A Diagnostic Analysis And Numerical Simulation Study On A Severe Weather Event In Dalian

Under enhancement of the global warming,extreme weather might increase.Heavy rain/snow is a typical extreme weather and climate event.Almost every year,there are extreme heavy rainfall/snowfall events in China which have high impacts on people's life and production of industry and agriculture and cause a great loss of life and property. Thus,it is significant for disaster prevention and reduction to analyze causes of heavy rain/snow and improve accuracy of forecast.During the period of March 3-5 2007,a sever cold weather process occurred in the northeast of China,which brought various disastrous weather,such as snow,frozen rain, gust wind and storm surge etc.In order to fully understand its dynamic and thermodynamic characteristics,meso-scale structure and mechanism,we investigated the cause of this process with an emphasis on heavy rain(snow)based on analyses of general circulation,physical diagnostics,features of satellite images and radar reflectivity by using data from conventional and automatic observations,satellite,and new generation radar,and found key factors for forecasting as well,in addition, numerical simulation based on a meso-scale model was analyzed.The main results are as followings:(1)This extreme weather process was a result of combined effects of a strong northward cyclone and strong southward cold air mass,i.e.,interaction between moist warm air from south and cold air from north is a main factor producing this extreme weather event.(2)The steadily maintenance of the low and high level jets is an important cause for heavy rain to last.The existence of low level jet played a key role in water vapor transport,and provided a very favorable water vapor condition for area of the storm.(3)The rain was mainly from large-scale,continuous and stable straitform clouds.From radial velocity field,it was shown that the strong meso-scale reflectivity was induced by the convergent shear between eastern and western jets in the upper boundary layers and above continuous transport of deep moist warm air.The measurement of radial velocity from Dopple radar is well corresponded to the surface observations.(4)The PSU/NCAR MM5 produced a relatively accurate forecast.The hourly predictions for precipitation,gust wind,and drop in temperature show the model's capability of refined forecasts. (5)Diagnostic analysis shows low level strong positive vorticity and convergence,high-level strong negative vorticity and divergence which were all favorable to occurrences and development of powerful convection.