Abstract:

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.

Key words: rainstorm in northern Shaanxi, water vapor transport, water vapor budget, water vapor trajectory, comparative analysis

摘要：

为深入认识西北半干旱区暴雨的水汽特征及来源，提高该地区暴雨预报能力，利用高空及地面观测资料、欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts， ECMWF）第5代全球大气再分析产品——ERA5（0.25°×0.25°）对2022年7月11日、8月9日陕北两次不同环流背景下、不同强度大范围暴雨过程的水汽输送及收支特征进行分析，并利用HYSPLIT （Hybrid Single-Particle Lagrangian Integrated Trajectory）模型，定量分析水汽来源及贡献率。结果显示：高空槽、低层切变线及低涡、低空急流是7月11日暴雨过程的主要影响系统，700 hPa气旋式辐合、850 hPa低涡加强并缓慢移动造成区域性暴雨；短波槽、低层切变线是8月9日暴雨过程主要影响系统，切变线两侧次级环流抬升西太平洋副热带高压（简称“西太副高”）外围暖湿气流，触发不稳定能量释放，形成大范围对流性暴雨天气。7月11日地面至300 hPa水汽输送更强，700 hPa西南急流和850 hPa东南急流形成两支明显的水汽输送带，强辐合维持时间更长，湿层深厚，以稳定性降水为主；8月9日受副热带高压控制，陕北高温、高湿，整层可降水量大，水汽输送较弱，强辐合维持时间短，湿层较薄，但能量充足，以对流性降水为主。7月11日水汽净收入主要来自地面至500 hPa，其中800~500 hPa占比52%，降水加强阶段800 hPa以下东边界的收入迅速增加，纬向收入增加和强的经向收入共同作用使区域净收入维持高值，产生区域性暴雨；8月9日净收入几乎全部来自经向收入，水汽净收入主要来自地面至800 hPa（占比88%），700 hPa切变线南压，榆林北部辐合增强，南风出流减少，水汽经向收入明显增多，暴雨加强。HYSPLIT模型水汽输送轨迹显示7月11日水汽主要源自热带海洋，其中来自南海的水汽贡献率最大，本地及周边近地层高比湿大气也有重要贡献；8月9日水汽主要源自内陆近地层高比湿大气，其次为源自南海的水汽。

关键词: 陕北暴雨, 水汽输送, 水汽收支, 水汽轨迹, 对比分析