Diagnostic study of hurricane asymmetries using empirical normal modes.

摘要

Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls and mesovortices, have long been observed in radar imagery, many aspects of their dynamics still remain unsolved, particularly in the formation of the secondary eyewall. To fill this gap, a simple 2D barotropic "dry" model and the high-resolution PSU-NCAR non-hydrostatic mesoscale model (MM5) are used to study hurricane asymmetries. The Empirical Normal Mode (ENM) and the newly developed Space-Time Empirical Normal Mode (ST-ENM) techniques, together with Eliassen-Pahn (EP) flux calculations, are used to extract wave modes from the model generated datasets to investigate their impact on the changes in the structure and intensity of the simulated hurricanes.The role of internal dynamics on Concentric Eyewall Genesis (CEG) is further evaluated using the full physics MM5 simulation. The leading modes of the ST-ENM diagnostics exhibit mainly characteristics of VRWs and their contribution to the EP flux divergence induced two regions of maximum tangential wind acceleration one inside the primary eyewall which accounts for eyewall contraction and the other outside the primary eyewall which explains the development of the secondary eyewall. A signal of maximum eddy angular momentum propagating outwards to the critical radius of the mode suggests a redistribution of angular momentum and potential vorticity (PV) re-arrangement around that area. The fact that the critical radius for some of the leading modes is close to the location where the secondary eyewall eventually develops suggests that a wave-mean flow interaction mechanism may be suitable to explain important dynamical aspects of the CEG.From the ENM diagnostics of the 2D simulations, it is shown how an incipient storm described by a vortex monopole intensifies by "inviscid damping" of a "discrete-like" vortex Rossby wave (VRW) or quasimode. The critical radius, the structure, and the propagating properties of the quasimode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. The fastest growing wavenumber-4 unstable VRM modes of a vortex ring reminiscent of a mature hurricane are extracted and their relation with the polygonal eyewalIs, mesovortices, and the asymmetric eyewall contraction established. When asymmetric disturbances are placed outside a strong vortex ring with a large vorticity skirt they relax to form concentric rings of enhanced vorticity that contain a secondary wind maximum.