Analysis of the evolution mechanism of convective organisation pattern during an extreme heavy precipitation process in the Xiangjiang River valley

In order to deepen the understanding of mesoscale convective systems under the special topography of Hunan Province, this paper uses high temporal and spatial 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. The results indicate that this extreme heavy precipitation event occurred in the convergence zone between the northerly dry and cold airflow and the southwesterly warm and moist jet at 700 hPa. In the initial stage, clustered convection was sporadically triggered and gradually organized into a banded mesoscale convection system. During the development of the banded mesoscale convection system, the positive vorticity formed by the environmental vertical wind shear and the negative vorticity generated by the cold pool gradually approached balance, coupled with the enhancement of the southwest jet that transported a large amount of moisture, the banded mesoscale convection system developed rapidly, with extreme precipitation reaching 103 mm in one hour. In the maintenance stage, the compensatory downdraft of the upflow within the banded mesocale convection system suppressed convection generation in the middle of the system. Additionally, the downdraft enhanced the evaporation of raindrops in the lower and middle levels and transferred horizontal westward momentum to the near-surface, intensifying convection in the eastern part of the banded mesocale convection system and resulting in the splitting of the convection system into a clustered mesoscale convective system. As the convection moved south, the low-level southwesterly was blocked by Mingyang and Xuefeng mountains, making the new convection mostly triggered on the west side of the Xiangjiang River valley, resulting in the stronger distribution of total precipitation.