Intensive Observational Study on Evolution of Formation Location of Thunderstorms in Beijing under Complex Topographical Conditions

A group of thunderstorms locally formed heavy rain in Beijing on August 14, 2008. The locations of the thunderstorms initiated were complex and variable, forming multiple γ-mesoscale heavy precipitation centers. Using high spatial and temporal resolution data obtained from the ground automatic weather stations, Doppler radar, wind profiler, and ground-based microwave radiometer, combined with the analysis data retrieved by the four dimensional variational Doppler radar analysis system (VDRAS), the characteristics of the ground (boundary layer) wind and temperature evolution are analyzed, and the mechanism of the location change of the newly formed thunderstorms are discussed in this paper. The following results are given: (1) The cold pool outflow of pre-existing thunderstorms combined with a complex terrain played a leading role in determining the location of newly formed thunderstorm, thereby affecting the location of γ-mesoscale heavy precipitation center. (2) The inconsistency between the synoptic scale vortex at upper levels and trough at lower levels, the slow movement of the synoptic scale systems, and weak environmental vertical wind shear in the lower troposphere combined to form an important precondition. (3) The complex terrain caused the cold air flow within a certain range, and the collision and convergence of these cold air in the boundary layer triggered and enhanced convection; thus, a close relationship is shown between the shapes of the convective storms and terrain. (4) A certain intensity of the cold pool flow, pre-existing warm and humid air in the boundary layer, and accumulation of convective instability energy were essential conditions for the cold pool outflow to trigger the new thunderstorm. (5) Finally, the cold pool outflows of thunderstorms around Beijing flowed along the valley terrain or to the plain of city, then formed a boundary convergent uplift mechanism with the warm and moist air, which subsequently triggered the thunderstorms.