高空槽脊对台风“天兔”（0705）变性过程中非对称降水的影响

本文基于NCEP再分析资料、TRMM卫星降水资料和RSMC最佳路径集（Best-track Dataset）资料，分析了热带气旋（TC）“天兔”变性过程中降水分布和大尺度环流的演变，并结合片段位涡反演进行修改槽脊的数值试验，研究了环流调整对TC变性过程中非对称降水的影响。结果表明:（1）“天兔”变性前降水集中在路径左侧，伴随着纬向型向经向型调整的背景环流，副热带高压南退的同时减弱西进；（2）TC降水的非对称分布与冷暖锋的相对强弱、水汽输送情况以及高空冷空气下传的落区有直接关系；（3）加强环流调整后，系统斜压性加强，与LOT（降水集中于TC路径左侧）型降水相关的环流指数增大过程随之加强，有利于LOT型降水分布进一步加强。

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.