与东北冷涡相伴的高空急流诱发平流层重力波的数值模拟研究

使用中尺度数值模式WRF-ARW，针对2010年6月发生在中国东北地区一例伴随对流层高空西风急流（位于~9 km高度）演变过程出现的平流层重力波活动特征开展了数值模拟. 事件发生期间，对流层区域环流处在一个东北冷涡系统的控制之下. 模拟结果再现了该东北冷涡的发展和维持过程，以及与之相伴的高空急流的特征. 模拟结果揭示出在急流区域上空的平流层中存在显著重力波活动现象. 分析结果显示，重力波活动与急流存在紧密联系，在水平方向上，重力波呈显著的二维结构，出现在急流出口区上部并逆背景流向西传播. 功率谱分析结果表明盛行波动具有~700 km水平尺度、9~12 h时间尺度以及4~5 km垂直波长. 由于急流的存在，造成其与平流层中下部之间存在显著的水平风速垂直切变，与切变相伴的耗散使得上传的重力波动量通量数值随着高度升高而递减. 同时，在18~20 km高度间出现的西风-东风转换带极大地抑制了波动在垂直方向的传播，形成显著动量通量沉积效应. 估算结果表明，在11~20 km高度之间，这种效应的整体作用相当于对该层背景流施加强度为0.86 m·s^-1·day^-1的动力阻曳.

Simulation of the generation of stratospheric gravity waves in upper-tropospheric jet stream accompanied with a cold vortex over Northeast China

The stratospheric gravity waves (GWs) activity accompanying an upper-tropospheric jet stream is simulated by using a meso-scale weather forecast model, i.e., the WRF-ARW. The simulation is conducted with a Northeast Cold Vortex, i.e., a cut-off-low weather system, which develops over northeastern China in June 2010. The simulation reproduces the key features of the development of the cold vortex, as well as the upper tropospheric jet stream evolving at around 9 km height. The results further reveal that pronounced stratospheric GWs are generated by the jet stream as they emerge from the exit region of jet stream. These GWs exhibit 2-D structure, and propagate preferentially in the upstream of the background winds which is just over the jet stream. Spatio-temporal spectral investigation results show that the predominant waves exhibit horizontal wavelength of ~700 km, period of 9~12 h, and vertical wavelength of 4~5 km. The presence of the jet stream results in strong vertical shear in background flow in the lower and middle stratospheric range over the jet stream. The shear further results in wave dissipation as the momentum flux of the GWs declines with height. Inhibition of GW propagation is disclosed by the great attenuation of GW momentum flux at around 18~20 km where the transition of easterlies and westerlies happens. It is anticipated that the upward propagating GWs may approach their critical level in the region thus deposit all their momentum fluxes there. Consequently, the bulk GWs drag on the mean flow in 11~20 km height range is estimated as around 0.86 m·s^-1·day^-1.