Comparative Analysis of Unstable Conditions of a Two-day Continuous Rainstorm Process in Midsummer in Heilongjiang Province

Meng Yingying; Zhao Ling; Li Shuling; Wu Yan; Zhang Min

doi:10.16765/j.cnki.1673-7148.2025.01.003

Abstract：Based on the meteorological observations and ERA5 reanalysis data,the unstable condi-tions and convective triggering mechanism of a continuous torrential rain process from July 25 to 26,2021 in western Heilongjiang Province are comparatively analyzed.The results show that the two-day torrential rain occurred in the situation of west-low and east-high circulation.The northwest Pacific subtropical high and high-level cold vortex were stable with little movement.The low-level southwesterly warm-moist air and the long-distance water vapor transport by the outer airflow of Typhoon"In-Fa"provided sufficient water vapor and thermodynamic conditions for the torrential rain event.The upward motion of atmosphere was stronger on the 25th,while the moisture conditions were better on the 26th.The strongest rainstorm cloud clusters in the two days were both generated at the superposition zone of convergence area in the left-front side of the low-level jet and the tongue area of specific humidity,and then developed vigorously in the superposition zone of the divergence front and the humidity front.The short-time severe rainfall of about 40 mm·h-1 appeared in the area of large Black Body Temperature(TBB)gradient behind the rainstorm cloud cluster.The precipitation on the 25th was mainly convective and unstable,while the 26th precipitation was conditionally symmetric and unstable.The main triggering mechanisms of the occurrence and development of the two-day torrential rain cloud clusters were the upward motion of the high-level cold vortex,the coupling effect of high-level and low-level circulations,the wind field convergence of low pressure trough on the ground,the forced uplift effect from the windward slope of the Lesser Khingan Mountains,and the meso-scale convergence in the left-front side of the low-level jet with the effects of the humidity front and the divergence front.However,the characteristics of atmospheric instability and con-vective development processes over the two days were different.On the 25th,the strong upward motion caused the release of convective unstable energy and the development of cloud clusters was more vigorous than on the 26th.And then on the 26th,the warm-moist air climbed up along the dry and cold air in the north side,forming an oblique upward air flow and triggering conditional symmetric instability.