Abstract:Affected by the westerly trough, subtropical high pressure and Typhoon Lichma, a localized heavy rainstorm occurred in Gaotang County on the West Shandong Plain from 20:00 BT 9 to 08:00 BT 10 August 2019. Based on FY-4A satellite cloud images, ECMWF numerical forecasts, dual-polarization Doppler radar data, ADTD lightning system positioning data, regional station minute rainfall, raindrop size distribution as well as the Beijing 3 km regional model products, this paper analyzes the causes for the localized heavy rainstorm in Gaotang and the microphysical characteristics of the severe precipitation. The conclusions are as follows. Before the localized heavy rainstorm, obvious cracks appeared in the westerly trough cloud belt at the bottom of which cold airs diffused to the front of the trough, forming a meso-α scale low-altitude shear line with the warm and humid airflow in front of the trough. In the early stage of the localized heavy rainstorm, due to the superposition of the downdraft outflow of strong shear line convection and the cold air diffusing from the bottom layer, the outflow boundary echo zone quickly moved away from the parent body so that the strong shear line echo zone was placed on the bottom cold air cushion and weakened quickly. The convection triggered by the outflow boundary and the quasi-linear pair of echoes formed by the warm and humid air advancing to the northwest were all dominated by convective precipitation. The precipitation intensity was high, the duration was short and the accumulated rainfall was small, but due to the different properties of the bottom layer, there were obvious differences in their raindrop size distribution. The low-altitude shear line was maintained for a long time, causing warm and humid air to accumulate continuously and forming a large value area of θse on the warm side of the shear line. At the beginning of the severe precipitation, the vertical upward movement on the shear line was significantly enhanced, and the maximum upward speed at 925 hPa was higher than 1.5 Pa·s-1, and two meso-β scale cyclone disturbances were formed, triggering the release of environmental instability energy and the deep moist convection. Cloud images and echoes show that the severe precipitation cloud clusters moved through the rainstorm area successively along the shear line, producing the “train effect” and thus resulting in the severe precipitation near Gaotang. After the westerly trough and shear line echoes combined, relatively abundant supercooled water kept maintaining at 6-10 km above Gaotang, promoting the growths of ice crystals and precipitation particles and making the precipitation intensity stronger, the peak of heavy precipitation widened, and the ground precipitation increased significantly. During the localized heavy rainstorm, there was an obvious double-peak structure in the scale spectrum of ground raindrops. The peak position with raindrop diameter at 1.2 mm was stable relatively while the other peak was at the end of the small raindrops with diameters at 0.3-0.5 mm. Statistical analysis suggests that the time series of large raindrops is synchronized with the time series of minute precipitation, and the correlation coefficient reaches 0.9867. The time series of small raindrops lags behind the time series of minute precipitation by 2 minutes.