中文摘要：

利用三维数值云模式和雷达资料四维变分（4DVar）同化技术,通过对4部新一代多普勒天气雷达探测资料进行快速更新同化和云尺度模拟,初步分析了2009年8月1日发生在北京地区的一次短生命史、突发性增强风暴的低层动力和热力影响机制。此次风暴过程处于弱天气尺度背景和弱层结背景下,冷池和低层环境风场相互作用是造成山上对流风暴增强传播下山的关键机制,而风暴的短生命史和平原地区上空弱风垂直切变环境有关：在对流风暴产生的初期,由于平原地区局地热力、动力场分布的差异,在平原地区西部近地面形成冷池结构,而冷池的“障碍物”作用进而阻碍环境风场的传播。在此机制下,导致在冷池东南边缘附近形成辐合中心、较强的低层水平风垂直切变和全螺旋度大值中心,有助于风暴传播下山。在风暴临近山边阶段,平原地区原有冷池的“绕流”等机制仍然有助于形成有利于主体风暴传播下山增强的近地面辐合中心、强低层水平风垂直切变和全螺旋度大值中心等环境。此外,随着山上风暴降水产生若干冷池,由于风暴形成的阵风锋抬升作用以及新生冷池与老冷池的逐渐发展并相互靠近,使冷池之间暖空气不断抬升,在冷池之间低层形成较强的辐合中心、全螺旋度大值中心。并且,由于冷池边缘的热力场分布不均匀,同样在冷池边缘形成较大扰动气压和扰动温度,增大了垂直加速度,在冷池之间中高层形成上升气流区,这些机制使北部风暴重新增强和新生风暴产生的同时,最终也导致这些风暴互相靠近,合并组织成带状回波。风暴在平原传播阶段,带状回波产生的冷池进一步增强,并明显扩展。低层风场指示冷池出流（阵风锋）更加强烈且存在明显的“前冲”特征,显现出部分飑线系统特征。但是,由于此时平原地区处于弱风垂直切变环境,此时冷池强

英文摘要：

With a three-dimensional cloud-scale numerical model and the rapid update cycling 4DVar assimilation technique applied to the 4 new-generation radars, a preliminary thermo-dynamical mechanism analysis is made on a short-duration case which suddenly intensified during the period from the mountains to the plains. The analysis indicates that the precipitation episode is under the weak weather background and stratification in the low levels, the cooperation of the cool pool and environment wind fields is a trigger and strengthening mechanism for the storm during the period from the mountains to the plains, and the short duration of the storm is due to the weak low-level vertical wind shear. At the stage of the initiation of storm over mountains, cool pool generates due to the uneven distribution of thermo-dynamical field, and blocks wind propagating at the southern brim of cool pool. This mechanism results in relative high convergence, relatively large shear and helicity, which are conducive to storm propagating from the mountains to the plains. At the stage when storms reached at the foothill of the mountains, the original long-duration cool pool still provides relatively high convergence, large shear and helicity for storm propagating from the mountains to the plains, and new storms trigger some cool pools. Squeezes between new-born and original cool pools lead the northern weakening storm to re-intensifying and trigger new storms, and while these storms are drifting toward each other, it will eventually lead to organized line-echoes. At the stage of line-echoes spreading over the plains, the perturbed temperature field shows that the cold pool further intensifies and expands. The wind field shows that gust-front intensifies, becomes tilting forward, and begins to be away from the storm. The thermo-dynamical field of line-echoes shows some characters of squall line. But there is so weak wind shear in the path of propagating of the storms that it is not in equilibrium with cool pool with the result that the gust fro