Comprehensive Analysis on Snow and Freezing-rain Events Based on Doppler Weather Radar in Ningbo

Seven snow processes in Ningbo are analyzed using conventional observational data, sounding and Doppler radar data in aspects of weather background, temperature stratification, and characteristics of radar echo and so on. The necessary conditions for snowfall are summarized as follows: The temperature inversion in the lower and middle level, the northerly winds in the lower and the surface temperature is below 4 ℃. The strength and influencing ranges of southwest current determine the strength of the snowfall. When freezing rain occurs, it is above 0 ℃ in the lower-middle level forming a melting layer, and the surface temperature is below 0 ℃. During snow weather, the depth of freezing layer is much thicker than the melting layer or there is no melting layer at all. Generally speaking, the reflectivity of snow is less than 30 dBz, the horizontal and vertical gradient of reflectivity is small and the structure is uniform.The spectrum width is below 4 m/s and the echo top is basically below 6000m. It is found the height of 0 ℃-layer bright band is closely related with the precipitation nature. The distribution of the zero velocity line under the layer of 600 hPa can be divided into three types: Forward-inverse, forward-inverse-forward-inverse, and inverse-forward-inverse. And the analysis on the distribution of the zero velocity line is of great significance to forecast the end time of snow precipitation.