Typhoon “Rumbia” was the most disastrous tropical cyclone, triggering rare floods in Shandong. Based on the conventional meteorological observation data, the reanalysis data from the National Centers for Environmental Prediction, and the precipitation data from automatic weather stations, the frontogenesis mechanism of an extreme rainstorm in Shandong Province caused by Typhoon“Rumbia” from 17 to 20 August 2018 was studied in this paper. The results indicate that the precipitation affected by Typhoon “Rumbia”can be divided into three stages: the precipitation of the typhoon outer cloud system, the precipitation of the interaction of the middle and low latitude weather systems and the precipitation triggered by the typhoon trough. The frontogenesis area of the typhoon rainstorm is mainly in the lower level, and the location of the frontogenesis area is closely related to the location of the cold air. The locations of the heavy precipitation are consistent with the frontogenesis area. The large-value center of frontogenesis intensity corresponds well to the center of the heavy rainstorm, and the intensity of frontogenesis can well indicate the rainfall in the next 6 hours. Favorable convergence flow field on the south side of the dense area of pseudo-equivalent potential temperature (θ_se) lines was the key to cause frontogenesis. The location of the elongation deformation frontogenesis is consistent with the convergence center of the divergence, the large value center of θ_se and the total frontogenesis area in this rainstorm process is consistent. The elongation deformation term, shear deformation term and divergence term all contribute positively to the total frontogenesis. The typhoon rainstorm is caused by frontogenerative dynamics, and the area with the strongest ascending motion of the frontal secondary circulation corresponds to the area of the strongest rainstorm. Under the conditions of strong water vapor transport, convergence and strong convective instability, the convergence of typhoon trough and strong frontogenic secondary circulation together produce strong upward movement, and the dynamic uplift effect is rapidly enhanced, resulting in strong convergence of water vapor and transport to the upper level and causing extremely heavy rain in Shandong Province. The rainstorm area is located at the 700 hPa positive helicity center and its right side, and the period of rapid enhancement of positive helicity corresponds to the period of heavy precipitation, and the maximum value center of positive helicity moves down to the vicinity of 900 hPa, which indicates the weakening of typhoon heavy precipitation.

Based on the data from three wind profile radars, the rain and snow weather process in the central part of Shandong Province from February 12 to 13, 2016 was analyzed. The results are as follows: (1) This rain and snow weather was the return-flow precipitation mainly caused by the combination of low-altitude vortex and surface cyclone. Heavy precipitation occurred in the cold and north side of the cyclone center. The temperature on 925 hPa and near-surface  dropping below 0 ℃ was the important indicator of the conversion from rainfall to snowfall. (2) The change of wind field below 1 km was the signal of precipitation start. The change of strength of low-layer cold pad determined the transition of precipitation phase. The maximum detection height of the radar during snowfall was significantly lower than that during rainfall. The low-altitude wind-shear index would increase significantly at the beginning and end of precipitation. The small increase of the low-altitude wind-shear index during the continuous rainfall period corresponded to the increase of rainfall intensity. (3) The vertical velocity of the wind profile radar could not only reflect the change of precipitation intensity, but also reflect the change of precipitation phase. The vertical velocity near ground gradually decreased during phase transition. (4) Both structure constant of atmospheric reflective index and signal-to-noise ratio showed a decreasing trend with height, and the changes of them could reflect the change of precipitation intensity. They all had zero-level bright band characteristics, and the disappearance of bright band corresponded to the beginning of snowfall.