一次暴风雪过程中的中尺度重力波特征及其影响

An investigation into the features and effects of the mesoscale gravity waves as in a snowstorm event

应用地面自动气象站观测资料、数字化多普勒天气雷达探测资料和WRFV2.2.1中尺度数值模拟资料,分析了中尺度重力波与基本气流的相互作用,以及重力波活动对暴雪和大风天气的重要影响。结果表明,在波导中传播的中尺度重力波能够与基本气流进行动量交换,使得对流层中上层4.5—8 km气层内的水平平均风速趋于均匀,形成斜穿整个对流层的饱和湿空气急流,即"湿急流"。在高空急流出口区激发的垂直向下传播的重力波,使基本气流的水平风速在垂直方向上出现了加速和减速的交替变化,水平风加速的气层,反射率增大;水平风减速的气层,反射率减小。随着波动下传及其随基本气流的移动,反射率回波强度沿高空风的方向(由西南向东北)出现周期性变化,回波带呈西北—东南走向,强回波中心之间为宽约40 km的弱回波区。重力波下传期间,当地面气压迅速下降时,东北风快速增长,风向有明显的改变,反射率强度开始减弱;气压脊线过后,反射率降低到最低点。地面大风中心出现在反射率回波强度周期性变化的地带,沿西南—东北方向间隔着分布。雷达探测表明,对流层低层风速在风向切变层上下边界对称相等,因此推测在重力波与切变层汇合的高度层存在垂直环流,由风切变层上下边界附近的西南气流和东北气流与受重力波影响形成的垂直方向上的上升和下沉气流共同组成。切变层上方的动量通过垂直环流的下沉支到达地面,强风中心对应着下沉气流,出现在降水回波开始减弱之际。

Based on the data from the surface automated stations,the digital Doppler weather radar,and the WRFV2.2.1 model simulation,the interaction between the mesoscale gravity wave and the basic flow in the troposphere is studied,and the role of the waves in the severe snowfall and high surface winds is also investigated.It is shown by the radar data,the mean horizontal wind velocity in the 4.5-8 km layer of the troposphere tended to be uniform because of the momentum exchange between the wave motion and the basic current.At the same time,a saturated moisture jet stream was established extending almost throughout the troposphere as seen in the simulation,with the most severe precipitation area located just ahead of the 850 hPa jet center.Waves inspired under the exit region of an upper layer jet stream near the tropopause propagate downwards,resulting in an alternative acceleration or deceleration of the horizontal wind velocity in vertical over the station.The divergence or confluence caused by the wind change with the height has a substantial effect on the base reflectivity,and the radar echoes show the gravity wave signature.It is found that considerable increase in the intensity of the echoes occurred in the layers with the accelerating flow,but decreased in the layers with the decelerating flow,with about 40 km wide weak precipitation band imbedded in the major cloud bands,in a northwest-southeast orientation.While the waves propagate downwards,it was observed that,the gusty surface northeasterly wind appeared as the surface pressure drops rapidly,with an obvious change in the wind direction,and the intensity of the echoes begins to be weakened;the minimum reflectivity occurred just after the ridge passage.Observations from the radar show,the wind velocities beyond the boundary layer of the wind shear have the similar values,suggesting that the vertical circulation should be formed in the layer where the waves jointed to the wind shear.These vertical circulation would be established by the updrafts and downdrafts due to the vertically propagating gravity waves,together with the airflows moving in the opposite direction beyond the shear layer boundary.The observed severe surface winds centers distributed in a southwest-northeast orientation as in the gravity wave signature similar to the features of the echoes.The surface winds should be accelerated significantly by the downdrafts of these vertical circulation.