The low-level meso-γ-scale vortices during the extreme rainfall in Shenyang on 16 August 2019: Formatiom, merging, and rain-producing mechanisms

The urban area of Shenyang was affected by an intense rainfall system with low-level meso-γ-scale vortices on 16 August 2019. This system caused record-breaking hourly precipitation (102 mm) since the observations started in 1951. In order to improve the analysis and forecasting ability of heavy rain caused by such vortices, multi-source observations and ERA5 reanalysis data are comprehensively used to analyze the characteristics of the low-level meso-γ-scale vortices in this process, the environment for their generation and their roles in the formation of the torrential rain. The results show that during this process, Shenyang was located in the front of the Northeast Cold Vortex at 500 hPa and in the water vapor conveyor belt on the west side of the residual vortex of typhoon "Likima" at low levels. Moreover, a low-level nearly dry adiabatic lapse rate prevailed in the urban area of Shenyang on the afternoon of 16 August with lower lifted condensation level and increasing vertical wind shear. At 16:00 BT (Beijing Time), the meso-γ-scale convergent wind field in the urban area of Shenyang triggered the local storm, and the cold vortex storm then entered Shenyang. The storm group merged in the area with anticyclonic rotation of the local storm. The merged storm strengthened precipitation, and a meso-γ-scale vortex pair with a lifetime of 30 min appeared in lower levels, followed by a rare phenomenon of the strengthening of the converging anticyclonic vortex. Compared with the statistical characteristics of mesocyclones in China, this low-level shallow vortex demonstrated a short life span with small scale, slow moving speed and strong vertical vorticity. All the automatic weather stations with precipitation exceeding 10 mm in 5 min after the vortex were located in the area between the vortex pair, and the record-breaking intense hourly precipitation occurred since observations started in 1951. The strength and extent of the heavy precipitation can be characterized by the strength of the vortex's rotation, the height of its extension, and the distance between the two vortices. The occurrence of extreme precipitation events requires strong rainfall intensity and long duration time. The merged storm in this process had the characteristics of warm clouds and low centroid echo in radar observations. The early local storm precipitation also reduced the difference between ground temperature and dew point, which ensured high precipitation efficiency. Furthermore, the updraft near the ground generated by low-level vortex promoted the growth and collision of raindrops, thereby enhanced the rain intensity. The strong rotation of the vortex caused an updraft near the ground, which was conducive to the re-development of storms and prolonged the precipitation duration time. The reason for the emergence and strengthening of the low-level anticyclonic vortex is likely attributed to the following factors. The low-level quasi-linear outflow boundary of the cold vortex storm formed a horizontal vortex tube from north to south. Under the downward twisting action of the downdraft in the initial precipitation, meso-γ-scale vortex pair appeared near the ground, and since the environmental wind shear vectors rotated counterclockwise with altitude, the clockwise rotation was more conducive to the strengthening of anticyclonic storms. This is consistent with the theoretical research. In addition, this anticyclone was closer to the strong updraft area in the merged storm, and it could also be reinforced under its stretching. Finally, the formation mechanism of this low-level meso-γ-scale vortices and the rainstorm model are summarized, which provide a reference for future weather analysis and forecasting research.