The Characteristics Of Wind And Rainfall Of Tropical Cyclone During Its Extratropical Transition 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 system.This study firstly analyzes the activities of the extratropical transitioned tropical cyclone(ETTC)from 1987 to 2016 over the Northwest Pacific.Then the wind and rainfall distribution of ETTCs under the influence of strong vertical wind shear is studied.Furthermore,we compare the cloud structure,microphysical features and thermal structures of ETTCs during their mature and ET stage of the lifecycle,to explore its impact on precipitation changes.Statistical results show that:(1)There are 228 ETTCs in the Northwest Pacific during the 30 years,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 the 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 the northern quadrant than that in the southern quadrant.(2)The convective activity and precipitation in inner 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(the right or back side along TC track),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.(4)Strongest rainfall tends to be in downshear to downshear left,and the strongest wind tends to be in shear left after removing the motion vector.We compare the cloud structure,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 rainbands 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 rainbands and outer region.(2)During ET,in the northeast quadrant of ETTCs,the reflectivity,droplet effective radius and particle number concentration in the inner core region reduced significantly above 5 km height.While those increased a lot at the height of 5 to 7 km in the 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 become more convectively stable at the ET stage,especially in the north quadrants.Numerical simulation data from the Weather Research and Forecasting model(WRF)shows that:(1)Haitang completes its extratropical transition on the southwest side of subtropical high without significant interaction with a midlatitude trough.(2)During ET,the averaged wind speed decrease,and stronger wind turned to the northeast quadrant from southeast quadrant,mainly because the larger pressure gradient in northeast quadrant in the later period associated with subtropical high ridge on the northeast side.(3)The rainfall on the south side of Haitang during ET is related to the VWS.In addition,the low-tropospheric frontogenes is contributed to the rainfall appeared on the north side of Haitang after ET.(4)The rainfall on the north side of Haitang occurs in water vapor flux convergence area with convectively instable stratification.And the results revealed that the frontogenesis was mainly forced by the tilting forcing.