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Instabilities in hurricane-like boundary layers

机译：类飓风边界层的不稳定性

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Recent advances in observational technology have led to a more detailed knowledge of the low-level flow in hurricanes. In particular, quasi-streamwise rolls on a variety of scales have been observed. Some of these rolls have radial wavelengths of 4-10 km, which is comparable to rolls associated with instabilities inherent to Ekman-type boundary layers. The evolution and stability of the swirling boundary layer underneath a hurricane-like vortex is studied using both a nonlinear model and linearized stability analysis. The nonlinear model is an axisymmetric model of incompressible fluid flow, which is used to simulate the development of boundary layers underneath vortices with hurricane-like wind profiles. Axisymmetric rolls appear in these boundary layers, which have some similarities to the observed rolls in hurricanes. The axisymmetric flow is also used as the basic-state for a linearized stability analysis. The analysis technique allows for arbitrary variation in the radial and vertical directions for both the basic-state flow and the perturbations. Thus, the strong radial variations and curvature effects common to strong vortices are part of the analysis. The analysis finds both symmetric and asymmetric instabilities that are similar to those in the nonlinear simulations and in observations. The instabilities acquire some of their energy from the vertical shear associated with a reversal of the radial inflow at the top of the boundary layer, and some of their energy from vertical shear of the azimuthal flow. The radial flow energy conversion tends to increase for flows with less inertial stability and for modes oriented across the low-level shear; the azimuthal flow conversion increases for larger inertial stability and for modes aligned with the low-level shear.
机译：观测技术的最新进展已导致对飓风的低空流量有了更详细的了解。特别地，已经观察到各种尺度的准流向辊。这些轧辊中的一些轧辊的径向波长为4-10 km，这与与埃克曼型边界层固有的不稳定性相关的轧辊相当。利用非线性模型和线性化稳定性分析，研究了飓风状涡旋下旋流边界层的演化和稳定性。非线性模型是不可压缩流体流动的轴对称模型，用于模拟具有飓风状风廓线的涡旋下边界层的发展。轴对称轧辊出现在这些边界层中，与飓风中观测到的轧辊有一些相似之处。轴对称流也用作线性稳定性分析的基本状态。分析技术允许基本状态流和扰动在径向和垂直方向上任意变化。因此，强旋涡常见的强径向变化和曲率效应是分析的一部分。分析发现对称和非对称不稳定性都与非线性模拟和观测中的不对称相似。这些不稳定性从与边界层顶部的径向流入逆转相关的垂直切变中获取一些能量，并从方位角流的垂直切变中获取一些能量。对于惯性稳定性较差的流以及在低剪切方向上定向的模式，径向流能量转换趋于增加。对于较大的惯性稳定性以及与低剪切力对齐的模式，方位角流量转换会增加。

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