On the symmetrization rate of an intense geophysical vortex |
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| Institution: | 1. Advanced Study Program, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA;2. Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA;1. Department of Bioengineering, University of Utah, USA;2. Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, USA;3. Department of Biology, University of Utah, USA;4. Department of Cardiology, University of Washington, USA;1. School of Science, Chongqing University of Posts and Telecommunications, Chongqing, China;2. Atmospheric and Planetary Sciences, Hampton University, Hampton, VA, USA;3. ESSIC, University of Maryland, College Park, USA;4. State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China;5. College of Mathematics and Information Science, Henan Normal University, Xinxiang, China;1. College of Physical Sciences and Technology, Yangzhou University, 22500, Yangzhou, China;2. Chinese Academy of Meteorological Science, 100081, Beijing, China;3. Henan Meteorological Institute, Dongminglu, Beiduan, Zhengzhou, Henan, China;4. Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing 100081, China;1. A. M. Obukhov Institute of Atmospheric Physics RAS, 109017 Moscow, Russia;2. Space Research Institute RAS, 117997 Moscow, Russia |
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| Abstract: | Numerical models demonstrate that a broad class of geophysical vortices freely evolve toward vertically aligned, axisymmetric states. In principle, this intrinsic drive toward symmetry opposes destructive shearing by the environmental flow.This article examines the case in which a discrete vortex-Rossby-wave dominates a perturbation from symmetry, and symmetrization occurs by decay of the wave. The wave is damped by a resonance with the fluid rotation frequency at a critical radius, r*. The damping rate is proportional to the radial derivative of potential vorticity at r*. Until now, the theory of resonantly damped vortex-Rossby-waves (technically quasi-modes) was formally restricted to slowly rotating vortices, which obey quasigeostrophic (QG) dynamics. This article extends the theory to rapidly rotating vortices.The analysis makes use of the asymmetric balance (AB) approximation. Even at a modest Rossby number (unity), AB theory can predict damping rates that exceed extrapolated QG results by orders of magnitude. This finding is verified upon comparison of AB theory to numerical experiments, based on the primitive equations. The experiments focus on the decay of low azimuthal wave-number asymmetries.A discrete vortex-Rossby-wave can also resonate with an outward propagating inertia-buoyancy wave (Lighthill radiation), inducing both to grow. At large Rossby numbers, this growth mechanism can be dynamically relevant. All balance models, including AB theory, neglect inertia-buoyancy waves, and therefore ignore the possibility of a Rossby-inertia-buoyancy (RIB) instability. This article shows that a large potential vorticity gradient (of the proper sign) at the critical radius r* can suppress the RIB instability, and thereby preserve balanced flow, even at large Rossby numbers. |
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