Mechanism analysis for the evolution of convective organization pattern during an extremely heavy precipitation process in the Xiangjiang river valley

To deepen understanding of mesoscale convective systems under the special topography of Hunan province, this paper uses high spatiotemporal resolution data obtained from the Variational Doppler Radar Analysis System to study the extreme precipitation process that occurred in Hunan province on 29 May 2022. Results indicate that this extremely heavy precipitation event occurred in the convergence zone between dry, cold northerly airflow and warm, moist southwesterly jet at 700 hPa. In the initial stage, clustered convections were sporadically triggered and gradually organized into a banded mesoscale convection system. During the development of the banded mesoscale convection system, positive vorticity formed due to environmental vertical wind shear and negative vorticity generated by the cold pool gradually approached equilibrium, which, coupled with the enhancement of the southwesterly jet that transported a large amount of moisture, resulted in rapid development of the banded mesoscale convection system with extreme precipitation reaching 103 mm in one hour. In the maintenance stage, the compensatory downdraft for the updraft within the banded mesoscale convection system suppressed convection generation in the central part of the system. Additionally, the downdraft enhanced raindrops evaporation in the middle and lower levels and transferred horizontal westward momentum to the near-surface, intensifying convection in the eastern part of the banded mesoscale convection system and resulting in splitting of the convection system into a clustered mesoscale convective system. As the convection moved southward, the low-level southwesterly was blocked by Mingyang and Xuefeng mountains. As a result, new convections were mostly triggered on the west side of the Xiangjiang river valley, resulting in larger total precipitation there.