Stimulated generation: extraction of energy from balanced flow by near-inertial waves
Stimulated generation: extraction of energy from balanced flow by near-inertial waves
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DOI：
10.1017/jfm.2018.308
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发表时间：
2018-05
影响因子：
3.7
通讯作者：
C. Rocha;G. Wagner;W. Young
中科院分区：
文献类型：
--
作者：
C. Rocha;G. Wagner;W. Young
我们利用正压准地转流和近惯性波耦合的渐近模型研究了受激波的产生--能量从平衡流到现有内波的转移。通过对垂直平面波模型守恒定律的详细描述，阐明了垂直涡度和横向应变的激发产生机制。波势能和相应的平衡动能汇有两个来源：波作用密度向反气旋的折射会聚(和从气旋发散)；通过地转应变增强波场梯度。我们利用初始均匀惯性振荡与成熟的自由演化二维湍流相互作用的数值解来量化这些能量传递并描述受激产生的现象。在所有的解中，刺激生成与平衡动能通过涡旋合并向大尺度的转移共存。此外，地转应变占波势能产生的大部分，相当于初始平衡动能的10%-20%。然而，折射是基本的，因为它在近惯性场中产生初始的涡度横向梯度，然后被平流增强。在这些准无粘解中，波的频散是扰乱激发产生的唯一机制：在正压平衡流动中，横向应变增强了波群速度，因此波加速并迅速逃离应变区。这种波的逃逸防止了波能向耗散尺度的级联。
We study stimulated generation – the transfer of energy from balanced flows to existing internal waves – using an asymptotic model that couples barotropic quasi-geostrophic flow and near-inertial waves with $\text{e}^{\text{i}mz}$ vertical structure, where $m$ is the vertical wavenumber and $z$ is the vertical coordinate. A detailed description of the conservation laws of this vertical-plane-wave model illuminates the mechanism of stimulated generation associated with vertical vorticity and lateral strain. There are two sources of wave potential energy, and corresponding sinks of balanced kinetic energy: the refractive convergence of wave action density into anti-cyclones (and divergence from cyclones); and the enhancement of wave-field gradients by geostrophic straining. We quantify these energy transfers and describe the phenomenology of stimulated generation using numerical solutions of an initially uniform inertial oscillation interacting with mature freely evolving two-dimensional turbulence. In all solutions, stimulated generation co-exists with a transfer of balanced kinetic energy to large scales via vortex merging. Also, geostrophic straining accounts for most of the generation of wave potential energy, representing a sink of 10 %–20 % of the initial balanced kinetic energy. However, refraction is fundamental because it creates the initial eddy-scale lateral gradients in the near-inertial field that are then enhanced by advection. In these quasi-inviscid solutions, wave dispersion is the only mechanism that upsets stimulated generation: with a barotropic balanced flow, lateral straining enhances the wave group velocity, so that waves accelerate and rapidly escape from straining regions. This wave escape prevents wave energy from cascading to dissipative scales.