Jet stream is an elongated belt of high speed air , typically found in upper tropospheric westerly flow round the globe . a jet stream may stretch over tens of thousands of kilometres in length from west to east and a few hundred kilometres in width from north to south 高空急流是围绕地球的一条强而窄的高速气流带，集中在对流层上部或平流层中，在中高纬西风带内或在低纬度地区都可出现。
The windshear and turbulence warning system wtws was not able to warn the windshear associated with the elevated low level jet since the system used the lowest elevation scan 0 . 6 deg of the tdwr to calculate the windshear , which did not intercept the jet stream within the warning area 因为wtws利用tdwr的最低仰角0 . 6度扫描来计算风切变，而该扫描在警报范围内未遇到急流，所以这系统未能就这低空急流引起的风切变发出警报。
The analysis for the evolution of the upper westerly field , north - south wind field show that the upper jet stream is a good indication to sandstorm - occurring areas , the strong sandstorm appears at the right side of the entry domain of the jet stream and the ascension domain of secondary circulation 对高空东西风场、南北风场的演变进行分析，发现高空急流对沙尘暴发生区有很好的指示作用，强沙尘暴位于200hpa高空急流入口区右侧、 500hpa正涡度中心下风方和次级反环流的上升区域内。
We can use the mm5 model in definite time , definite area weather forecasting . we conclude that : : the moving route of this necw in its developing progress was direct - south along latitude , the cold air is rough , and just this made the rough vertical velocity ; the necw southern moving and conflicting with the west pacification sub - tropical high makes rough jet steam . the plenty south - west warm and wet air made by the jet stream gave the rainstorm demanded vapor ; . the rough vertical velocity by jet stream is a main reason in this rainstorm ; the mm5 model uses four - direction analysis data as its input , this avoids some errors occurred in observe and transfer , thus made the result more useful 综合分析认为： ( 1 ) 、这次冷涡在其发生发展过程中，其移动路径是沿经圈直线南压，携带的冷空气势力较强，形成槽后较强的垂直运动； ( 2 ) 、西太平洋副热带高压偏北位置与东北冷涡南压形成的急流轴，把源源不断的西南暖湿气流输送到辽西地区，也就是急流轴的左前方位置，为这次暴雨的产生提供了充足的水汽来源； ( 3 ) 、低空急流不但为暴雨输送水汽，其造成的较强的垂直环流也是产生强降水的一个主要原因； ( 4 ) 、 mm5模式以四维同化资料作为初始场，最大限度地避免了观测误差可能造成的积分不稳定，提高了模拟结果的参考价值。
