Dynamic mechanism and water vapor transportation characteristics of two extreme rainstorm events in Ili River valley in summer of 2016

Based on routine upper-air and surface observations, hourly rainfall from automatic weather stations (AWSs) and ECMWF reanalysis data with 0.25°×0.25° resolution, we have conducted a comparative analysis of the two extreme rainstorm events (hereinafter referred to as "0617" event and "0801" event, respectively) occurred in Ili River valley from June 16 to 17 and from July 31 to August 1 in 2016, emphasizing their similarities and differences in dynamic mechanism and water vapor transportation characteristics. Main results are as follows. The similarities show that (1) both rainstorm events occurred under the background with two troughs and one ridge pattern at 500 hPa. (2) The southward moving by 40°N of Central Asia low trough, establishment and maintenance of the westerly low-level jet stream over Ili River valley, and the wind shear, wind velocity convergence and forcing uplifting under topographic effect are observed. All these factors provide favorable environment conditions for the formation of extreme rainstorm. (3) The combined effect of the low-level westerly jet and the southwest air flow in the mid-and upper-level causes the maintenance of ascending movement in the windward slope, which is the dynamic mechanism of the two rainstorm events. (4) The water vapor for rainstorm comes mostly from the Central Asia nearby 40°N between Aral Sea and the south of Balkhash Lake. The differences show that (1) "0617" event is characterized by a more favorable thermodynamic condition, including strong local convection, short severe rainfall duration, large hourly rainfall, scattered rainstorm areas and serious disaster. The primary water input for this event is from the west boundary, and the secondary source is from north one. (2) In "0801" event, the southerly air flow in front of the trough at 500 hPa, and the convergence and shear of wind at 600 hPa are distinctly stronger. Especially, the easterly jet maintains over East Tibetan-Qinghai Plateau, Hexi Corridor and South Xinjiang in the mid-and low-level, which causes the dynamic and the water vapor convergence enhancement significantly. The primary water input is from the south boundary, and the secondary source is from the west and east. Precipitation increases obviously with the enhancement of the water input from the west and east boundary in the middle level. Unusual transport and strong convergence of water vapor result in the maximum area rainfall value which broke historical records during this event.