Revisiting the evolution of downhill thunderstorms over Beijing: A new perspective from a radar wind profiler mesonet

Abstract

Downhill thunderstorms frequently occur in Beijing during the rainy seasons, leading to substantial precipitation. The accurate intensity prediction of these events remains a challenge, partly attributed to insufficient observational studies that unveil the thermodynamic and dynamic structures along the vertical direction. This study provides a comprehensive methodology for identifying both enhanced and dissipated downhill thunderstorms. In addition, a radar wind profiler (RWP) mesonet has been built in Beijing to characterize the pre-storm environment downstream of the thunderstorms at the foothill. This involves deriving vertical distributions of high-resolution horizontal divergence and vertical motion from the horizontal wind profiles measured by the RWP mesonet. A case study of an enhanced downhill thunderstorm on 28 September 2018 is carried out for comparison with a dissipated downhill thunderstorm on 23 June 2018, supporting the notion that a deep convergence layer detected by the RWP mesonet, combined with the enhanced southerly flow, favors the intensification of thunderstorms. Statistical analyses based on radar reflectivity from April to September 2018-2021 have shown that a total of 63 thunderstorm events tend to be enhanced when entering the plain, accounting for about 66% of the total number of downhill thunderstorm events. A critical region for intensified thunderstorms lies on the downslope side of the mountains west to Beijing. The evolution of a downhill storm is associated with the dynamic conditions over the plain compared to its initial morphology. Strong westerly winds and divergence in the middle of troposphere exert a critical influence on the enhancement of convection, while low-level divergence may lead to dissipation. The findings underscore the significant role of an RWP mesonet in elucidating the evolution of a downhill storm.