The analysis of the difference of forming environment of thunderstorm gales in different regions of Shaanxi Province is helpful to better understand thermal, dynamic and circulation characteristics of this kind of processes, and provide reference for the forecast and early warning of this kind of weather. Based on the surface observation data, lightning data and ERA5 reanalysis data of European Centre for Medium-Range Weather Forecasts from 2017 to 2022, the temporal and spatial distribution characteristics of thunderstorm gales in Shaanxi are analyzed, and the environmental parameters and circulation characteristics of warm type thunderstorm gales are compared and analyzed in different regions. The results show that northern Shaanxi and eastern Guanzhong are high-occurrence areas of thunderstorm gales, and warm type process is significantly more than cold type one. In summer this kind of weather process is much more than that in other seasons, and from June to August, the frequency of warm type processes in northern Shaanxi is significantly higher than that in Guanzhong and southern Shaanxi. The high occurrence period of thunderstorm gales is from 15:00 BT to 21:00 BT, and from 14:00 BT to 18:00 BT, the frequency of warm type processes in northern Shaanxi is significantly higher than that in Guanzhong and southern Shaanxi. There are some differences in thermal and dynamic conditions before the occurrence of warm type thunderstorm gales in different regions. Before the occurrence of warm type processes in northern Shaanxi, the energy and water vapor conditions are relatively weak, while the dynamic condition is relatively strong. But in southern Shaanxi, the energy and water vapor conditions are relatively strong, and the dynamic condition is relatively weak. The circulation types with frequency higher than 15% are northern Shaanxi west wind type and anticyclone with west wind type, Guanzhong west wind type and anticyclone with west wind type, southern Shaanxi cyclone with west wind type and anticyclone with west wind type. For the northern Shaanxi west wind type and anticyclone with west wind type, northern Shaanxi is located at the bottom of cold vortex or low trough, or between the bottom of low trough and the subtropical high, and the temperature difference between 850 and 500 hPa is larger, which provides certain unstable conditions for the occurrence of convective weathers. There is shear near the average occurrence position of warm type thunderstorm gales, which is conducive to the triggering of convective weather. For the west wind type in Guanzhong, the southerly airflow in the lower layer is stronger, and the dew point depression is smaller. For the cyclone with west wind type in southern Shaanxi, the T-ln P diagram shows a near-V shape and the energy condition is better. For the anticyclone with west wind type in Guanzhong and southern Shaanxi, the T-ln P diagram shows a near-V shape and the water vapor conditions are better.
Based on the precipitation data from Shaanxi Province from 2016 to 2021 and real-time products of two-source and three-source precipitation from the CMA Multi-source Precipitation Analysis System (CMPAS-V2.1) during 2018-2021, the spatial and temporal characteristics of short-term heavy rainfall in Shaanxi Province over the past 6 years were analyzed. Furthermore, statistical tests were applied to evaluate the accuracy of the multi-source precipitation products, with the aim of providing reference for correction of multi-source precipitation products in short-term heavy rainfall processes. The results are as follows:(1) In Shaanxi Province, the short-term heavy rainfall frequency peak was at 19:00 pm, with heavy precipitation primarily occurring from 16:00 to 02:00 in a day and predominantly in the months of June to August. The diurnal extreme value of heavy rainfall shows relatively higher from 17:00 to 01:00 and from 04:00 to 07:00 in the morning. Short-term heavy rainfall is more frequent in southern Shaanxi compared to Guanzhong and northern Shaanxi. The regions with relatively larger extreme values include northern Shaanxi, eastern Guanzhong, and central-eastern of southern Shaanxi. (2) Both two types of precipitation products tend to underestimate precipitation compared to observed values. The mean absolute errors of the three-source product exhibit smaller in southern of northern Shaanxi, most part of Guanzhong and southern of southern Shaanxi, while the two-source product performs relatively better in other region.The mean absolute error increases with precipitation increase. For heavy precipitation ranging from 20 to 50 mm, three-source product is recommended, while two-source product is more suitable for heavy precipitation above 50 mm. (3) The diurnal variations in mean absolute errors for both two types of multi-source products are relatively larger from 13:00 to 19:00, 23:00 to 01:00 and from 04:00 to 06:00, and relatively smaller from 08:00 to 12:00, 20:00 to 22:00 and from 02:00 to 03:00. The three-source product outperforms two-source product from May to August, while two-source product performs better from September to October. (4) The accuracy rate of multi-source precipitation products increases as the threshold of short-term heavy rainfall decrease. Both mean absolute error and accuracy rate indicate that the three-source product outperforms two-source product. Multi-source precipitation products exhibit higher accuracy from September to October compared to May to August.
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.
In order to get a deeper understanding of the water vapor characteristics and sources of rainstorms in semi-arid areas in Northwest China, and improve the ability of rainstorm forecasting in this area, based on upper-air and surface observation data and European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation global atmospheric reanalysis (ERA5) (0.25 × 0.25),the characteristics of water vapor transport and budget of two large-scale rainstorm processes with different intensities occurring in northern Shaanxi under different circulation on July 11 and August 9, 2022 were analyzed. Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to quantitatively analyze the source and contribution rate of water vapor. The results show that the high level trough, low level shear line and vortex and low level jet were the main influence systems of the rainstorm occurring on July 11. 700 hPa cyclonic convergence and 850 hPa low vortex strengthened and moved slowly, causing regional rainstorms. The short-wave trough and low-level shear line were the main influence systems of the rainstorm process on August 9, and the secondary circulation on both sides of the shear line lifted the warm and humid air flow outside the West Pacific Subtropical High (referred to as "West Pacific Sub-high") triggering the release of unstable energy and forming a large-scale convective rainstorm weather. On July 11, the vertically integrated water vapor fluxes from ground to 300 hPa was stronger, 700 hPa southwest jet and the 850 hPa southeast jet formed two obvious water vapor transport belts. The strong convergence lasted longer, the wet layer was deep and the weather process was dominated by stable precipitation. On August 9, under the control of subtropical high, the atmosphere over northern Shaanxi had high temperature and humidity, and the total amount of precipitable water was large. The water vapor transport was weak and the strong convergence maintenance time was short, the wet layer was thinner, but the energy was sufficient, the weather process was dominated by convective precipitation. On July 11, the water vapor net income mainly came from the ground to 500 hPa, of which accounted for 52% during 800-500 hPa. The water vapor income of the eastern boundary below 800 hPa increased rapidly during the precipitation intensification stage and the combination of increased zonal income and strong meridional income keeps the regional net income at a high value, resulting in regional heavy rain. On August 9, almost all of the water vapor net income came from meridional income, and the water vapor net income mainly came from the ground to 800 hPa (accounting for 88%). The 700 hPa shear line moving southward and the convergence in the north of Yulin increased, reduced the outflow of the southerly wind, significantly increased the meridional income of water vapor, and strengthens the rainstorm. Water vapor backward trajectories with HYSPLIT model showed that on July 11, water vapor mainly came from tropical oceans, and the South China Sea contributed the most, the local and surrounding near surface atmosphere with high specific humidity also contributed significantly. On August 9, the water vapor mainly came from the high specific humidity atmosphere in the near-surface layer of the inland, followed by the South China Sea.
