Mesoscale characteristics and triggering mechanism of a sudden heavy rainstorm in Shaanxi

A local sudden heavy rainstorm occured on 5-6 August 2020 in the plain area of the central and mid-eastern Shaanxi and the montainous area Luonan of Qinling Montains, causing 6 people death and 10 missing cases. Both global numerical model and Meso-scale numerical models had biases in predicting the location and intensity of this process. To deepen our understanding of such weather processes, based on conventional surface and upper-air observation data, ERA5 reanalysis data, and Doppler weather radar data, we analyze the circulation background, environmental conditions characteristics of the meso-scale convective systems (MCS) and triggering mechanisms of this heavy rainstorm process. The results are as follows. (1) The heavy rainstorm occurred under a weak synoptic-scale forcing, influenced by a sub-tropical high at 500 hPa, with no obvious synoptic scale weather system dominating at mid-lower levels, and no cold fronts on the ground. There was no strong water vapor transport at 700 hPa in the central and eastern Shaanxi, but there was a strong water vapor transport at the boundary layer, and the local water vapor content was abundant in the rainstorm area. The total atmospheric precipitable water reached 50~70 mm which deviated 1.5~2 \sigma from the climate average, while the convective available potential energy (CAPE) over 3 000~4 800 J·kg⁻¹ and the vertical wind shear between 0 and 6 km was weak, which provided favorable environmental conditions for the occurrence of the sudden heavy rainstorm. (2) The physical processes of the heavy rainstorms in the plain area and the montainous area Luonan of Qinling Montains were different. The thunderstorm in the plain was triggered respectively by the convergence line on the ground and Lishan mountain in the south of plain. The thunderstorm triggered by Lishan Montain propagated towards the plain, merging with the thunderstorm in the plain, and furthermore enhanced to be a meso-β-scale (M_βCS) convective system, which moved northeastwards along southwesterly on upper-level, led to a local sundden rainstorm in the plain. The strong thunderstorms were mainly generated and maintained at the northeast side of the M_βCS convective system, in which a M_βCS vortex maintaining nearly 1 h at lower layer directly induced an extreme severe precipitation of 103.7 mm∙h⁻¹ in a short time in the plain. The favorable configuration of cold pool driving and direction of low-level vertical wind shear relative to the thunderstorm outflow boundary is the reason for the strengthening and maintenance of thunderstorms in the plain. (3) The thunderstorms adjacent to Luonan of Qinling Montains were triggered by the southeasterly wind at the topographic uplift boundary layer, and key factor was the southeasterly wind on the boundary layer maintaining stable from afternoon to night. The thunderstorm cells propagating backwards formed a train effect under the influence of southwestward advection passing over the north of Luonan, thus caused heavy rainstorm.