Analysis on causes of a warm-sector torrential rain event in the Beijing-Tianjin-Hebei region

Based on conventional meteorological observations, observational data from the regional automatic weather stations, NCEP reanalysis data with spatial resolution of 1°×1° and temporal resolution of 6 h, satellite images, Doppler weather radar and wind profiler data, as well as the retrieval results from BeijingVariational Doppler Radar Analysis System (VDRAS), we have conducted analysis of the precipitation characteristics, atmospheric circulation, mesoscale system evolution features and the causes of a regional warm-sector torrential rain event in the Beijing-Tianjin-Hebei region on 12 August 2020. The results show that the event occurs in the warm air mass below 500 hPa at the edge of the subtropical high, and the main influencing systems are the low-level jet and the warm shear line at the 850 hPa. The whole severe precipitation event presented clearly the phased characteristics; it can be divided into two stages corresponding to the two different rainstorm areas, i.e., the southern Hebei and the north part of the Beijing-Tianjin-Hebei region with different formation mechanisms. The severe precipitation in the first stage is caused by a meso-α-scale convective system (MαCS), which is triggered by the surface convergence line. Before the precipitation starts, the local water vapor and unstable energy is rich. When the surface convergence line triggers the release of unstable energy, a strong linear echo is formed. The severe precipitation is mainly caused by the continuous development of convective cells in the spiral echo corresponding to the surface cyclonic circulation. The severe precipitation in the second stage is generated by several β-scale cloud clusters, and the southerly low-level jet strengthened northward provides sufficient water vapor and better dynamic conditions for the occurrence of torrential rain. The low-level jet in the boundary layer and the mesoscale low vortex system are the crucial influence systems at the second stage of this event. The hourly extremely severe precipitation of 125.9 mm at a station in Xiongan New Area is caused jointly by the warm cloud precipitation at the tail of comma-shaped radar echo and the "train effect".