应用湿Q矢量分解诊断梅雨锋暴雨

摘要

A heavy rain process of the Changjiang-Huaihe Meiyu front (MYF) is diagnosed by the agency ofthe traditional Q vector partitioning (QVP) method to decompose the wet Q vector (Q) in a naturalcoordinate system that follows the isoentropes and by using the numerical simulation results of the revisedMM4 meso-scale model. The technique shows that the partitioned wet Q vectors can lead to a significantscale separation of vertical motion related to the torrential rain. The results not only verify the existingconclusion that different scales interact throughout the rainstorm but also indicate the largely differentroles of these scales during differing phases of the heavy rainfall on a quantitative basis. Specifically, duringthe developing stage, the large-scale plays a predominant role in forcing vertical motion, while frontal-scaleforcing is secondary; during the intense stage, the frontal-scale evolves into the primary factor of forcingvertical motion, whereas the large-scale forcing is minor and plays a diminishing role and can even beignored; and during the decaying stage, the large-scale once again serves as the main forcing of verticalmotion in such a way that the forcing of the frontal-scale decays quickly and is of secondary importance.Furthermore, the partitioned wet Q vectors are suggested to be more suitable than the total wet Q vectorfor evaluating the potential physical mechanism of rainstorm genesis. The first step is that the forcingof large-scale 27 @ Q*s gives rise to the genesis of meso-scale 2 @ Q*n forcing; and then, accordingly as27 @ Q*n forcing increases, whereby the secondary circulation is reinforced, the intensity of the rainfall isstrengthened; and at last, the secondary circulation caused by 2 @ Q*n forcing is directly responsible forgeneration of the MYF heavy rainfall.%采用传统的Q矢量分解方法将湿Q矢量(Q*)分解在沿等位温线的自然坐标系中,并结合改进的MM4模式(MMM4)模拟资料,对一次江淮梅雨锋暴雨过程进行诊断分析.结果表明,分解湿Q矢量可将梅雨锋暴雨的垂直运动场进行一个有意义的尺度分离,这不仅验证了梅雨锋暴雨过程中存在不同尺度相互作用的已知结论,而且从定量的角度揭示出不同尺度在梅雨锋暴雨不同阶段所起的作用不同,有明显的主、次之分:在梅雨锋暴雨的开始形成阶段,大尺度对垂直运动场的产生起着主要的强迫作用,锋区尺度的强迫作用处于次要地位;在梅雨锋暴雨的强盛阶段,锋区尺度已经演变为垂直运动场的主要强迫因子,而大尺度的强迫作用则处于次要地位,其至多起着背景场的作用,甚至个别时次可以忽略不计;在梅雨锋暴雨的衰亡阶段,大尺度又逐渐演变为此时垂直运动场的主要强迫动力,而锋区尺度的强迫作用则迅速衰减,其又仅处于次要地位.此外,我们认为分解湿Q矢量比"总"的湿Q矢量更有利于对产生梅雨锋暴雨的潜在物理机制的评估:先是在大尺度2 @Q*s的强迫作用下诱发中尺度2 @Q*n强迫作用的产生,随着2 @Q*n强迫作用的增强,其所强迫作用产生的次级环流增强,降水的强度也同时随着增大,最终由2 @Q*n强迫作用所产生的次级环流直接导致梅雨锋暴雨的发生.