Influence of Upper-Level Trough and Ridge on the Asymmetric Precipitation during Extratropical Transition of Typhoon Usagi

Based on the reanalysis data from NCEP/NCAR, rainfall data from TRMM (Tropical Rainfall Measuring Mission), and database from RSMC (Regional Specialized Meteorological Centre) best-track, this paper presents an evaluation of the precipitation distribution and large-scale circulation of tropical cyclone (TC) Usagi during its extratropical transition (ET). Based on this, a numerical experiment was conducted, in which the upper-level trough and ridge were modified using piecewise potential vorticity inversion to consider the influence of the trough and ridge on the asymmetric precipitation of the TC during ET process. The results are as follows: (1) Precipitation from Usagi during ET is concentrated on the left side of the path, accompanied by an adjustment in the meridional-to-zonal circulation, with the subtropical high retreating southward, while weakening and being propelled westward. (2) The asymmetric distribution of TC precipitation is directly related to the relative strength of the cold and warm fronts, the water vapor transport, and the area of falling upper-level cold air. (3) When the circulation adjustment is strengthened, the baroclinity of the system is enhanced. This means that the increased westerly, as indicated by the increased circulation index, promotes an increase in the LOT (left of track) rainfall.