Cases Study On The Synoptic Conditions And The Physical Characteristics Of Raindrop Size Distribution Of Freezing Precipitation In Lushan Area

During the icing weather(Feb 7 to Feb 15,2014),we observed the physical characteristics of winter precipitation with PARSIVEL,AWS,MICAPS and other instruments.In this paper,two cases(Feb 10 and Feb 13)that longtime winter precipitation events(>5h)were collected.By analyzing the characteristics of precipitation particles,freezing rain drops were majority of total precipitation part-icles in two cases and freezing rain drops with low speed would gradually transform to ice pellets and dry snow;the distribution of terminal fall velocity and particle size deviated from the G-K curve and here still remained some differences with the conventional liquid precipitation.The results showed that:(1)When icing occurred,the rough of low pressure near east of the Lake Baikal on 500hPa extended from northwest to southeast which continuously transport the cold air in high latitudes to the south,simultaneously influenced by low pressure center on 700hPa and thus causing water vapor transport.In addition,the cold and warm air converged under the trough bottom and the small temperature ridge gradually extended to the north,which provided the conditions for the occurrence of an inversion layer over the Mount Lu area and freezing process.(2)The width of size distributions for total precipitation particles in Case 1 was greater than that in Case 2,whereas the peak number density in Case 2 was greater than that in Case 1.in Case 1,the width of distri'bution was l10mm and the peak number density was 484 m-3·mm-1;in Case 2,the width of distribution was 4.25mm and the peak number density was 1664 m"3·mm-1;the width of different precipitation particles size distribution in Case 1:dry snow>freezing raindrops>ice pellets and in Case 2:freezing raindrops>dry snow>ice pellets.(3)Gamma distribution was more appropriate to describe and parametrize the size distribution of freezing rain and mixing phase precipitation.For total precipitation particles,Case 1:N(D)=20D-3.61exp(-0.08D),Case 2:N(D)=30D-4.68exp(-0.75D);and for freezing raindrops,Case 1:N(D)=76D-2.18exp(-1.11D),Case 2:N(D)=30D-4.67exp(-0.75D).(4)Mixing phase precipitation combined with solid precipitation particles showed an obvious tendency to a different and non-empirical direction,which can provide a new idea for distinguishing contemporary precipitation types in the future.