The abundant water vapor supply is a necessary condition for heavy rainfall, especially in arid area with complex terrain on southern slope of the Tianshan Mountains. Based on daily precipitation at sixteen weather stations of southern slope of the Tianshan Mountains and NCEP/NCAR GDAS reanalysis data from May to September during 1981-2020, the circulation situation of rainstorms in warm season on southern slope of the Tianshan Mountains was analyzed, and the water vapor source and transport trajectory was simulated and clustered by using HYSPLIT model and cluster analysis. The results show that the rainstorms in warm season on southern slope of the Tianshan Mountains mainly occurred over the overlap area of the South Asia high pressure with double pattern, southwesterly jet (airflow) above 500 hPa, 700 hPa shear convergence and topographic force lifting of the Tianshan Mountains. Water vapor mainly came from Central Asia, the Atlantic Ocean and its coasts, the Mediterranean Sea and the Black Sea and adjacent areas, through the key areas of TKAP (Tajikistan, Kyrgyzstan, northeastern Afghanistan, northern Pakistan and northwestern India), southern and northern Xinjiang, it mainly transported into rainstorm areas by the westerly, southerly, northerly channels, respectively. The water vapor above 700 hPa and below 700 hPa over rainstorm areas mainly came from the westerly and northerly channels, respectively, and the contribution of water vapor in the key area of southern Xinjiang to rainstorms was the maximum. The water vapor from central Asia mainly transported to 700 hPa and below, the contribution to rainstorms was greater, and the damage of water vapor was more along the transport way, while the water vapor from the Atlantic Ocean and its coastal areas, the Mediterranean Sea and the Black Sea mainly transported above 700 hPa, and the contribution to rainstorms was relatively smaller. In additional, the water vapor in middle and lower troposphere came from northern and southern Xinjiang, eastern North America and Mongolia. Based on the above characteristics, a three-dimensional fine structure model of water vapor for rainstorm processes in warm season on southern slope of the Tianshan Mountains was established.

A heavy rain process occurred in Yining of Yili valley on 30 September 2019. Based on the observation data of micro rain radar (MRR), terrestrial OTT-PARSIVEL raindrop spectrometer and rain gauge at Yining station, the reliability of MRR data was compared and tested. And on this basis, the vertical distribution characteristics of microphysical quantities retrieved by MRR were investigated at different stages of the heavy rain process. The results show that the precipitations from three kinds of instruments were close, and their change trends were consistent. The rainfall intensity of MRR near ground level (35, 70 and 105 m) had a good correlation with the observation of OTT-PARSIVEL raindrop spectrometer, the coefficients were 0.9233, 0.9289 and 0.9186, respectively, and the convergence degree of rainfall intensity between two kinds of instruments was higher when the rainfall intensity was less than 3 mm·h^-1. The vertical distributions of microphysical quantities from MRR were different at different stages of rainfall intensity. At the low rainfall intensity stage, the environmental humidity was lower and evaporation was stronger during the initial period of rainfall, the radar reflectivity factor, liquid water content and rain intensity from MMR decreased with decrease of height, while the environmental humidity was higher and evaporation was weaker during the middle period of rainfall, and the vertical changes of radar reflectivity factor, liquid water content and rain intensity weren’t obvious. During the late period of rainfall, the rain intensity near ground decreased significantly under the low rainfall intensity condition because the supply of water vapor and power was lack. At the moderate and high rainfall intensity stages, the vertical distribution of particles falling velocity was stable, while the radar reflectivity factor, liquid water content and rain intensity increased with decrease of height due to the collision and merger interaction between raindrops. The small raindrops dominated during the heavy rain process in Yining, the percentage of average number concentration of small raindrops to total number concentration exceeded 90%, and it decreased with decrease of height. The contribution of medium raindrops to total rain intensity was the greatest, the contribution rates at different stages of rainfall intensity were 60% and above, and it increased with decrease of height. The number concentration of large raindrops was the smallest in proportion of total number concentration, and its contribution to total rain intensity was the smallest.