2002年6月20～24日梅雨锋中尺度对流系统发生发展分析

AN ANALYSIS ON MCSs IN MEIYU FRONT DURING 20—24 JUNE 2002

利用“973”中国暴雨试验获得的加密观测资料 ,首先对 2 0 0 2年 6月 2 0～ 2 4日发生在长江中下游地区的中尺度对流系统发生发展进行了分析 ,有 7个尺度较大的α中尺度对流系统在长江中下游地区发展 ,造成了大范围的强降雨。然后针对安徽南部发生的中尺度对流系统发生发展过程进行了详细分析 :MCS生成于切变线南侧的西南暖湿气流中 ,其源地是大别山区和大别山与九华山之间的长江河谷地带 ;低层西南风的水汽输送是安徽南部对流不稳定和对流有效位能积聚的主要原因 ;在降雨发生前整层可降水量有明显增加 ,开始降雨后则逐渐减小。雷达探测的回波显示α中尺度对流系统中有 β和γ中尺度系统的活动 ,β和γ中尺度系统与整个中尺度对流系统的移动方向不一致 ;中尺度对流系统中的带状回波有地面中尺度辐合线配合 ,对流带随地面的中尺度辐合线移动 ,对流带中的对流单体沿辐合带移动。

The routine observational data and intensive observational data of “China Heavy Rain Experiment and Study-CHeRES' were used to analyze MCSs in Meiyu front during 20—24 June, 2002. It was found that some MαCSs produced severe heavy rainfall over middle and lower reaches of Yangtze River basin, the heaviest rainfall occurred at Huangshan station. The duration of these MCSs was longer than 10 h, some of them longer than 24 h. Afterwards, the MCS1 triggered over southern part of Anhui province and produced heavy rainfall was further analyzed by using intensive observational data, especially, GPS, radar and wind profiler data. During 20-21 June 2002, the Meiyu front stagnated to the north of 30°N, while shear line on 850 hPa between southwesterly along rim of subtropical high and easterly from North China located at 31°N. The Meiyu front and shear line provided favorable synoptic condition for the development of convections. In addition, the analyses show that MCS1 formed in warm-moist southwesterly to the south of shear line over Dabieshan mountain and gorge between Dabieshan and Jiuhuashan mountains. During the evolution of MCS1, two systems originated over Dabieshan mountain area and moved eastwards. The first system moved eastwards quickly, whereas the second system stagnated and developed deeply. The severe rainfall occurred at rearward of MCSs. The GPS observation indicates that the precipitable water increase obviously before occurrence of rainfall about 2—3 h and decrease after it. Although the observation sites of GPS are limited, but time resolution is very high, thus, it could be employed to predict local rainfall. The wind field from wind profiler reveals that LLJ extended to 4000 m accompanied the development of convection. The analysis of sounding data of 6 h interval at Anqing station captured the releasing and accumulating of CAPE during the evolution of convection. The abundant moisture transportation by southwesterly is favorable to the maintenance of convective unstability and accumulation of CAPE. The monitoring of radar reveals that meso β and γ systems were very active in MαCS and its propagation direction was not consistent with that of MαCS. Several convection lines developed during the evolution of MCS1, which associated with surface convergence lines. The outflow boundary of convection line could trigger another convection line. The convection line moved with mesoscale surface convergence line, but the convective cells embedded in convergence line propagate along the line.