Analysis On The Effective Factors In Of Secondary Eyewall Formation Over The Western North Pacific

Intensity forecast for tropical cyclones (TCs) has little progress at present and during eyewall replacement cycle (ERC) process, the intensity change of severe typhoons with concentric eyewalls (CEs) is hard to forecast. For this, environment factors and internal dynamic processes which affect secondary eyewall formation(SEF) of TCs are investigated. The Morphed Integrated Microwave Imagery(MIMIC) data obtained from Cooperative Institute for Meteorological Satellite Studies (CIMSS) was used to research severe TCs with SE during the period from the year of 2005 to 2014 and the best track data from Joint Typhoon Warning Center(JTWC) was regarded as the reference of strong typhoons during this period over the Western North Pacific (WNP). Then the differences of environment and structure between these cases are analyzed, the results show that:Both the environment and structure affect the formation of SE. For thosewhich can form SEs, there are significant differences in initial structures but not in large scale factors. In details, larger relative humidity, higher ambient SST and larger initial size lead to faster SEF. The larger initial size the TC has, the faster the SEF is.The variances exist in typhoons with SEF. On the whole, TCs with larger initial vortex size is apt to form the larger SE, experience longer ERC period and more significant intensity fluctuations.Based on the above observational results, the impacts of initial wind profile on SEF are mainly discussed. Hence, vortexes with different initial structure are designed. In details, two groups of ideal experiments are investigated. By comparing model results, it is found that when initial vortex size is larger, it is apt to form the SE and larger outer eyewall. Both unbalanced boundary layer and balanced processes, which lead to SEF,are examined to reveal the underlying mechanisms.The results of ideal experiments vindicate observational facts, which provide theoretical basis for operational forecast.