A study on the characteristics and causes of a warm-sector heavy rain event during June 2022 in Shandong

By multiple observations from regional automatic weather stations, S-band Doppler weather radar, L-band wind profile radar, and ERA-5 reanalysis data, the characteristics and formation mechanism of a warm-sector heavy rain event during June 2022 in Shandong province are analyzed. Results are as follows: (1) This process occurred under the background of an unstable warm air mass with obvious enhancement of low-level jet, the westward and northward movement of subtropical high and no obvious cold air forcing, with weak baroclinicity and abundant water vapor transport. (2) The development and evolution of the rainstorm convective system can be divided into two stages: the first stage corresponds to the rapid growth of local convections (daytime of 26th) and the second presents back-building of band-like convections (the night of 26th). Before the precipitation began, the southwesterly wind under 2000 m gradually strengthened, forming a low-level jet, which had a good correspondence with the beginning time of the precipitation at the first stage. After 20 o"clock of 26th, the jet stream was significantly strengthened, and the level of the jet stream was also significantly deepened. The depressions in the outflow area of the jet stream , the cyclonic wind shear in the lower level, the differential vorticity advection, the warm advection and diabatic heating are conducive to the development of the upwelling movement. The convection instability stratification with high temperature, high humidity and high energy is reconstructed repeatedly due to the warm and wet transport of low level jet, which is an important reason for the development and strengthening of convection.. (3) The surface convergence line is the key factor of convective triggering and gradually organizing into a band-like convective system. Especially in the second stage, the similar direction of the ground convergence line, the low-level jet stream axis and the movement direction of the radar echo is a favorable condition for backward propagation of convection and then causes the train effect. (4) The large value area of IPI (Ingredient-based propagation index) corresponds well with the propogation of convective generation in the two stages, which can effectively indicate the potential of convective development.