Abstract:By using the data of conventional surface and sounding observations as well as the associated NCEP FNL and GFS reanalysis data, the features of precipitation type of heavy rain snow process in North China during 3-4 November 2012 are studied in this paper. It is found that in the periphery of low level vortex, the rain and snow separatrix is identical with the 0℃ isotherm and streamline convergence zone between northerly and southerly wind at 925 hPa. However, near the center of low level vortex, the rain and snow separatrix takes on a feature of anticlockwise revolving from the northwest quadrant to the center of the vortex. In the early stage of cyclone developing, snow is located at the north northeast quadarant to northwest quadrant of the 850 hPa vortex while the precipitation type at southeast quadrant is rain. As the extratropical cyclone intensifies, temperature at the southwest quadrant of low level vortex drops quickly due to the low level cold advection. Therefore, the snow area gradually expands southward along the wind flow of the west side of low level vortex and takes on a cyclonic shaped distribution ultimately. The temporal evolution of precipitation type depends on the temperature advection of the whole low level troposphere (850-950 hPa). When there is warm temperature advection in either 900-850 hPa or 950-900 hPa level, the precipitation type may still be rain even if cold advection exists in other levels. The cold temperature advection of northwest wind at the west side of low level vortex is very important to snow. When the low level wind transforms to eastly wind, the low level cold advection disappears, and finally, the precipitation type involves to rain.