摘要
使用纬向基流下横波型扰动的Boussinesq近似方程组,分析了这种沿着基本气流方向传播的中尺度扰动发生不稳定时,大尺度背景流场在垂直方向上的各种分布特征。在大气层结比较稳定的情况下,如果基本气流在低层和高层较大(有可能存在低空急流和高空急流),此时产生的β中尺度不稳定扰动相对于基流向东传播,甚至于快速向东传播。基本气流在垂直方向上的风速切变对于中尺度横波型的扰动起着不稳定的作用。如果考虑基流的二次切变,可以得到涡旋Rossby波的相速度表达式,涡旋Rossby波相对于基本气流是单向传播的。涡旋Rossby波产生的物理根源是基本流场的风速二次切变,亦即基本流场y方向的平均涡度在空间z方向上的不均匀所致。涡旋Rossby波的相速度与纬向波数也有关,它的能量是频散的,其在纬向x方向也存在群速度。在基本流场的风速存在二次切变时,横波型不稳定可能是混合的涡旋Rossby重力波的不稳定;而在基本流场的风速仅仅存在线性切变,不存在二次切变时,横波型扰动的不稳定则是重力惯性波的不稳定。
In the context of the Boussinesq approximation equations for transversal wave-type （TWT） disturbances in zonal basic flow, it is analysed that the various features in vertical distribution of a large-scale atmospheric background flow field when a meso-scale disturbance propagating along basic flow becomes unstable. Evidence suggests that 1） under more stable stratification, the basic flow, if moving faster at low and high levels （particularly in the presence of jets there）, allows a resulting meso-βunsteady wave to propagate eastward with respect to basic flow and even at greater velocity compared with it; 2） vertical windspeed shear of basic flow causes instabilities of the TWT perturbation; 3） considering the second derivative of basic-flow wind with respect to z （denoted by Uzz≠0 which is simply given as β＊ hereafter） the expression for the phase velocity of vortex Rossby wave （VRoW） is ob- tained, which is unidirectional in propagation with,respect to basic flow; 4） VRoW has its physical origin from β＊. ,i. e, , from z-varying heterogeneities of y-direction averaged vorticity of the basic flow field; 5） VRoW phase velocity is associated with zonal wave number k, its energy is dispersive and the group velocity exists in the x direction; 6） when windspeed meets the condition of β＊ , TWT disturbance instability may be that of mixed VRoW and gravity wave; 7） if basic flow is subject to linear shear but does not meet the condition of β＊ , the TWT instability is that of inertia-gravity wave.
出处
《大气科学》
CSCD
北大核心
2005年第5期727-733,共7页
Chinese Journal of Atmospheric Sciences
基金
国家重点基础研究发展规划项目2004CB418301
国家自然科学基金资助项目40433007
40075023
上海台风研究基金课题
关键词
β中尺度暴雨系统
横波不稳定
涡旋ROSSBY波
基流二次切变
meso-β, rainstorm system, transversal-wave-type instability, vortex Rossby wave, the second deriva-tive of basic flow with respect to z
