Potential vorticity diagnosis of a mesoscale vortex development and associated vertical motions during the 20— 22 July 2021 torrential rainfall in northern Henan

A persistent torrential rain process occurred in Henan province from 17 to 23 July 2021 (hereafter referred to as Henan "21.7" torrential rain). During 20—22 July, the center of the Henan "21.7" torrential rain moved to northern Henan, causing severe floods in Xinxiang, Hebi and other places on the east side of the Taihang mountains. Based on rainfall observations collected at automatic weather stations and ERA5 atmospheric reanalysis product, the thermodynamic mechanism for the mesoscale vortex and vertical motion development during the torrential rain event in northern Henan is diagnosed by using the Potential Vorticity (PV) tendency equation and the vertical motion decomposition method. The results show that the atmospheric diabatic heating caused by latent heat release during the precipitation process increased with height in the middle and lower troposphere, leading to the genesis of local PV and the development of mesoscale vortex. On the other hand, an intensified mesoscale vortex caused the isentropic surfaces to become more concave with more inclined surface, which enhanced the convergence and uplift of large-scale warm and moist southeasterlies in northern Henan. In particular, the isentropic-gliding ascending motion and the ascending motion induced by condensational latent heat release intensified, leading to further increases in precipitation. Therefore, the positive feedback loop formed between the mesoscale vortex and vertical motion and heavy precipitation development, resulting in the persistent torrential rain process in northern Henan.