Characteristics of γ-mesoscale vortices during an extreme warm-sector rainstorm under weak synoptic forcing in Hunan

Mesoscale weather systems that cause extreme precipitation often include γ-mososcale vortices with horizontal scales of ≤20 km. Due to their small scale, they are difficult to forecast, their characteristics and formation mechanisms are currently one of the hot topics in mesoscale meteorology. In this study, we analyze the triggering, evolution and three-dimensional wind field characteristics of γ-mesoscale vortices associated with a warm-sector extreme rainstorm under weak synoptic forcing that occurred in Taojiang County, Yiyang City, Hunan Province on June 2, 2022. This analysis uses ERA5 reanalysis, Doppler radar observations, and surface regional station observations, with synoptic diagnostics, moist potential vorticity theory and multi-radar 3D wind retrieval technique. The results are as follows. (1) This case occurred under a weak synoptic forcing. The warm and moist conditions in the mid-lower troposphere were highly conducive to the development of deep and moist convection. The near-vertical distribution of low-level pseudo-equivalent potential temperature isolines, together with the positive baroclinic component of the moist potential vorticity, favored the generation of low-level vertical vorticity. (2) The low-level wind convergence during the merging of two linear convective systems provided the initial vorticity necessary for γ-mesoscale vortices formation. As the tilted updraft intensified, the γ-mesoscale vortices developed upward from the lower to upper levels. (3) Three γ-mesoscale vortices with rotation diameters≤10 km formed during this process, eventually merging into a β-mesoscale vortices with a rotation diameter >60 km. During the mature stage, the γ-mesoscale vortices exhibited rotational velocities of 6~10 m s⁻¹ at various heights, vorticity of 1.5~2·10⁻³ s⁻¹, and a vertical depth of at least 8 km. The feedback of condensation latent heat release caused by heavy rainfall, together with the interaction between the enhanced southwest airflow on the south side of the β-mesoscale vortex and the cold pool outflow from the vortex, was the main reason why the vortex could persist for 7 hours.