Based on hourly precipitation data at 144 regional automatic stations and national stations in Lanzhou of Gansu Province from April to September during 2010-2018 and geographic information data, the temporal and spatial distributions of short-term strong rainfall were analyzed in detail, and the relationship between frequency of short-term strong rainfall and geographic elements was discussed quantitatively. The results show that the threshold of short-term strong rainfall in Lanzhou was 10 mm·h-1, and the short-term strong rainfall events concentrated from late July to August, its frequency was the highest from 21:00 BST to 22:00 BST. The frequency of short-term strong rainfall had obvious spatial difference in Lanzhou, and it was more in the south and less in the north as a whole. Although the frequency of short-term strong rainfall varied significantly at different stations in Lanzhou, it wasn’t obvious dispersion and fitted the normal distribution. The frequency distribution of short-term strong rainfall was closely related with altitude, windward side and slope, so the high frequency area mostly distributed in the bell-mouth of valley, the windward slope, the urban heat island area and alpine mountainous area in Lanzhou.

Based on the conventional observations，NCEP / NCAR reanalysis data，FY － 2E satellite images and radar data，the synoptic analysis and physical field diagnostic analysis methods were used to explore the triggering mechanisms and vapor sources of the heavy rainstorm that occurred in Northwest China． By analyzing circulation patterns，vapor，dynamic and unstable conditions，the results show that the southwestern low level jet and the eastern low level jet played significant roles during the heavy rainfall process，they brought abundant water vapor and unstable capability． Forward trough and warming humidification in the low layer made stratification convective instability．The overlay of convergence line on 700 hPa and the shear line on 850 hPa was the triggering condition，which resulted in the release of unstable capability and induced the heavy rainstorm． Meanwhile，convergence at low level and divergence at upper － level formed upward motion over the entire layer，which was the dynamic condition．The study not only provides focus for forecasting heavy rainstorms in arid Northwest China，but also supplies references to control flood and mitigate disasters for local government．