Study On Numerical Simulation And Forecasting Method Of Freezing Rain In Hunan

Wire ice caused by freezing weather has a significant influence in Hunan province.In this study,we analyze the spatio-temporal distributions of freezing rain in China and the spatial distributions,inter-annual variations,monthly variations and vertical structure of freezing rain in Hunan province using the ground observations and vertical sounding observations from 1997 to 2016.The Ramer algorithm is improved by using the statistical thresholds of meteorological elements which are suitable for the occurrence of freezing rain in Hunan province.The meteorological elements such as liquid water content,temperature,hydrate and precipitation intensity in two typical cases: from 1to 5 January in 2011 and from 27 to 29 January in 2015,are simulated successfully and analyzed using the Weather Research Forecast model(WRF).The original Ramer scheme and the improved scheme are used to simulate the results and the AFWA algorithm is also compared with the improved Ramer algorithm.The conclusions are shown as follows:(1)In China,the freezing rain frequently occurs in Yunnan-Guizhou Plateau,Hunan Province and Jiangxi Province,and most of the freezing rain occurs in winter and early spring especially in January(from December to March).The number of freezing rainy days in southern part of Hunan is obviously more than that in northern part of Hunan.From 1997 to 2007,the number of freezing rainy days in Hunan Province is very limited and has a little variation.After the rapid increments of freezing rainy days in 2008,the number of freezing rainy days is fluctuated by year.The monthly variations of the freezing rain in Hunan Province is in accordance with that in China,which peaking in January.The thresholds of meteorological elements which suitable for the occurrence of freezing rain in Hunan province are shown as follows: wet bulb temperature between-6 ~0?;relative humidity higher than 85%;wind speed within 0~5 m/s;precipitation within 0-0.6 mm.The values of meteorological elements at high altitude are basically consistent with the limitations of the Ramer algorithm and AFWA algorithm.(2)The simulation results of temperature and precipitation in Hunan Province using WRF model are generally consistent with the observations.In the simulation of Ramer algorithm,compared with the parameterized scheme before modification,although the missing report rate increases slightly after optimization,the missing report rate decreases greatly from 0.08-0.53,and the TS score rises to 0.11-0.36.The accuracy of the scheme has been greatly improved which is more suitable for forecasting rainy weather in Hunan province.The high-altitude structure usually is "three-layer model" when rainstorm occurs in Hunan Province,and the thresholds of high-altitude meteorological elements are different under different rainfall patterns.(3)As a whole,the accuracy of the modified Ramer algorithm is higher than AFWA algorithm.