The Characteristics of Wind and Rainfall Variation of Tropical Cyclones during Its ET Process over the Western North Pacific

Tropical cyclones (TC) undergoing extratropical transition (ET) pose a forecasting challenge in strong wind and rainfall distribution because of the structural changes under the complex interaction with the mid-latitude synoptic systems. In this study，the activity characteristics of extratropical transitioned tropical cyclones (ETTCs), their wind and rainfall distribution variations and precipitation structure features before and after ET process are investigated based on the best track data from China Meteorological Administration and Joint Typhoon Warning Center, the multiplatform tropical cyclone surface wind analysis product, CMORPH (Climate Prediction Center morphing technique) satellite data during 1987 to 2016, and tropical cyclone crossing dataset of Cloudsat from 2006 to 2016.

Statistical results show that: (1) There are 228 ETTCs in the Northwest Pacific during 1987 to 2016, accounting for about 26.2% of the total TCs. ETTCs mostly appear in summer and autumn and peak in September. TCs usually complete ET in the north of 30 °N, only 9.6% in the south of 30 °N occurring in the alternation of spring and summer or autumn and winter. (2) The major tracks of ETTCs are northwestward in early stage and then turn northeastward or northward, and they complete ET after their turning. After ET, 57% TCs move faster and 90% TCs become weaker or maintain their intensity.

During ET, the wind and rain distribution of TCs shows obvious asymmetry, which is closely related to strong vertical wind shear. Results show that: (1) The TC structure becomes much asymmetric and the inner core becomes loose during the extratropical transition: the maximum wind of ETTCs decreases rapidly with its radius increasing. And the 34-knot wind radius of the ETTC is larger in northern quadrant than that in the southern quadrant. (2) The convective activity and precipitation in in-core region significantly weaken during the ET period, while the precipitation in the outer region of the northeast quadrant increases near the completed time. (3) The stronger wind tends to be on the east side of the ETTCs, while the stronger rainfall tends to be on the north side. And the strongest rainfall move to outer region from inner-core region during the ET process.

We compare the cloud structures, microphysical features and thermal structures of ETTCs in their mature and ET stage of the lifecycle and find that: (1) At the mature stage, the inner core region of ETTCs is dominated by single-layer cloud with low cloud base and deep thickness. While in the rain bands region, multi-layer clouds have a higher proportion. At the ET stage, the occurrence frequency of multi-layer clouds increased and the thickness of clouds decreased significantly in inner core region, while the occurrence frequency of single –layer with a thickness more than 10 km increased significantly in rain bands and outer region. (2) During ET process, in the northeast quadrant of ETTCs, the reflectivity, droplet effective radius and particle number concentration in inner core region reduced significantly above 5 km height. While those increased a lot at the height of 5 to 7 km in outer region. (3) The range and intensity of warm and wet core of ETTCs weakened during ET. The negative anomalies of temperature and specific humidity occur in the core region are more significant, indicating the intrusion of dry and cold air. (4) At the mature stage, atmospheric stratification of ETTCs is convectively instable below 5 km. The atmospheric stratification becomes more convectively stable at the ET stage, especially in the north quadrants.