Medium and small scale characteristic analysis of a short-time heavy rain under northwest flow in Urumqi

Based on Doppler radar, wind-profiler radar, GPS/MET water vapor detector, FY-2 E/G, conventional observation data and GFS/ NCEP 6 h interval reanalysis data with 1°×1° resolution, short-term heavy rainfall from 18:30 BT to 20:00 BT on 9 June 2015 in Urumqi is analyzed, with emphasis on the enviornmental conditions and the mesoscale convective system. The results show that there are four rainfall concentrated periods, and each period lasted about 10-15 minutes. Rainfall occurred under the control of northwest stream ahead a ridge and behind a trough in the northwest stream. γ-scale convective cells along β-scale northwest jet stream on 700 hPa was the direct impacting system of rainfall. Water vapor of the rainfall came from around Urumqi and was fast converged within 2-3 hours. Convective unstable potential energy began to develop at 14:00 BT, and the development only lasted four and half hours before rainfall. It occurred thatγ-scale convective cells along northwest low-level jet stream brought about short-time heavy rainfall in Urumqi by"train effect"style. Reflectivity composite of the convective cells was 45-50 dBz with a life-time only 10-15 minutes. Rainfall occurred where the big value area of infrared brightness temperature (TBB) gradient at the southwest of local new-forming meso-scale convective clouds with a maximum TBB of -44℃and a life-time of about 2-3 hours.