Statistical Characteristics And A Case Study Of Snowstorms Associated With Northward Extratropical Cyclones In Northeast China

Extratropical cyclones were important weather systems for heavy snowstorms.Many a extreme snowfall processes in northeast China were related to the northward extratropical cyclones.Up to now,research on the extratropical cyclone snowstorm affecting Northeast China mainly focused on individual case analysis,laking systematic and indepth study.In addition,some cyclones had the characteristics of back-bent front and occluded rolling,which were different from the traditional frontal cyclone structure.Therefore,it is necessary to further study the characteristics of the frontal region and the occurrence and development mechanism of cyclones.In this paper,conventional observation data,site precipitation data and NCEP-FNL data were used to make statistics on the northward extratropical cyclone snowstorm affecting Northeast China from 2000 to 2016.According to the circulation situation of 500 hPa,the snowstorm was divided into low vortex type,shallow trough type and deep trough type.Composite analysis shows that Water vapor flux convergence region of low vortex and deep trough pattern snowstorms were related to the convergence of cyclonic circulation and those of shallow trough pattern were mainly associated with convergence in front of troughs in low level.Snowstorms of low vortex and deep trough pattern occurred in warm tongue of pseudoequivalent potential temperature while that of shallow trough pattern concentrated in relatively flat pseudoequivalent potential temperature field.Front area of deep trough and shallow trough pattern were more intensive than that of low vortex pattern.Low vortex and deep trough pattern snowstorm had one and two branches of upper level jet streams respectively,while shallow trough pattern had one or two upper level jet streams.The snowstorm centers were located on the convergence area of right side to the entrance area of north branch or left side to the exit area of the south branch of upper jet stream.In addition,FY-2D satellite data,doppler radar data and WRF mesoscale numerical model,HYSPLIT trajectory model were used to study the environment field,frontal structure and the occurrence and development mechanism of cyclones in a case of Jianghuai Cyclone snowstorm from November 23-26,2013.The results showed that:(1)Cyclone snowstorm occured in the background of the support of north,middle trough and coupling of upper and low level jetstream.There was isallobaric wind convergence in front of 700 hPa vortex,with strong frontogenesis.Snowstorm area corresponds well to the frontogenesis area.Analysis of water vapor transport showed that Sea of Japan was the mean source of water vapor for the snowstorm.Heavy snowfall in the central and eastern part of Heilongjiang Province was related to the mesoscale band echo with a length of 150 km,a width of 30-40 km radar whose reflectivity was greater than 30 dBZ.(2)Analysis of cyclone front area structure showed that: Cold front was separated from warm front continuously in the development process,forming a "T"-shaped structure with the appearance of warm nuclear isolation.Development of the cyclone was similar to the structure of the Shapiro-Keyser cyclone.Frontal area strengthened in the process of warm front bending to the rear of cyclone center,making the central and eastern part of Heilongjiang Province which was located in northwest quadrant of cyclone center had intense warm advection and deep wet layer accompanied with strong upward movement,which was conducive to the formation of heavy snowfall.The cold and wet conveyor belt to the north of the cyclone was dominated by saturated wet air and upward movement.(3)Piecewise potential vorticity inversion method was used to analyze the occurrence and development mechanism of this cyclone:Baroclinity of the low level troposphere was the decisive factor for the generation of the cyclone;In the explosive development period of the cyclone,with the enhancement of low level warm advection and precipitation,low level baroclinic and the release of latent heat of precipitation condensation became the reasons for the rapid development of the cyclone,and the release of latent heat gradually exceeded the role of low level baroclinic,becoming the most favorable factor for the development of the cyclone.As the disturbance height center of baroclinic in low level and release of latent heat moved eastward,the cyclone began to slow down.Tropopause fold played a secondary role in the development of the cyclone.