西北太平洋热带气旋变性过程中的风及降水分布变化特征分析

利用中国气象局上海台风研究所（CMA/STI）整编的热带气旋最佳路径资料、美国飓风联合警报中心（JTWC）最佳路径资料、美国国家海洋与大气管理局（NOAA）的全球多平台热带气旋风场资料（MTCSWA）和CMORPH降水资料、日本卫星云顶黑体辐射温度（T_BB）资料等，分析1987～2016年30年间西北太平洋228个变性热带气旋（ETTC）的活动规律、风与降水分布及其演变特征。结果表明:（1）ETTC年均7.6个，除1～2月，各月均有分布，峰值在9月。约90.4%的ETTC变性位置在30°N以北，仅约9.6%在30°N以南较低纬度，且多发生于春夏和秋冬交替季节。（2）TC（热带气旋）变性通常发生在其转向后，半数以上移速加快，大多数中心气压升高或维持，仅10.5%降低。（3）变性过程中ETTC近中心最大风速减小，最大风速半径增大，内核趋于松散。其34节风圈半径北侧明显大于南侧，风场结构非对称性增强。（4）ETTC强风和强降水呈显著非对称性分布，其强风区主要出现在ETTC中心东侧，即路径右后方；强降水区主要出现在北侧，且变性后在东北象限向外扩张。（5）较强的环境水平风垂直切变（VWS）是影响ETTC风及降水分布的重要因子。强降水主要出现在顺风切方向及其左侧，强风（去除TC移速时）出现在切变左侧。

Characteristics of Wind and Rainfall Distribution of Tropical Cyclones during Their Extratropical Transition Processes over the Western North Pacific

The characteristics of ETTCs (extratropical transitioned tropical cyclones) activities and their wind and rainfall distribution were studied based on the best track data from CMA/STI (China Meteorological Administration and Joint Typhoon Warning Center), the multiplatform tropical cyclone surface wind analysis product, CMORPH (Climate Prediction Center morphing technique) satellite data provided by NOAA (National Oceanic and Atmospheric Administration) and T_BB (cloud-top black body temperature) calculated by satellite observation during 1987 to 2016 over the western North Pacific. The results show that: (1) The average annual number of ETTCs was 7.6. The ETTCs could appear in every month except January and February, and peak in September. About 90.4% of the ETTCs occuring to the north of 30°N, while the others (9.6%) to the south of 30°N always occurred in the alternation of spring and summer or autumn and winter. (2) The major tracks of the ETTCs were northwestward in their early stage and then later turned northeastward or northward, and the extratropical transitions occurred after their turning. After ETs, most TCs moved faster and became weaker or maintained their intensity. Only 10.5% of ETTCs were strengthened. (3) The maximum wind of ETTCs decreased rapidly with its radius increasing, and the 34-knot wind radius of the ETTC was larger in the northern quadrant than in the southern quadrant. These indicate that the TC structure became much asymmetric and the inner core became loose during the extratropical transition. (4) The asymmetry of the ETTCs wind and rainfall distribution was enhanced during the ET process. The stronger wind tended to be on the east side of the ETTCs (right or back side along TC track, while the stronger rainfall tended to be on the north side, and the strongest rainfall moved to the outer region from the inner-core region in the northeast quadrants. (5) The asymmetric characteristics of wind and rainfall distribution had a close relationship with the strong VWS (vertical wind shear). Strong rainfall tended to be in the downshear to downshear left, and strong wind (motion-removed) tended to be in the shear left.