Abstract:Using hourly precipitation, radar, FY-4A satellite and other observational data, and ERA5 hourly reanalysis data, this paper analyzes the mesoscale characteristics, moisture and circulation anomalies of the large-scale regional torrential rain process in the Qinling and Daba Mountains on 23 April 2021. The results showed that the heavy rain occurred in a large area, and the precipitation at many stations broke through the historical extreme values. Meanwhile abrupt extreme torrential rain attacked the Daba Mountains. At 500 hPa, the abnormal “high in the east and low in the west” circulation situation over the Qinling and Daba Mountainous area increased the pressure gradient between Shaanxi and the western Qinghai-Tibet Plateau, causing the mid-level westerly airflow to have increased. At the same time, the superimposition of the mid- and low-level multi-scale weather systems provided the background dynamic condition for the torrential rain. The southeast airflow in front of the southwest vortex and the easterly wind on the north side formed a shear line that caused the convergence ascending motion of the environmental field while the other side transported the water vapor brought by the southwesterly airflow and the southerly airflow to the Qinling and Daba Mountains to converge. The circulation associated with the long-distance typhoon and the northeast cold vortex activity in the same period caused the abnormal water vapor transportation (30%) of the northeast path to be the unique source of water vapor for this torrential rain process. Compared with the same period in history, the specific humidity on the water vapor path of the south passage and the northeast passage was abnormally positive, indicating that there was abundant water vapor during the torrential rain period. The torrential rain process was mainly caused by the activity of a mesoscale convective system (MCS) in front of the southwest vortex. The topographical uplift of the windward slope of the Daba Mountains and the convergent ascending of the southerly airflow in the environmental field were superimposed. Sufficient water vapor at the lower level was transported vertically to the upper level by strong ascending motion, with strong convective activity, severe hourly rainfall and strong abruptness. However in the Qinling Mountains, the easterly wind converged and rose in front of the mountain, and the ascending movement of the environment field with southwestern airflow in the middle layer was weaker than that in the Daba Mountainous area during the severe precipitation period, and was dominated by stratiform precipitation. But the severe precipitation lasted for a long time, so the accumulated precipitation was a lot. In a word, multi-scale system interactions, abnormal water vapor transport combined with topographical influence under abnormal circulation conditions are the main reasons for the occurrence of this torrential rain process in the Qinling and Daba Mountains.