Initiating and intensifying mechanism of a local thunderstorm over complex terrain of Beijing

Using data obtained from the four-dimensional Variational Doppler Radar Analysis System (VDRAS), Doppler radar, surface intensive automatic weather stations, wind profile and 4 times daily sounding, the initiating and intensifying mechanism of a strong local thunderstorm on 9 August 2011 in Beijing area was analyzed. The results show that before the thunderstorm initiated, weak vertical wind shear, a large amount of unstable energy (CAPE to 2798 J·kg^-1) on atmosphere environmental field, high specific humidity (＞20 g·kg^-1) air accumulating at the foot of mountains and a warm meso-γ scale convergence centre over the western city proper of Beijing were favorable to the formation and development of the convection. Cold pool outflow was uplifted because of topographic forcing effect of thunderstorms moved eastwards from the upper reaches of Beijing, thus cold advection superposed on the warm meso-γ scale convergence centre in ground layer made the local atmospheric stratification more unstable; both northerly wind in boundary layer generated by strong cold pool outflow and weak southerly wind in ground layer formed vertical wind shear favorable for convection geneses; the interaction of the surface warm convergence zone and the increasing hermodynamic and dynamic instability in boundary layer was main mechanisms. Upstream cold pool outflow boundary (gust front) with strong temperature gradient and upward motion of convergence in boundary layer were the main reasons which led to the significant strength of the original genesis local thunderstorm, while upward motion of convergence generated by collision and merging of multicell thunderstorms was a key factor of the local thunderstorm continuation.