Mechanism Analysis of an Abrupt Torrential Rain Event under Weak Synoptic Forcing

Abstract

On July 18， 2021， abrupt rainstorms， torrential rain and local torrential rain occurred in areas of northeastern Xi'an and southern Weinan.The global numerical model and multiple Meso-scale numerical models failed to predict this torrential rain process.Based on data of aerological sounding， surface observation， ERA5 reanalysis， TBB data of FY-2G satellite， lidar wind vector data of Lintong station， radar data of Jinghe station， and microwave radiometer data， circulation patterns， environmental conditions and Meso-scale convective system evolution are investigated in this paper to enhance the understanding of mechanisms of this type of torrential rain and improve its predicting accuracy.Results show that the abrupt torrential rain process occurred under the forcing of a weak synoptic system， on the edge of the continental high at 500 hPa.There were no shear lines， warm advections and high-speed wind transporting water vapor at 700 hPa and 850 hPa， and neither cold front nor warm front on surface.Before the torrential rain process， on environmental fields， vertical wind shear was weak from 0 to 6 km on T-lnP chart， and 0 ℃-level was high， which was conducive to formation of deep warm clouds.A light rain appeared on surface， increasing atmospheric humidity near ground and lowering atmospheric lifting condensation level and lower cloud base， which was favorable for developing Meso-scale convective systems and improving precipitating efficiency.On radar wind profile map and wind lidar map， the low-level easterly wind was increased to 6~10 m·s-1， transferring abundant water vapor and energy to the rainstorm area， giving the atmosphere a certain amount of convective available potential energy（CAPE）.Satellite image， ERA5 reanalysis data and microwave radiometer data show a weak ascent was found in the middle and low levels which produced a deep wet layer and a large value of total atmospheric precipitable water.TBB data indicates that quasi-stationary isolated Meso-β-scale convective systems caused the abrupt torrential rain.On radar composite reflectivity factor map， the heavy rainfall was induced by a Meso-γ-scale and Meso-β-scale convective cell or a Meso-β-scale convective system.The Meso-γ-scale convective cell developed near Lishan Mountain with loose structure and organization.The maximum center of reflectivity factor demonstrates a consistently vertical structure from top to bottom with a low centroid， showing characteristics of highly efficient warm cloud precipitation.The abrupt torrential rain was caused by convections triggered by interaction of the ground convergence line induced by weak cold air on surface， topographic uplift of the Lishan Mountain and funneling effect of Bahe Valley collectively.The initial heavy rainfall formed a cold pool， then， the outflow of the cold pool generated new convergence lines around， triggering new convective systems.The Meso-β-scale convective system on the northeast side of the Jinghe River propagated backwards and moved slowly southwestwards.In the Meso-β-scale convective system， multiple new Meso-γ-scale convective cells developed and created a “train effect” over Jinghe River， resulting in this torrential rain process.For prediction under the forcing of weak synoptic systems， it is necessary to strengthen the understanding of its formation mechanism and the use of new observation data， pay close attention to the influence of early weak precipitation on near-surface humidification and lifting condensation level， water vapor and unstable energy transport to rainstorm areas by increased low-level wind， and the triggering effect of weak cold air on the ground and terrain， combined with maximum values predicted by a number of Meso-scale numerical models， comprehensively consider the possibility of abrupt torrential rain.

Keywords

• abrupt torrential rain
• the forcing of weak synoptic system
• meso-β-scale convective system
• low-level easterly