A Study On Tropical Upper-tropospheric Trough Cells (TUTT Cells) Over The Western North Pacific And Its Influence On The Sudden Change Track Of Typhoons

The sudden change of tropical cyclone（TC）track,which is a great challenge in TC forecast,is influenced by various enviornmental systems.The upper-tropospheric cold-core low is one of systems which could cause the sudden track of TC.However,their influence on TC track has not been known clearly.Especially,people know little about the upper-tropospheric cold-core low cutted from tropical upper tropospheric trough（referred as TUTT cell）which is the comment upper-level systems over western North Pacific.In this study,the TUTT-cell events during the period of 2006 to 2015 are analyzed statistically.Then we study the atmospheric circulation background and structure of TUTT cell from its genesis to disspation.Furthermore,we study the the dynamic mechanics between TUTT cell and the movement of its eastern TC.Finally,using the ensemble analysis,the sudden change of typhoon“Meranti”（2010）track influence by the nearby TUTT cell is studied.The statistical results show that:（1）A total of 369 TUTT-cell events more than 24 h or6836 TUTT cells are studied in this region.Most TUTT-cell events are observed from March to December like TC.（2）Most TUTT cells form to the east of 150°E and then move westward to Chineses offshore area with a mean speed of 6.6 m s^-1 and a mean lifespan of 4.4 days.（3）75%of the TUTT cells have radii of<5°latitude–longitude.In general,TUTT cells exhibit the height of cold center is 250 hPa.And significant cyclonic vorticity in the 550-125 hPa layer and its peak amplitudes at 225 hPa.A few samples’positive vorticity columns could even extend to700 hPa.The characteristics of TUTT cells during genesis,mature stage and dissipation were conducted using the dynamic composite method.Results suggest that the TUTT cell events usually form near the TUTT base.The TUTT cell is always cutted off from TUTT after moving along the southwest of subtropical high.Then it usually dissipates after arriving in the south of subtropical high at the Chinese offshore area.In this process,its geopotential height will increase.The TUTT-cell radius is about 3-4°latitude/longitude.When the TUTT cell dissipating,the maximum warming is as large as 4°C,which is not avail to cold center.In the TUTT-cell genesis stage,the north and west part of TUTT cells are dominated by descending motion,while the south and east are asending motion.In the dispattion of TUTT cell,the zonal cross section is the opposite to the section in the genesis stage of TUTT cell.We study the typical TCs experienced abrupt turning in the east of TUTT cell.The diagnostic results show that,when the typhoons move near the strong TUTT cell,their circulations are overlayed influencing TC steering flow.The results of dynamic analysis show that the easterly pressure gradient force in the west of typhoon is reduced by the influence of TUTT cell,which result in the astmmetry of forcing in TC circulation.And the force convergence in the east of TUTT cell and the divergence area in the western of TC provide the force leading the westward movement of TC.The force convergence in the northwest of the TUTT-cell periphery hinders the typhoon northward motion,which is helpful for the looping motion of typhoon when the steering flow is weak.Typhoon Meranti,which experienced northward track change under the influence of a TUTT cell located in its west,is selected to be studied further.Using model ensemble data,the good members are compared to the poor group.Results show that:（1）The abrupt northward turning of typhoon Meranti is related to the coupling between Meranti and the nearby TUTT cell in north-south direction.（2）TUTT cell could change the upper tropospheric environmental flow around the typhoon,and then influence the typhoon steering flow.（3）The TUTT cell with wide horizontal advection and vertical deep cyclonic circulation is more favorable to the track change.（4）The diagnostic analysis based on the potential vorticity tendency equation showed that,the horizontal advection potential vorticity in the northwest of typhoon is strengthened,which is helpful for the northward movement of Meranti.