Diagnosis of the Heavy Rain near a Meiyu Front Using the Wet Q Vector Partitioning Method

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.