Impact Of Outflow On Rapid Intensification In Tropical Cyclone

Tropical cyclone(TC)outflow layer has relatively few research achievements,and the mechanism of the interaction between TC outflow and the environmental field affecting TC intensity change is still unclear.In our study,the structure and evolution characteristics of TC outflow layer and the mechanism of the interaction between the outflow and the environmental field are explored,using synthetic analysis,dynamic diagnostics and numerical simulation.Firstly,259 TC samples with the rapid intensification(RI)process in Northwest Pacific during 1979-2012 are analyzed using the TC Best-Track data from IBTrACS(International Best Track Archive for Climate Stewardship),the high-resolution satellite images from CIMSS(Cooperative Institute for Meteorological Satellite Studies)Tropical Cyclones and Digital Typhoon and the atmospheric reanalysis datasets from ERA-I(European Center's Interim Re-Analysis).The results show that TC outflow channel is mainly divided into four types:single poleward channel(SP),single equatorward channel(SE),double channel(D)and no channel(N).The outflow is strongest at 175 hPa,the thickness is 100-300 hPa,and the radius is more than 2000 km.SP TC has obvious interaction with the mid-lattitude trough.Secondly,outflow-jet interaction of tropical cyclone Roke(2011)is preliminarily analyzed by using four atmospheric reanalysis datasets CFSv2(Climate Forecast System,version 2),ERA-I,MERRA2(Modern Era Retrospective Analysis for Research and Applications,version 2)and JRA-55(Japanese 55-year Reanalysis).It is found that during the outflow-jet interaction period,the poleward outflow jet and divergence were enhanced.Meanwhile,the positive PV(Potential Vorticity)anomaly extending from the southern side of upper-level jet stream gradually approached and partially superposed with the positive PV region of Roke(2011),which leads to the RI process of Roke(2011).Finally,the numerical simulation of Roke(2011)is carried out.The structural evolution characteristics during RI process and the impact of the outflow-jet interaction on TC intensity are carefully analyzed by the high-resolution simulation results.It is found that the later stage of RI is influenced by the strong divergence in the entrance of the mid-latitude jet stream,and there is a strong convection propagating inward near 400-500 km north side of the TC center,which maintains the development of the TC intensity.The weak positive PV from the mid-latitude jet stream gradually approaches the center of Roke(2011),which promotes the development of the positive PV in the upper troposphere.REFC(Eddy flux convergence of relative angular momentum)was transported inward along the TC radius,which increases the angular momentum output of the TC outflow layer under the circumstance of relatively low vertical wind shear,so that the TC outflow reached the peak earlier than the TC intensity.