An Investigation of the Characteristics and Mechanism of the High Precipitation Supercell in the Beijing “623” Severe Rainstorm

In this paper, an observational and diagnostic analysis of an extreme heavy precipitation (EHP) event in Beijing on 23 June, 2011 is performed. This analysis used radar-observed data and automated weather station observations with high time and space resolution, combined with NCEP 1°×1° reanalysis data and conventionally observed data. Results showed that the EHP event was attributable to the high precipitation supercell (HPS) at the right end of the squall line moving southeast, which occurred at the highest latitude recorded among all HPS events documented in China thus far. The HPS had distinctive V-shaped inflows in both in the right-front and right-rear flanks, which differed from those in the known HPS model. This indicated that the cold dry air flow at mid- and lower levels and the warm moist air flow at lower levels were significant. In terms of environmental conditions, there was an inversion layer (IL) in the lower levels of the troposphere, indicating that the thunderstorm would intensify explosively when the cap was broken. However, the IL formed within 6 hours from 0800 BT (Beijing time) to 1400 BT, which was difficult to forecast. Compared with other thermodynamic parameters, storm-relative helicity and bulk Richardson number sharply increased from 0800 BT to 1400 BT before the occurrence of the HPS, which had certain indicative effects. The westerly upper-level jet and low-level easterly wind were significant, which enhanced the vertical shear of horizontal wind over Beijing, facilitated the convective instability with the dry upper layer and moist lower layer, and formed the secondary circulation. Meanwhile, the upper-level jet caused obvious differential vertical advection of equivalent potential temperature, which maintained and strengthened the convective instability. Combined with the terrestrial effects, there existed both an obvious dry-moist boundary line and a wind convergence line near the 100 m topographic elevation line in western Beijing. The water vapor supply was mainly from the low-level easterly wind and local water vapor. When the squall line invaded Beijing from the northwest and moved to the southeast, in the northern mountainous areas, the thunderstorm did not develop significantly due to insufficient conditions. In contrast, in the western mountainous areas, supported by a lake, the urban heat island effect, low-level easterly winds, a cold pool outflow, and the other favorable environmental factors, the thunderstorm at the right end of the squall line strengthened significantly. Notably, it developed into the HPS when it reached the 100 m topographic elevation line, which, in turn, caused the rainstorm center at the Moshikou station of Shijingshan District.