On the evolution of mesoscale environment and influential factors of the
heavy rainfall in Beijing on 23 June 2011

Based on 14:00 BT sounding data, conventional observation data, and high resolution data, the spatial and temporal evolution of the mesoscale environment 2-12 hours prior to the event are analyzed for the storm produced heavy rainfall in Beijing on June 23, 2011 using ingredients based forecasting methodology. The associated physical processes are also discussed. The results are as follows: 1) Influenced by low level moisture advection (low level southeasterly wind continuously transports moisture) and thermal advection (warm and cold advections attribute to high lapse rate and inversion), moisture is accumulated at low levels; 2) Because of the inhomogeneous distribution of thermal parameters, the most unstable and moist regions are confined to the foothill and urban area of Beijing; 3) The convection initiated at the northwestern part of Hebei Province strengthens when it moves toward southeast. Cold mesoscale high and strong outflow are then formed. When the strong outflow moves down from the mountains at about 16:00 BT, it converges with the moist southerly and forms a strong uplift. The strongest convergence is formed in the most unstable and moist region. Meanwhile, vertical wind shear in the Beijing area strengthens in the
afternoon. The convective system strengthens when it moves down from the mountains causing the heavy rainfall event; 4) Since the Beijing sounding is influenced by the advections from northwestern part of Hebei Province, the morning sounding in Zhangjiakou is a good indictor for the variation of Beijing sounding in the afternoon.