一次强降水过程的中尺度对流系统模拟研究

1998年5月23～24日在珠江三角洲地区发生的特大暴雨过程是华南暴雨试验(HUAMEX)加密观测期间的一个典型个例,卫星云图与降水分布表明这是锋面附近与锋前暖区发生的两个中尺度对流系统(简称MCS)造成的强降水。使用非静力原始方程模式MM5较为成功地模拟了这次暴雨过程。根据数值模拟的结果,本文着重分析了发生在锋面上和锋前暖区的两类MCS的中尺度特征,并探讨了这两类MCS的差别。结果表明,两类MCS具有某些共同的中尺度特征,即对流系统的底层和顶部分别存在β尺度的低压和高压中心;低层流场辐合而在对流雨团的顶部辐散出流;对流系统内部具有暖心结构等,但锋面上的MCS较暖区中的对流系统具有更强的斜压性;二者内部的流场与三维运动结构也具有不同的特征,来自西南和偏南方向的空气从底部流入锋前暖区MCS时受到中低压的气压梯度力作用而加速;而锋面上MCS中不仅有来自锋前的暖湿空气,而且还有来自锋后的冷空气参加对流。MCS高空反气旋式发散气流和空气的加速运动反映出MCS顶部存在中尺度高压及向外的气压梯度力,轨迹分析也证明了MCS上空气流的这种非地转特征。

NUMERICAL SIMULATION STUDY ON MCS OF A HEAVY RAINFALL PROCESS IN SOUTH CHINA

A heavy rain process occurred in South China on 23 May 1998 was a typical case during the intensified observation period of HUAMEX. Satellite images revealed that two types of MCSs caused the precipitation, Type A was generated on the Mei-yu front, and Type B was generated in the prefrontal region. Using the PSU/NCAR MM5 model, a 36-hour numerical simulation of this case was conducted initiating at 00:00Z 23 May 1998. It was verified that the amount and area of precipitation, cloud distribution and the configurations of synoptic systems matched quite well with observations. The model also reproduced the process of cold front moving southward and MCSs occurring both on the front and in the warm area simultaneously. Features of the two types of MCSs' structure associated with Mei-yu front were investigated. Both of them have some common characteristics of MCS. There are meso low and high centers located to the bottom and top of MCS respectively. Correspondingly, wind fields converged at the low level, while diverged at the top of MCS. Additionally, both of them have warm core structures. However, their difference was also distinct. Type A was triggered by the cold front, and the prevailing lifted southeastly flow was its main provider of water vapor and unstable energy. Type B was a deep convective system occurring in the warm area ahead of the front, which was triggered by meso-vortex. Consequently, Type B was suggested to be more baroclinic than type A. Using trajectory techniques, three dimensional flow structures of the MCSs were revealed. It shows considerable acceleration presence in the inflow in the low troposphere and the anti-cyclonic outflow in the top of both MCS, which proving the existence of the meso high and outward pressure gradient force locating on the top of MCSs. And the air stream originating from the cold area was identified to penetrate the front and move upward into Type A.