中文摘要：

2005年6月20-21日，一个活动在锋前暖区的长生命史的中尺度对流系统（Mesoscale Convective System，简称MCS）给广东地区带来一次大范围的暴雨降水过程。天气学分析表明，从北部湾指向广西东部的低空急流核前方的水汽通量辐合对MCS初始的发展加强有重要作用。作者利用MM5模式成功地模拟出与此MCS相联系的天气尺度背景场和中尺度的降雨分布，利用高分辨的模式输出结果分析了成熟阶段MCS的β中尺度系统的结构和演变特征，结果表明：（1）在MCS的发展加强过程中，其内的β中尺度对流中心经历了此消彼长的更替过程，伴随着口中尺度对流中心的发展，与之相联系的正涡度中心由对流层的下部向中上部伸展，形成高达300hPa附近的下宽上窄的强涡柱；在成熟阶段，沿系统移动方向上的对流层中部的涡度场呈正负相间，宽约50km，且与对流带近于平行的带状分布。（2）MCS的海平面气压场呈跷跷板型的中尺度扰动，即由前置中低压和后置中高压组成，最强的对流带位于中低压和中高压之间的过渡区内。（3）成熟阶段MCS的β中尺度环流特征主要包括：强对流区内是一支近于垂直的深厚的对流尺度的上升流，紧接在其后方的对流层下部有一支对流尺度的下沉气流；系统前部有深厚的入流，其中较强的两支入流分别出现在900hPa和300hPa高度附近；层状降雨区内有中尺度的上升气流和下沉气流，二者的分界线在0℃层高度附近；对流层下（上）部强大的水平辐合（散）流中最大的水平辐合（散）约出现在900hPa（200hPa）。

英文摘要：

The Mesoscale Convective System （MCS） that produced the heavy rainfall during 20 - 21 Jun 2005 in Guangdong Province is examined. The observations reveal that the MCS initiated and developed in front of a cold front. The convergence of vapor flux in the ahead of the jet stream core at 925 hPa level appears to be a crucial factor for its development. A 24-hour numerical simulation of the developing and mature as well as decline stages of the MCS is performed using the fifth generation Penn State/NCAR mesoscale model （MMS）. The synoptic scale circulation evolutions and the mesoscale precipitation distributions associated with the MCS are reproduced successfully by the model. Using the higher-resolution output of the model, the meso-β-scale structure and evolution of the MCS at the mature stage are examined. The results show that： （1） with the development of the strong convective core, the associated positive relative vorticity core in the lower troposphere extends to the mid-upper troposphere, so as to form a deep and intense cyclonic air-column penetrating to the upper troposphere in the severe convective region of mature MCS. In the meantime, bands of positive and negative vertical vorticity, paralleled to the convective zone, are found in the middle troposphere. The maximum positive vorticity is near the severe convective zone, the negative vorticity near the rear of the convective zone and submaximum positive vorticity farther back. （2） On the surface map, the MCS is composed of a mesolow and a mesohigh which are located in the front of severe convective zone and behind the zone, respectively, i. e. , the severe convective zone occurred at the transition zone between the mesolow and the mesohigh. （3） The mesoscale circulation characteristics viewed in the coordinate system moving with the MCS include： a strong, nearly perpendicular convective scale updraft penetrates the troposphere of the severe convective zone; a convective scale downdraft within the lower troposphere is behind severe