Analysis of a server convective rainstorm in the weak background

Using conventional and unconventional meteorological observation dataes, RMAPS-NOW( RR4DVar cloud model), the severe convective rainstorm occurred over the Beijing-Tianjin-Hebei junction area on May 17, 2019 was analyzed. Taking the observation of  Tongzhou 101 farm rainstorm (17: 30-20h precipitation 179.1mm) as an example, the results showed that this server convective rainstorm took place under a weak background, (1) Boundary conditions : The Beijing-Tianjin-Hebei area had high temperature and high humidity. The θse energy front was located in the middle of Beijing-Tianjin-Hebei. The CAPE at Beijing Observatory reached 1113 J · Kg-1 at 08h, and correction value reached 2669 J · Kg-1 at 14h. Convection cloud streets appeared around 12 o'clock in the visible image of the Himawari-8 satellite; the southeast wind jet in the boundary layer provided sufficient water  for convection development; at 20 o'clock, sounding showed that the vertical wind shear of 0-6 km increased to 17.5 m · s-1. (2) Trigger conditions: The southeast wind at the rear of the offshore High merged with sea breeze and pushed inland, formed a local convergence line with the local southerly wind in the central part of Beijing, Tianjin, and Hebei, and convection occurred at the convergence line and the θse energy front. (3) Tongzhou heavy rain was caused by two convective cells. Cell 1 was generated at the convergence line and the θse front. It developed into a server storm within 1 hour, and the composite reflectivity was > 60dBz. Subsequently, at its downstream (northwest side, the leading airflow is the southeast airflow), Cell 2 developed rapidly, and the two Celles revolved, moved over Tongzhou successively, accompanied by heavy rainfall ,hail and strong winds. (4) RMAPS-NOW data can describe the refined process of cells formation and evolution, that is, the θse field in the Beijing-Tianjin-Hebei region is extremely uneven, even in the θse front area. In the boundary layer convergence line and θse high-energy region, the convective bubble stimulated the formation of cell 1, the airflow spined up, and the development of the convective cell was strengthened. Half an hour later, a single cell was gradually separated into two cells (cell 1 and cell 2) in the upper layer, and the updraft gradually separated from the center into two rotating oblique updrafts. Seen from the echo profile, the two cells were connected by a cloud bridge and rotated clockwise. The convergence line in the boundary connected the two cells and rotated organically.