In order to effectively develop the cloud water resources in the Liupan Mountain area and improve the scientificity of artificial precipitation enhancement, it is necessary to grasp the spatial and temporal distribution characteristics of atmospheric water vapor in the region and its causes. In this paper, the precipitation observation data of national basic meteorological stations in the Liupan Mountain area from 1989 to 2018 and the Fifth Generation Atmospheric Reanalysis Data (ERA5) of European Centre for Medium-Range Weather Forecasts (ECMWF) during the same period are used to analyze the spatial and temporal changes of atmospheric water vapor elements such as precipitable water, specific humidity, relative humidity and water vapor flux in this area. The reasons for the difference of water vapor conditions and precipitation in different areas of the Liupan Mountain area are analyzed from the aspects of water vapor transport, terrain effect and influence of buoyancy frequency. The results show that the water vapor conditions over the top and the east slope of the Liupan Mountain are better than those over the west slope in most of the year. The large value areas are mainly concentrated near the main peak of the Liupan Mountain, and water vapor condition has obvious seasonal variation characteristics. Over the eastern slope of the Liupan Mountain, the dynamic field of 500 hPa divergence and 700 hPa convergence caused by the uplift of the terrain is the most obvious in summer and the weakest in winter. The buoyancy frequency is the highest in winter and the lowest in summer. The higher buoyancy frequency and steeper terrain on the eastern slope make the gravity wave effect more obvious, resulting in more favorable vertical upward diffusion conditions and greater precipitation potential.

The WRF (weather research and forecasting) mesoscale numerical model was used to simulate a typical rainstorm process in the Liupan Mountain region on July 10, 2018. The dynamic and moisture field, the evolution of cloud and precipitation micro-physical structure were analyzed in this paper. A sensitivity test was conducted by varying the height of the Liupan Mountain topography in initial field of the model, and mechanism of the Liupan Mountain topography affecting precipitation there was discussed. The results show that the rainstorm was caused by the cold air at the bottom of the Mongolian cold vortex and the warm-moist air at the west side of the WPSH (western Pacific subtropical high) intersecting at the Liupan Mountain region and the 700 hPa shear line at the lower level. The rainstorm zone, the center of heavy precipitation and the shear line at 700 hPa were well simulated by the WRF model in the control test. At the stage of development and prosperity of precipitation, the southeasterly warm and wet air was affected by topographic forced uplift and topographic circumfluence, and there were updrafts in western and eastern slopes of the Liupan Mountain. Cloud water was brought to negative temperature layer and formed supercooled water. Cloud water, ice crystals, snow and graupel coexisted between 0 ℃ layer and -40 ℃ layer, which was conducive to collision growth of ice particles and the Bergeron process. The terrain sensitivity test shows that the change of terrain had little effect on rainfall area, and elevation of the terrain made rainfall level increase significantly, especially the heavy precipitation was more concentrated on the windward slope side. Forced uplift of terrain further strengthened vertical transport of water vapor and updraft, and the ice phase process in clouds developed fully. Supercooled cloud water provided favorable conditions for snow and graupel growth, thus increasing surface precipitation.