Mesoscale convective systems (MCSs) are severe disaster-producing weather systems. Radar data and infrared satellite image are useful tools in MCS surveillance. The previous method of MCS census is to look through the printed infrared imagery manually. This method is not only subjective and inaccurate, but also inefficient. Different from previous studies, a new automatic MCS identification (AMI) method, which overcomes the above disadvantages, is used in the present study. The AMI method takes three steps: searching potential MCS profiles, tracking the MCS, and assessing the MCS, so as to capture MCSs from infrared satellite images. Finally, 47468 MCSs are identified over Asia and the western Pacific region during the warm seasons (May-October) from 1995 to 2008. From this database, the geographical distribution and diurnal variation of MCSs are analyzed. The results show that different types of MCSs have similar geographical distributions. Latitude is the main control factor for MCS distribution. MCSs are most frequent over the central Tibetan Plateau; meanwhile, this area also has the highest hail frequency according to previous studies. Further, it is found that the diurnal variation of MCSs has little to do with MCSs' size or shape; MCSs in different areas have their own particular diurnal variation patterns. Based on the diurnal variation characteristics, MCSs are classified into four categories: the whole-day occurring MCSs in low latitude, the whole-day occurring MCSs in high latitude, the nocturnal MCSs, and the postmeridian MCSs. MCSs over most places of China's Mainland are postmeridian; but MCSs over the Sichuan basin and its vicinity are nocturnal. This conclusion is coincidental with the hail climatology of China.
The Advanced Research WRF(Weather Research and Forecasting)model is used to simulate the evolution of a mesoscale convective vortex(MCV)that formed on the Meiyu front and lasted for more than two days.The simulation is used to investigate the underlying reasons for the genesis,intensification,and vertical expansion of the MCV.This MCV is of a type of mid-level MCV that often develops in the stratiform regions of mesoscale convective systems.The vortex strengthened and reached its maximum intensity and vertical extent(from the surface to upper levels)when secondary organized convection developed within the mid-level circulation.The factors controling the evolution of the kinetic and thermal structure of the MCV are examined through an analysis of the budgets of vorticity,temperature,and energy.The evolution of the local Rossby radius of deformation reveals the interrelated nature of the MCV and its parent mesoscale convective system.
2003年04月12日江西、福建交界处发生一次飑线过程,该飑线过程造成了雷暴、冰雹和大风等强烈天气.本文使用NCEP1°×1°逐6 h分析资料及卫星、雷达观测资料对这次飑线过程发生的大尺度背景进行了分析,之后本文使用Advanced Research WRF(ARW)2.2对该过程进行了模拟,并利用模拟结果对飑线系统的演变过程、近地层结构、对流层结构进行了分析,结果表明:1)本次飑线发生于大尺度鞍型场中,它是在冷锋、切变线、低空西南急流、高空西风急流以及中尺度对流复合体等系统共同作用下生成,从位涡角度看,它是高空正的湿位涡移至低层斜压不稳定的冷锋上所诱生的一次强对流过程;2)位于江西、福建交接处的武夷山地形对本次飑线的形成、移动和发展有较大影响,敏感试验表明若将武夷山地形高度减至一半则无法生成飑线系统,但对流单体依然能够生成;3)近地层风场表明本次飑线过程主要为南北向气流的辐合,东西向气流的辐合较小,温度场和海平面气压场上则可以清晰的看出冷空气丘和中高(低)压结构,冷空气丘的存在对风场有明显的加速作用,风速在飑锋处达到最大,且飑锋出流与飑线系统的移速和方向较为匹配,这是本次飑线过程维持和发展的主要因子;4)对流层结构的分析则显示,本次飑线介于前部层云降水飑线(LS)和平行层云降水飑线(PS)之间,其移动方向上(纬向)的垂直结构类似于Front-Fed LS(FFLS)结构,垂直于其移动方向(经向)的垂直结构则与典型的尾部层云降水飑线(TS)结构一致.
