Gravity Waves Generated by Sheared Three-Dimensional Potential Vorticity Anomalies
Gravity Waves Generated by Sheared Three-Dimensional Potential Vorticity Anomalies
复制标题
剪切三维位涡异常产生的重力波
DOI：
10.1175/jas-d-11-0296.1
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发表时间：
2012
影响因子：
3.1
通讯作者：
Plougonven R
中科院分区：
文献类型：
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作者：
Plougonven R
在恒定垂直切变、恒定分层和无界域假设下，研究了三维位涡异常产生的重力波。与前面文章分析的二维情况一样，PV异常附近的扰动可以用准地转理论很好地模拟。在较大的距离上，扰动的性质在位于异常上方和下方的两个惯性层之间发生变化，并且它采取垂直传播GW的形式。对于深度无穷小的水平单色PV异常，用精确解和WKB近似对扰动进行了解析描述；后者包括一个指数小项，它捕获了远场辐射边界条件引起的PV异常附近解的变化。分析结果表明，发射对理查德森数和水平波数的方向有很强的敏感性：惯性层的吸收特性使得北半球的波数在与切变负夹角时发射最大。对于局域PV异常，数值计算给出了GW场的时间演化。分析和数值结果还用于建立Eliassen-Palm通量的显式形式，该通量可用于参数化gcm中的GW源。Eliassen-Palm通量矢量的性质表明，在西风切变中，GWs在上层惯性层向西南方向施加阻力，在高空消散的高度向西北方向施加阻力。
The gravity waves (GWs) produced by three-dimensional potential vorticity (PV) anomalies are examined under the assumption of constant vertical shear, constant stratification, and unbounded domain. As in the two-dimensional case analyzed in an earlier paper, the disturbance near the PV anomaly is well modeled by quasigeostrophic theory. At larger distances the nature of the disturbance changes across the two inertial layers that are located above and below the anomaly, and it takes the form of a vertically propagating GW beyond these.For a horizontally monochromatic PV anomaly of infinitesimal depth, the disturbance is described analytically using both an exact solution and a WKB approximation; the latter includes an exponentially small term that captures the change of the solution near the PV anomaly induced by the radiation boundary condition in the far field. The analytical results reveal a strong sensitivity of the emission to the Richardson number and to the orientation of the horizontal wavenumber: the absorptive properties of the inertial layers are such that the emission is maximized in the Northern Hemisphere for wavenumbers at negative angles to the shear.For localized PV anomalies, numerical computations give the temporal evolution of the GW field. Analytical and numerical results are also used to establish an explicit form for the Eliassen–Palm flux that could be used to parameterize GW sources in GCMs. The properties of the Eliassen–Palm flux vector imply that in a westerly shear, the GWs exert a drag in a southwest direction in the upper inertial layer, and in a northwest direction at the altitudes where the GWs dissipate aloft.