1215号“布拉万”台风暴雨及降水非对称性分布的成因分析

A diagnostic analysis of the causes of the torrential rain and precipitation asymmetric distribution of Typhoon Bolaven (2012)

中文关键词:  台风暴雨  数值模拟  非对称性  水汽散度通量  湿位涡  倾斜涡度发展  条件性对称不稳定

英文关键词:Typhoon torrential rain  Numerical simulation  Asymmetric structure  Moisture divergence flux  Moisture potential vorticity  Slantwise vorticity development  Conditional symmetric instability

中文摘要:

      利用中尺度非静力模式WRF对2012年第15号台风“布拉万”在中国东北地区造成的暴雨过程进行了数值模拟,结合观测资料对模拟结果进行了验证,利用模式输出的高分辨率资料,对“布拉万”台风造成的强降水及其非对称性分布的成因进行了诊断分析。结果表明,模式很好地再现了台风登陆过程中的路径、强度变化和降水分布,受中纬度西风槽带来的干冷空气影响,“布拉万”台风登陆后的降水和环流结构具有明显的不对称性,降水主要集中在台风中心西北侧的能量锋区附近。水汽散度通量和水汽螺旋度能够较好地描述强降水过程的发生、发展及其非对称性分布的时空特征,在强降水区,水汽散度通量表现为正值强信号,而水汽螺旋度表现为负值强信号,在非降水区和弱降水区,两者均表现为弱信号。等熵位涡分析显示,对流不稳定只是此次台风暴雨前期和初始阶段的不稳定条件,而湿位涡(MPV)的湿斜压项(MPV₂)则是暴雨增强和出现非对称性分布的主要机制。在暴雨形成过程中,由于冷空气侵入造成了在台风环流西北侧湿等熵面的陡立倾斜和水平风垂直切变的增强,导致了气旋性涡度的显著增强,气旋性涡度增强造成的强烈上升运动将降水区东南侧输送过来的暖湿空气向上输送,从而导致了暴雨的发生,这其中条件性对称不稳定是降水得以加强的一种重要不稳定机制。

英文摘要:

      A numerical simulation has been performed with the mesoscale non-hydrostatic model WRF (Weather Research Forecast) to study the torrential rain associated with Typhoon Bolaven(2012) in Northeast China, and the detailed comparison between the simulated results and the observations has been conducted. Furthermore, by using the high resolution model outputs, a diagnostic analysis has been also carried out to gain a better understanding of the torrential rain and precipitation asymmetric distribution mechanism. The results indicate that the simulated track, intensity and rainfall of Typhoon Bolaven all match with the observations well. After landfall of Bolaven, its precipitation distribution and its circulation showed clear asymmetric structure because of the intrusion of the north cold-dry current from the middle latitude trough into the typhoon circulation. The rainfall mainly occurred near the energy front zone located in the northwest of the typhoon. The moisture divergence flux and moisture vertical helicity can well describe the genesis and spatial-temporal variation of the torrential rain and its asymmetric distribution of Bolaven. The moisture divergence flux shows strong signals in positive value and the moisture vertical helicity shows strong signals in negative value over the torrential rain areas. And they display weak signals over the non-rainfall or weak rainfall areas. Convective instability provided a favorable condition for the precipitation only in the initial stage of the typhoon rainfall, and the moisture baroclinic item MPV₂ of MPV was the major mechanism for the torrential rain enhancement and its asymmetric distribution. During the process of torrential rain formation, the vorticity over the precipitation area was increased greatly by the isentropic surface slantwise and the vertical shear of horizontal wind characterized by MPV₂. The strong ascending motion caused by the increased cyclonic vorticity transferred the warm-moist air mass coming from the southeast of the rainfall area upward, and therefore, the torrential rain occurred. The conditional symmetric instability was the important instability mechanism for the torrential rain enhancement.