A Numerical Study of the Beijing Extreme Rainfall of 21 July 2012 and the Impact of Topography

【摘要】：The extreme rainfall on 21 July 2012 is the heaviest rainfall that has occurred in Beijing since 1961. Observations and WRF(Weather Research and Forecasting Model) model are used to study the effect of MCS(mesoscale convective system) and topography on the event. In this high-impact event, a quasi-stationary MCS developed in a favorable moist environment that including southwesterly monsoonal flow and northeast flow of subtropical high(SH) as well as remote effect by typhoon Vicente. The numerical simulation successful reproduced the amount, location and time evolution of the rainfall despite 4–6-h delay. Especially, the model reproduced the repeat passage of convective cells at the leading convergence line region along Taihang Mountain and the trailing stratiform region, producing the rainfall at nearly the right position. Results indicate the important roles of meso-low and low-level jet in maintaining conditional instability, lifting of moist air to trigger deep convection, and the repeated initiation and movement of the line shaped convective cells to produce the rainfall. The sensitive experiment was then further carried out to examine the effect of topography to this heavy rainfall. The reduction in the model elevation field significantly influenced above meso-scale systems, which lead to the convective cells becoming less organized. In this simulation the results showed that the peak rainfall amount in Beijing was decreased by roughly 50%.