Three-dimensional instabilities in tornado-like vortices with secondary circulations
Three-dimensional instabilities in tornado-like vortices with secondary circulations
复制标题
具有二次环流的龙卷风状涡旋的三维不稳定性
DOI：
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发表时间：
2012
影响因子：
3.7
通讯作者：
D. Nolan
中科院分区：
文献类型：
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作者：
D. Nolan
龙卷风和其他强烈的大气涡旋在其大环流中偶尔会转变为具有多个涡旋的流动结构。这种现象一直被认为是由于内核环流的流体动力学不稳定性，许多先前的研究已经诊断出类似于观测结构的龙卷风状涡旋的低波数不稳定模态。然而，相对而言，这些研究中很少将内部核心循环的强垂直运动纳入稳定性分析，并且没有使用完整的、摩擦驱动的、靠近地面的强径向流入的二次循环进行稳定性分析。利用理想化模拟的类龙卷风涡所产生的完整环流，给出了稳定性分析。在这些模拟中发现了与不对称结构一致的快速不稳定模式。Howard & Gupta和Leibovich & Stewartson推导出的轴向射流涡的不稳定条件，试图将这些模态的结构和位置与不稳定条件联系起来，结果却不一致。对扰动能量增长的分析表明，涡旋通量与方位风径向切变之间的相互作用对优势模态的增长贡献很小。相反，垂直风的径向切变和垂直风的垂直切变（对应于轴向的变形）是微扰增长的主要能量来源。相对较弱的轴对称不稳定性也被发现与在龙卷风中观察到的对称振荡有一些相似之处。
Abstract Tornadoes and other intense atmospheric vortices are known to occasionally transition to a flow structure with multiple vortices within their larger circulations. This phenomenon has long been ascribed to fluid dynamical instability of the inner-core circulation, and many previous studies have diagnosed low-wavenumber unstable modes in tornado-like vortices that resemble the observed structures. However, relatively few of these studies have incorporated the strong vertical motions of the inner-core circulation into the stability analysis, and no stability analyses have been performed using a complete, frictionally driven secondary circulation with strong radial inflow near the surface. Stability analyses are presented using the complete circulations generated from idealized simulations of tornado-like vortices. Fast-growing unstable modes are found that are consistent with the asymmetric structures present in these simulations. Attempts to correlate the structures and locations of these modes with instability conditions for vortices with axial jets derived by Howard & Gupta and by Leibovich & Stewartson produce only mixed results. Analyses of perturbation energy growth show that interactions between eddy fluxes and the radial shear of the azimuthal wind contribute very little to the growth of the dominant modes. Rather, the radial shear of the vertical wind and the vertical shear of the vertical wind (corresponding to deformation in the axial direction) are the primary energy sources for perturbation growth. Relatively weak axisymmetric instabilities are also identified that have some similarity to symmetric oscillations that have been observed in tornadoes.