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

Abstract

A heavy rain process of the Changjiang-Huaihe Meiyu front (MYF) is diagnosed by the agency of the traditional Q vector partitioning (QVP) method to decompose the wet Q vector (Q) in a natural coordinate system that follows the isoentropes and by using the numerical simulation results of the revised MM4 meso-scale model. The technique shows that the partitioned wet Q vectors can lead to a significant scale separation of vertical motion related to the torrential rain. The results not only verify the existing conclusion that different scales interact throughout the rainstorm but also indicate the largely different roles of these scales during differing phases of the heavy ramfall on a quantitative basis. Specifically, during the developing stage, the large-scale plays a predominant role in forcing vertical motion, while frontal-scale forcing is secondary; during the intense stage, the frontal-scale evolves into the primary factor of forcing vertical motion, whereas the large-scale forcing is minor and plays a diminishing role and can even be ignored; and during the decaying stage, the large-scale once again serves as the main forcing of vertical motion 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 vector for evaluating the potential physical mechanism of rainstorm genesis. The first step is that the forcing of large-scale $2⊊bla cdot {⊼ Q}_s^*$ gives rise to the genesis of meso-scale $2⊊bla cdot {⊼ Q}_n^*$ forcing; and then, accordingly as $2⊊bla cdot {⊼ Q}_n^*$ forcing increases, whereby the secondary circulation is reinforced, the intensity of the rainfall is strengthened; and at last, the secondary circulation caused by $2⊊bla cdot {⊼ Q}_n^*$ forcing is directly responsible for generation of the MYF heavy rainfall.