The Low-level Jet (LLJ) in the Yangtze River Basin during the Meiyu season is analyzed and studied mostly as the atmospheric circulation background of precipitation, which cannot adequately reflect the characteristics of the jet itself. In this paper a fusion of sounding observations and precipitation data from Wuhan Station during the Meiyu season in 2010 are used to analyze the characteristics of the LLJ in the middle reaches of the Yangtze River. The results show that: the vertical structure of the LLJ is characterized by the predominance of a Boundary-layer Jet (BLJ) with an occurrence height concentrated in the 300-1200 m. The BLJ occurs most frequently at 22:00 at night, but most strongly at 01:00 at night, with resultant wind velocities exceeding 14 m/s. A Synoptic-system-related Low-Level Jet (SLLJ) occurs most frequently at 07:00 during the day, but most strongly at 10:00, with resultant wind velocities exceeding 12 m/s. For both the BLJ and SLLJ, the wind direction is characterized by southwesterly winds. However, the wind direction of the SLLJ is more westerly relative to the BLJ, and the northeasterly direction of the SLLJ occurs significantly more frequently. The analysis of four typical cases of heavy precipitation in the middle reaches of the Yangtze River shows that before the onset of heavy precipitation, a LLJ exists in the precipitation center and at its south side. The SLLJ is dominated by southwesterly winds, and the BLJ has more southerly wind component with the BLJ developing earlier than the SLLJ.

Multi-scale Eastward-Moving Southwest Vortex (EMSV) inducing severe rainstorms frequently occurs in the middle and lower reaches of Yangtze River Basin (YRB). The Second-Step Terrain Region (SSTR) located in the middle reaches of YRB have significant role in strengthening this synoptic system. This paper systematically studies the topographical effect of SSTR based on the WRF synthetic simulation of three multi-scale EMSV cases that occurred in 2015 and 2016. Results show that the compound circulation simulated by WRF can be decomposed into the meso-scale balanced circulation and the local-scale perturbed circulation with the application of the Piecewise Potential Vortex Inversion (PPVI) technique. The cyclonic perturbed circulation has a closer relationship with the occurrence of local heavy precipitation compared to the balanced circulation. Moreover, the good agreement between the positive Potential Vortex (PV) anomalies and the cyclonic perturbed circulation suggests that the persistence of the cyclonic perturbed circulation highly depends on the positive PV anomalies. Besides, the qualitative sensitivity experiments reveal that the topographical effect stimulates the genesis of the positive PV anomalies mainly by strengthening the latent heat release associated with the updraft, and the latent heat release associated with the cyclonic eddy. The quantitative diagnosis of the source of the PV anomalies shows that the former one contributes more to the genesis of the positive PV anomalies than the latter one. Further quantitative diagnosis of the updraft reveals that the topographical lifting effect is identified as the main mechanism in strengthening the updraft within the topography region