一次特大暴雨形成中天气尺度和次天气尺度系统的作用

EFFECTS OF SYNOPTIC SCALE AND SUB SYNOPTIC SCALE SYSTEMS ON FORMATION OF A HEAVY RAINFALL PROCESS

采用中尺度数值模式MM 5V2对 1 998年 6月下旬发生在长江流域持续的暴雨过程进行分析研究。通过尺度分离与数值模拟对比试验 ,着重分析了暴雨过程中天气尺度与次天气尺度各物理量场的结构特征 ,提出本次暴雨过程形成的物理机制 :天气尺度流场与水汽场为降水提供持续的远距离水汽输送通道 ,次天气尺度流场形成稳定的经向强辐合 ,为水汽的抬升与凝结提供动力条件 ;在有利的高、低空急流的配置下 ,暴雨区落在高空急流轴以南、低空急流轴以北 ;次天气尺度温度场下暖上冷的热力不稳定层结促进了热力不稳定的发展 ,促使暴雨增幅 ;特大暴雨发生地区上空的次天气尺度湿度的高值中心 ,有利于湿空气在上升运动中释放潜热 ,形成暴雨的反馈机制。数值试验分离模式初始场不同尺度系统信息 ,揭示了不同尺度系统在暴雨发生过程中的动力作用 ,没有中尺度系统的配合 ,仅有天气尺度系统信息 ,或只有次天气尺度系统信息 ,没有大尺度系统的配合 ,暴雨的强度及范围都将有所消减。分析及数值试验结果表明大暴雨是在天气尺度和次天气尺度系统的共同作用下才得以产生和维持的。

By means of the mesoscale numerical model MM5V2, the heavy rainfall process during the later June of 1998 in the middle and lower reaches of the Yangtze River is analyzed. By utilizing the scale separating method and comparing the results of numerical experiments, the characteristics of physical elements of synoptic scale and sub synoptic scale systems are investigated. The physical mechanisms of heavy rainfall formation are presented as follows: synoptic scale wind and water vapor fields provided long distance water vapor transportation for the heavy rainfall while the sub synoptic scale wind field results in strong meridional convergence which provided the steady dynamic condition for the lifting and condensation of water vapor; under the favorable disposition of upper and low level jets, the rainfall region was located to the south of the upper level jet and north of the low level jet; the unstable thermal stratification, cooler at lower level and warmer at upper level, of the sub synoptic scale temperature field facilitated the development of the thermal unstable condition and further strengthened the heavy rainfall; the sub synoptic scale high value center of the specific humidity field over the rainfall area was favorable to the emission of latent heat and further enhanced the feedback mechanism of the heavy rainfall. In numerical experiments, message separation was conducted on the systems with different scales in the model initial field to indicate their dynamic effects in the heavy rainfall process. The results show that both intensity and coverage of heavy rainfall will decrease to some extent while there is only a synoptic scale or sub synoptic scale system during rainfall process. Analyses and results of numerical experiments suggest that the heavy rainfall can form and sustain under the comprehensive effects of both synoptic scale and sub synoptic scale systems.