A study on the warm-sector torrential rainfall during 15−16 July 2018 in Beijing area

The characteristics and formation mechanism of a warm-sector torrential rainfall event in Beijing area during 15—16 July 2018 are analyzed using observations from regional automatic weather stations, SA Doppler weather radar, L-band Wind Profile radar, the NCEP 0.25° reanalysis product and high-resolution (0.03°) terrain data. Results are as follows: (1) The torrential rain occurred within the warm air mass (high θ_se energy zone) on the edge of the sub-tropical high, while there was no significant cold air forcing and the baroclinicity was weak. Specific humidity was high and strong low-level water vapor convergence occurred below 850 hPa. (2) The meso-scale convection experienced three development and evolution stages during the torrential rain process. The first stage corresponded to convective band formation and local rainfall strengthening. The second was the mature and strengthening stage, when the "echo training" developed in the north and the cold pool and gust front in the south resulted in the intensification and moving of convection. In the third stage, the line-shaped convective system established. (3) Before the occurrence of the warm-sector torrential rain, southwesterly winds in the lower layer fluctuated and the low-level jet (LLJ) was established. The wind speed first increased at 2500—3500 m, and significant increase occurred at 2500 m 2 h later. 5 h later, the LLJ reached 700 hPa. The low-level pressure decreased in the outlet area of the LLJ, while cyclonic circulation and wind shear developed, which was conducive to ascending motion and triggered/strengthened convective activities. (4) The warm and moist air mass transport by the LLJ led to repetitive formation of low-level convective instability with high temperature, high humidity and high energy. This was an important reason for the generation and combination of convective cells, the formation of belt-shaped convection, and the rapid reconstruction of the line-shaped convection. (5) Convergence in the surface was an important factor that triggered convective cells, which gradually formed belt-shaped convection. The direction of the surface convergence line, the LLJ axis and the moving direction of echoes were almost the same, which was favorable for backward propagation of the thunderstorm and the formation of "echo training" along the convective belt. (6) The Taihang Mountain and Yan Mountain had important effects on convection triggering and heavy rainfall development. 77.4% of the stations with the heaviest rain reaching or exceeding 40 mm/h were located at altitudes of 200—600 m in the southwestern and northeastern mountainous areas. The easterly wind triggered convection on the windward slope in the west, while the southwesterly LLJ was more significant along the windward slope and the horn-shaped topography in the north, resulting in heavy rain.