ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Analysis of the organization of extreme severe rainfall MCS under a low-level frontogenesis background in western Hubei Province
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Abstract:
An extreme torrential rain event occurred in Yichang on 22 April 2018, which was mainly caused by highly organized severe rainfall mesoscale convection system (MCS).Based on conventional observations,the data from regional automatic weather stations, radar data and ERA5 reanalysis data,we have performed the organizational characteristics and formation mechanism of extreme severe rainfall MCS.Results are as follows.(1)The extreme rainstorm event occurred under the background of weak forcing at high level and weak forcing turning into strong forcing at low level which was accompanied by strong frontogenesis.High temperature, high humidity and extremely unstable atmospheric environment were conducive to the occurrence of heavy rainfall.(2)That, the warm and wet easterly air at low level was forced to lift by the ‘C’ terrain in the east of Yichang, triggered the severe rainfall echoes in mountainous plain transition area.The warm and wet southeast or easterly wind at top of the boundary layer reverted warm trough triggered dispersed flocculent convection in the plain area from Yidu to Gongan.(3)The organization of extreme severe rainfall MCS had experienced merging stage and vortex stage.Under the influence of weak synoptic-scale forcing, MCS in mountainous plain transition area spread to southeast along the low-lying terrain, and merged along three paths with the warm area flocculent echoes from the plain to north.The third one was the continuous merger of the east-west MCS formed by the low-level east-west frontal zone and the ground convergence line,which moved toward west under the guidance of the middle and low level easterly jet, and the south-north MCS in mountainous plain transition area, which resulted in the strongest precipitation stage.The mesoscale cyclonic circulation, composed of MCS cold outflow and ambient airflow, and the latent heat of heavy rainfall heating the middle atmosphere were conducive to the organization, development and strengthening of vortex MCS. The water vapor energy transport of the low-level warm and wet easterly jet stream was conducive to the long-term maintenance of the vortex MCS. (4)The extreme heavy precipitation mainly occurred in the merging stage and the long-duration vortex stage of MCS. Synoptic-scale system forcing, low-level strong frontogenesis, mesoscale topography and positive feedback of mesoscale weather system were the important causes of the formation of this extreme severe rainfall.
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