mm. Based on regular meteorological observations data, regional automatic station observations data, the reanalysis dataset from the NCEP/NCAR reanalysis project and the data of FY-2E/2G Stationary Satellite from China meteorological Agency, the mechanism and the cause of the extreme torrential rain process was analyzed. The results showed that the extreme torrential rain occurred under the background of double body of South Asia high(high pressure center located in Iran Plateau and the east of Tibetan Plateau) and the subtropical trough deepening to 25°N around. The cold air from the back of Tashkent vortex at 500hPa and Northern Xinjiang simultaneously invaded into the Tarim Basin. The high and low-level jets met over the west of southern Xinjiang. Under the circumstances, the strong divergence area from the upper air and the strong convergence area from the low level air were superimposed over the rainstorm area to enhance the vertical upward movement of the entire layer of the atmosphere. The whole precipitation water vapor was twice more than the climate and the rapid convergence of low-level water vapor provides abundant water vapor conditions for heavy rain. The oriental denatured warm air, the cold air moving eastward from the western part of the basin，and the cold air sliding from the front of Kunlun Mountain were converging in the western part of Hotan. The convergence of multiple air streams and the uneven local moderate convergence systems triggered the evolution of mesoscale convective system, leading to the generation of local heavy rainstorm. The local mesoscale convergence system was a good indicator of the rainstorm area and intensity. The activity of multipleγ mesoscale MCS caused this extreme rainstorm. The extreme rainstorm occurred in the region with the largest TBB gradient. The TBB value decreased by more than 30 °C in 3 hours before the start of extreme precipitation, which is similar to the change of TBB value of local heavy rain in central and eastern China.

Key words: extreme torrential rain，Tashkent vortex，spatial structure，low-level easterly jet，the west of southern Xinjiang