Analysis of Dynamic Process and Moisture Source on a Heavy Precipitation Event in Southern Xinjiang Associated with the Double Upper-level Jet

Based on the ERA-Interim reanalysis data and the HYSPLIT (hybrid single particle Lagrangian integratedtrajectory) model driven by the NOAA reanalysis data, this study examined the dynamical process and moisture source ofa heavy precipitation event that occurred in southern Xinjiang on 24 May 2015. The results indicated that the directinfluencing system of this heavy precipitation was the small perturbation along the southwesterly flow east of the centralAsian low vortex. The enhanced upper divergence pumping action caused by the two upper-level jets and the coupling ofthe upper- and low-level jet streams induced deep and intense ascending motions, which was the main dynamic upliftmechanism of the heavy precipitation. The evolution of TBB (black body temperature) corresponded well with theoccurrence and development of precipitation; the rainfall occurred when the low value center of TBB dropped below-50℃ and strengthened continually with the expansion of the low value center. A further study suggested that thepositive center of the vertical component of CVV (convective vorticity vector) at 850 hPa could roughly reflect thelocation of the heavy precipitation 6 to 12 hours ahead. Analysis of the moisture flux and trajectory tracking revealed thatthe most significant moisture sources of the heavy precipitation over the basin in southern Xinjiang were the Black Seaand the Caspian Sea, and the low-level jet entrained a part of moisture into the south of Xinjiang. The result of HYSPLITmodel tracking illustrated that two moisture paths both originated from Eurasia but differed in trajectories during theheavy precipitation; the moisture above and below 800 hPa was mainly transported by the western track and diversiontrack separately. Significant uplift occurred vertically in both trajectories before heavy precipitation, and the convergenceof moisture contributed to the event．