华南局地锋生及对流系统发展的模拟分析研究

2009年3月28日一条东西向的锋面出现在华南25°N附近。伴随着锋面的活动,对流降水回波午后开始在广西的梧州附近发展,并在随后几小时向东移动,组织发展成为中尺度对流系统(MCS),为广东中西部及珠三角地区带来了雷暴、暴雨、冰雹等灾害天气。应用地面自动站、雷达回波、卫星云图以及NCEP-FNL再分析资料和WRF模式的模拟结果,对锋面暴雨形成的天气特征进行了诊断分析,考察了中尺度对流系统的发展演变过程及其与局地锋生的相互关系。结果表明,锋面对流系统形成发展于一个东西向水平尺度约200km的地面中尺度辐合线附近,对流起始发展于具有较大对流有效位能(CAPE)和较小对流抑制位能(CIN)的区域。伴随着对流的发展,锋面强度增强。锋生函数的计算发现,非绝热项和倾斜项分别在由对流引起的次级环流的上升运动支和下沉运动支起锋生作用,是引发中尺度锋生的主要影响因子。而相对来说,水平辐合项和形变项的作用却比较小。这与大尺度的锋生过程不同,中尺度锋生更主要的是由热力直接触发的非地转环流所驱动。涡度场的演变分析还发现,沿着850hPa锋区大的正涡度区与500hPa的强上升运动区对应良好,对流系统发展与中尺度锋生之间存在着类似于第二条件不稳定机制的相互作用,对流增强了锋生过程,锋面则对中尺度对流系统的发展起组织作用。中尺度锋生对对流组织发展的作用作为此类灾害天气形成的原因值得关注。

Analysis and simulative study of the local frontogenesis and convection development over South China

On 28 March 2009, a west to east oriented cold front appeared in the southern part of China around 25°N. With the development of the cold front, convective precipitation echoes were firstly detected in the early afternoon near Wuzhou, Guangxi province, then it moved eastward and organized into an MCS, and brought thunderstorm, heavy rain, and hail to most central and western parts of Guangdong province and the Pearl River Delta in the late afternoon. The analyses of the AWS (Automatic Weather Station) data, the radar echo data, the satellite images, the NCEP-FNL reanalysis data, and the successful simulation results from the mesoscale research model (WRF) are used to examine the development of the convection and its relation to the frontogenesis. The results show that the frontal convection initiated along a mesoscale surface convergence line with almost 200 km in length, and the convective storm was likely triggered in the regions with large CAPE and small CIN values. With the development of the convection, the front intensifies. Frontogenesis calculations showed that both the diabatic process and the tilting terms were important in the lower troposphere frontogenesis, but the effects of the deformation and horizontal convergence processes were small. The diabatic process mainly caused frontogenesis in the ascending branch of a secondary circulation driven by the frontal convection, while the tilting term contributed to frontogenesis mostly in the descending branch, which are different from those frontogenetic processes driven by large scale forcing, and suggest that mesoscale frontogenesis is primarily driven by the thermally direct ageostrophic circulation. Analyses of relative vorticity and vertical motion along the frontal zone found that there was a good correspondence between the 850 hPa positive vorticity maxima and the upward motion at 500 hPa, demonstrating that the CISK (Conditional Instability of the Second Kind) like driving mechanism is responsible for the interaction between the convection and the mesoscale frontogenesis. The convection acted to enhance the frontogenetic process, while the front helped organize the convection into the MCS. As a main cause for this kind of severe weather, that the mesoscale frontogenesis acts to organize convection should be concerned.