Statistical Characteristics,Three-dimensional Structures And Evolutionary Mechanisms Of Explosive Extratropical Cyclones In The Northern Hemisphere

Using the 6-hourly,0.75° resolution ERA-Interim reanalysis data,the characteristics of the Northern hemisphere explosive extratropical cyclones over the period 1979-2015(37 years in total)are analyzed.According to the statistical results,these explosive extratropical cyclones were divided into Pacific Ocean type and Atlantic Ocean type,and each type of them was been synthesized.In addition,the differences and similarities between these two types are compared.Aiming at the evolutionary mechanisms of extreme intensity explosive extratropical cyclones,numerical simulation and piecewise potential vortex inversion methods are applied in this paper.The main results showed that:(1)In the past 37 years,the northern hemisphere explosive extratropical cyclones has a weak trend.Compared with the Pacific Ocean cyclones,the incidence of cyclones in the Atlantic Ocean is greater in high latitudes.The average live period of the explosive extratropical cyclones was 2-6 days,and the longer the live time,the more inclined the cyclones occurred to the west of the ocean.(2)The synthetical analysis of the explosive extratropical cyclones has shown that these cyclones have a larger extension in the east-west direction and the shape is similar to the Keyser-Shapiro pattern.During the rapid development stage of the cyclone,its center lies in front of the upper trough and the west side of the low level jet stream,which has favorable vorticity advection and temperature advection conditions.Among them,the explosive cyclones in Atlantic region has stronger jets,better moisture conditions,and larger temperature gradient and geopotential gradient,which may be the reason that the average deepening rate of the cyclone in the Atlantic region is slightly larger than that in the Atlantic region.(3)The WRF numerical model successfully reproduces the track of a cyclone and its associated background circulations.The rapid development of the explosive cyclone was closely related to the heat transport by the upper-level jet stream in the upper troposphere,the favorable quasi-geostationary forcings(e.g.,warm advection and cyclonic vorticity advection)ahead of a shortwave trough in the middle troposphere,and the warm advection accompanied by the warm front in the lower troposphere.The result of piecewise potential vorticity inversion shows that the tropopause-folding related downward transport of high-value potential vorticity in the stratosphere and the precipitation-condensation related diabatic heating governed the rapid development of this explosive cyclone,while the baroclinic process in the lower troposphere made the smallest contribution.The latent heat release of precipitation and tropopause folds play a major role in the early and strong stages of rapid development of cyclones respectively.