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A novel statistical technique called space--time empirical normal mode (ST-ENM) is applied in a diagnostic study of the genesis of a secondary eyewall in a simulated hurricane using the nonhydrostatic, high-resolution fifth-generation Pennsylvania State University (PSU)--National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). The bases obtained from the ST-ENM technique are nonstationary, dynamically relevant, and orthogonal in the sense of wave activity. The wave activity spectra of the wavenumber-1 anomalies show that the leading modes (1--6) exhibit mainly characteristics of vortex Rossby waves (VRWs). These modes together explain about 75%% of the total wavenumber-1 variance in a period of 24 h. Analysis of the Eliassen--Palm (EP) flux and its time-mean divergence corresponding to the total contribution from these modes indicated that in the lower troposphere VRWs not only propagate inward (outward) in the primary eyewall region where the radial gradient of the basic-state potential vorticity is large and positive (large and negative), but there is also wave activity propagating outside the primary eyewall. Consequently, maximum cyclonic eddy angular momentum is transported not only inside the radius of maximum wind (RMW) by VRWs in the primary eyewall region, but also close to the location where the secondary eyewall forms by VRWs propagating outside the inner eyewall. The fact that the critical radius for some of the ST-ENMs is contained inside the region where the secondary eyewall forms and the existence of a signal of maximum eddy cyclonic angular momentum flux propagating outward up to the critical radius suggests that a wave--mean flow interaction mechanism and redistribution of angular momentum may be suitable to explain important dynamical aspects of concentric eyewall genesis. [ABSTRACT FROM AUTHOR]

Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls, and mesovortices, have long been observed in radar and satellite imagery, many aspects of their origin, space--time structure, and dynamics still remain unsolved, particularly their role on the vortex intensification. The underlying inner-core dynamics need to be better understood to improve the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic ''dry'' model is used to perform two experiments starting respectively with a monopole and a ring of enhanced vorticity to mimic hurricane-like vortices during incipient and mature stages of development. The empirical normal mode (ENM) technique, together with the Eliassen--Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their space--time structure, kinematics, and the impact on the changes in the structure and intensity of the simulated hurricane-like vortices. From the ENM diagnostics, it is shown in the first experiment how an incipient storm described by a vortex monopole intensifies by ''inviscid damping'' of a ''discrete-like'' vortex Rossby wave (VRW) or quasi mode. The critical radius, the structure, and the propagating properties of the quasi mode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. In the second experiment, the fastest growing wavenumber-4 unstable VRW modes of a vortex ring reminiscent of a mature hurricane are extracted, and their relation with the polygonal eyewalls, mesovortices, and the asymmetric eyewall contraction are established in consistency with results previously obtained from other authors. [ABSTRACT FROM AUTHOR]

Despite the fact that asymmetries in hurricanes (e.g., spiral rainbands, polygonal eyewalls, and mesovortices) have long been observed in radar and satellite imagery, many aspects of their dynamics remain unsolved, particularly in the formation of the secondary eyewall. The underlying associated dynamical processes need to be better understood to advance the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic 'dry' model is used to simulate a hurricane-like concentric rings vortex. The empirical normal mode (ENM) technique, together with Eliassen-Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their impact on the changes in the structure and intensity of the simulated hurricane-like vortex. From the ENM diagnostics, it is shown that asymmetric disturbances outside a strong vortex ring with a large vorticity skirt may relax to form concentric rings of enhanced vorticity that contain a secondary wind maximum. The fact that the critical radius for some of the leading modes is close to the location where the secondary ring of enhanced vorticity develops suggests that a wave-mean flow interaction mechanism based on vortex Rossby wave (VRW) dynamics may explain important dynamical aspects of concentric eyewall genesis (CEG). [ABSTRACT FROM AUTHOR]