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arXiv 2008.02203physics.flu-dynphysics.ao-ph
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地转turbulence对惯性重力波的散射

Inertia-gravity-wave scattering by geostrophic turbulence

Miles A. C. Savva, Hossein A. Kafiabad, Jacques Vanneste

摘要:

在大气和海洋中的旋转分层流中,惯性重力波(IGW)常常与地转平衡的湍流共存。平流和折射由这个流导致波散射,重新分配IGW能量的位置-波数相空间。通过推导IGW相空间能量密度演化的动力学方程,详细描述了这一过程。推导依赖于表征地转流的Rossby数的小,它被视为具有已知统计量的随机场,并且不假设空间尺度分离。动力学方程描述的能量转移,被限制到IGW具有相同的频率,作为一个结果的时间尺度之间的分离波和流。我们制定的动力学方程上的恒定频率的表面-一个双锥波数空间-使用极球坐标,我们研究的两个散射截面的形式,其中量化IGW之间的能量转移,分别相同和相反的垂直传播方向。动力学方程捕获了水平各向同性和散射导致的跨尺度能量级联。我们把注意力集中在后者,以评估直接模拟的三维Boussinesq方程的动力学方程的预测,找到良好的协议。

英文摘要:

In rotating stratified flows including in the atmosphere and ocean, inertia-gravity waves (IGWs) often coexist with a geostrophically balanced turbulent flow. Advection and refraction by this flow lead to wave scattering, redistributing IGW energy in the position--wavenumber phase space. We give a detailed description of this process by deriving a kinetic equation governing the evolution of the IGW phase-space energy density. The derivation relies on the smallness of the Rossby number characterising the geostrophic flow, which is treated as a random field with known statistics, and makes no assumption of spatial scale separation. The kinetic equation describes energy transfers that are restricted to IGWs with the same frequency, as a result of the timescale separation between waves and flow. We formulate the kinetic equation on the constant-frequency surface -- a double cone in wavenumber space -- using polar spherical coordinates, and we examine the form of the two scattering cross sections involved, which quantify energy transfers between IGWs with, respectively, the same and opposite directions of vertical propagation. The kinetic equation captures both the horizontal isotropisation and the cascade of energy across scales that result from scattering. We focus our attention on the latter to assess the predictions of the kinetic equation against direct simulations of the three-dimensional Boussinesq equations, finding good agreement.

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