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The Observing System Research and Predictability Experiment (THORPEX) was a 10-yr, international research program organized by the World Meteorological Organization's World Weather Research Program. THORPEX was motivated by the need [...]

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. DOI: 10.1175/2010JAS3499.1

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). DOI: 10.1175/2010JAS3500.1

The nonlinear reflection of an isolated Rossby wave train at a low-latitude wave-breaking region is contrasted with the more familiar longitudinally periodic case. General theoretical arguments for nonlinear reflection based on absorptivity bounds do not carry over to the case of an isolated wave train, and detailed investigation is needed to determine the absorption-reflection behavior. Numerical experiments in a single-layer shallow-water model show that wave activity is reflected back into midlatitudes (rather than propagating longitudinally at low latitudes). Finite-amplitude wave activity diagnostics are used to analyze the nonlinear reflection. Further idealized numerical simulations and simple ideas concerning the propagation of Rossby waves in shear flows are used to give insight into the nonlinear reflection.

It has been shown that the linearized equations for disturbances to a parabolic jet on a [Beta] plane, with curvature [Mathematical Expression Omitted] such that the basic-state absolute vorticity gradient [Mathematical Expression Omitted] is zero, ultimately become inconsistent in the neighborhood of the jet axis and that nonlinear effects become important. Numerical solutions of the nonlinear long-time asymptotic form of the equations are presented. The numerical results show that the algebraic decay of the disturbances as [t.sup.-1/2] predicted by the linear equations is inhibited by the nonlinear formation of coherent vortices near the jet axis. These lead to a disturbance amplitude that decays only through the action of weak numerical diffusion but is otherwise as [t.sup.0]. The linear theory is extended to the case when the basic-state absolute vorticity gradient is nonzero but weak. When the gradient is weak and negative the decay is modified and is ultimately as [t.sup.-3/2]. When the gradient is weak and positive, on the other hand, a discrete eigenmode is excited and asymptotic decay is inhibited. In both cases linear theory may give a self-consistent description if the amplitude is small enough. Numerical simulation shows that for both signs of the gradient there is a range of amplitudes for which nonlinear effects become directly important. Coherent vortices may form and either inhibit the decay or disrupt the linear mode. The structure of the nonlinear analog of the linear eigenmode is analyzed and shown to have a propagation speed, relative to the jet axis speed, that is a decreasing function of amplitude, tending to zero as the amplitude approaches a finite limiting value.