乌拉特-达茂草原暴风雪天气特征及预报

Synoptic features and forecast of snowstorm on Wulate-Damao grasslands

暴风雪是强降温、大风和强降雪相伴出现的天气现象。由于形成暴风雪天气的条件十分苛刻,且其发生机率较低,所以具有一定预报难度。文中从内蒙古乌拉特-达茂草原近40年来出现的暴风雪天气个例出发,分析了暴风雪的气候学特征、天气学特征、动力学特征,总结出了几个预报着眼点。分析表明:近20年来乌拉特-达茂草原暴风雪灾害呈明显减少趋势;暴风雪天气过程的环流形势可分为小槽发展型、低槽东移型和横槽转竖型三个类型,且常与蒙古冷涡的发展加深和旋转南压相联系。从天气学特征来看,暴风雪的环流形势兼具寒潮、降水和大风天气的环流特征,高层经向环流十分明显,乌拉尔山高脊发展旺盛,脊前西北急流很强,在蒙古国常存在深厚低涡或横槽,冷中心强、锋区强;低层暖湿气流活跃,常存在偏南风低空急流和暖平流,低空急流左前方往往有切变线;地面图上为蒙古气旋或狭长倒槽,并有锋面存在。从动力学特征来看,高、低空急流耦合作用明显,高空急流入口区左侧的辐合产生下沉运动使动量下传到对流层低层,有利于产生大风;在高、低空急流相叠置的地区由于强烈不稳定而产生深厚对流,容易形成强降雪。在制作暴风雪天气预报时,应主要以乌拉尔山高脊、蒙古冷涡、高、低空急流及其耦合作用等为着眼点,根据它们的发生、发展、演变、配合等情况做出预报。

Snowstorm is one phenomenon with the sharp dropping of temperature,strong wind and heavy snowfall.It is certainly difficult to forecast because weather conditions of snowstorm formation are harsh and its occurrence frequency is lower.This research studied climatic,synoptic and dynamic features of snowstorm cases in recent 40years on Wulate-Damao grasslands in Inner Mongolia,and summarized several key predictors.The result showed that snowstorm took on the tendency of definitely decreasing.Weather circulation background for snowstorm could be divided into three types： small trough developing,low trough moving eastward and transverse trough rotating upright; these types also associated with the development and movement southward of Mongolian cold vortex.Form the view of synoptic characteristics,snowstorm had the same circulation features as cold wave,precipitation and strong wind,it was noted that meridional circulation was obvious and wulaer ridge developed vigorously in front of which was strong northwest jet,and also there was a deep low vortex or transverse trough in Mongolia characterizing as strong cold center and front.Warm and wet airflow were very active in low level accompanying warm advection and southerly low-level jet with a shear line in front left side.The surface chart was one Mongolian cyclone or long narrow transverse trough with front.From the aspect of dynamic features,the interconnection between upper and low-level jet was distinct,it was favorable for strong wind that descending motion out of convergence on left side of the exit of jet aloft made momentum transmitting downward to lower troposphere.The overlaying region between upper and low-level jet produced deep convection which helped for strong snowfall because of powerful instability.Wulaer ridge,Mongolian cold vortex,upper and low-level jet and their interaction according to which for the forecast of snowstorm will be for key points.