In order to explore the forecasting method of precipitation phases in the winter half-year under the condition of complex topography, and further improve the spatio-temporal resolution and forecast accuracy of precipitation phases, the observation data at 11 national weather stations of Baoji City in the winter half-year (from November to next March) during 2010-2019 were used to analyze the spatio-temporal distribution characteristics of precipitation phases including rain, sleet and snow. Combined with the fifth generation atmospheric reanalysis data from European Centre for Medium-Range Weather Forecasts in the same period, the identification factor and its threshold of precipitation phases was selected and confirmed. On this basis of that, a fine objective forecast method of precipitation phases was established, and the prediction effect was tested. The results show that the rainfalls were more during the early and late winter in Baoji City, the proportions of precipitation days with three phases were similar in rain-snow transformation period, while the snowfalls were more in midwinter period. The spatial distribution of precipitation phases was closely relevant to topography, with more rainfalls in low altitude Chuanyuan region on both sides of Weihe River and more snowfalls in southern and northern high altitude mountainous areas. The surface temperature (T₂), 850 hPa and 700 hPa temperature (T₈₅₀, T₇₀₀) and geopotential thickness from 1 000 hPa to 850 hPa and 850 hPa to 700 hPa (H_850-1000, H_700-850) were selected to identify precipitation phases in the winter half-year in Baoji City. The T₂ (H_850-1000) thresholds of rain were 2.9 ℃ (1 307 gpm), 2.1 ℃ (1 308 gpm) and 1.8 ℃ (1 310 gpm) in the early and late winter, rain-snow transformation and midwinter periods at Weibin station in Chuanyuan region, respectively, while the thresholds of snow were 0.7 ℃ (1 302 gpm), 0.3 ℃ (1 303 gpm) and 0.7 ℃ (1 308 gpm), respectively. However, the phase identifications at Taibai station in mountainous area were different from Weibin station, the T₂ (H_700-850) thresholds of rain were generally greater than 2.6 ℃ (1 551 gpm) during the rainfall and less than -0.3 ℃ (1 540 gpm) during the snowfall at Taibai station, and their thresholds of sleet were generally between rain and snow. In addition, they must be associated with T₈₅₀ and T₇₀₀ to determine the phase transformation of rain and snow. The fine objective forecast method of precipitation phases in each period of the winter half-year in different topography areas of Baoji City was established based on the combined criterion of temperature and geopotential thickness, which could predict accurately hourly precipitation phases in the winter half-year from November 2020 to January 2022, with threat score (TS) up to 100% at Weibin station and more than 80% at Taibai station, which was better than a single physical quantity ( temperature or geopotential thickness).

Based on daily precipitation data, upper-air observation data and ERA5 hourly reanalysis data with 0.25°×0.25° resolution from 2010 to 2020, the rainstorms caused by the northwest vortex in Shaanxi are counted, and the characteristics of rainstorms caused by the northwest vortex with and without influence of typhoon are comparatively analyzed. The results show that the rainstorms caused by the northwest vortex in Shaanxi mostly occurred in July and August, and it occurred more in northern Shaanxi. The night rain characteristics of the rainstorms are obvious. The rainstorm intensity was stronger under influence of typhoon, and the falling area was to the north of two latitudes than that without influence of typhoon. The northwest vortex, which caused rainstorms in Shaanxi, was located at about 7-8 latitudes to the north of the ridge line of the western Pacific subtropical high. The northwest vortex had the dynamic characteristics of convergence at low level and divergence at high level. The upward motion of the northwest vortex was strengthened by the topographic forcing uplift, and the low-level water vapor transport and convergence provided favorable conditions for the occurrence of rainstorms caused by the northwest vortex. Under the influence of typhoon, the subtropical high was westward and northward, the water vapor and energy in the periphery of the typhoon were transported to the northwest vortex with the southwesterly low-level jet, the atmosphere at lower layer of the northwest vortex was convective instability, and the positive vorticity advection in front of the high trough and strong divergence on the right side of the high-level jet promoted the development and enhancement of the vortex, the strong updrafts on the south and east sides of the vortex triggered the release of unstable energy and formed strong frontogenesis in northern Shaanxi, the frontogenesis further enhanced the vertical movement on the south and east sides of the vortex, which caused heavy rainstorms on the south and east sides of the vortex. When there was no influence of typhoon, the subtropical high was eastward and southward, the southwest wind speed was smaller and the water vapor transport was weaker, the southwest wind in front of the plateau trough transported water vapor from the Bay of Bengal and the South China Sea to Shaanxi, the atmosphere at lower layer of the northwest vortex was stable, the strong updraft was located in the south of the vortex, the cold and warm air converged in central and southern Shaanxi, which resulted in scattered weak frontogenesis and caused rainstorms in the south of the vortex center.