On the dynamics of the origin and impact of asymmetric polygonal eyewalls and mesovortices in rapidly intensifying realistic simulated hurricanes

In this study, the output data of a high resolution realistic simulation of hurricane Wilma (2005) are used, along with the theory of empirical normal modes (ENM), and the Eliassen-Palm (EP) flux theorem to address; the underlying mechanism responsible for the formation of prominent polygonal eyewalls and mesovortices found in Wilma while it was rapidly intensifying, and the impact of these asymmetries on the intensity evolution of the hurricane vortex. It is found that the eyewall of Wilma exhibited an early azimuthal wavenumber 4 (m = 4) asymmetry followed by a transition to lower wavenumber asymmetries. From the simulated reflectivity and the spatial structure of potential vorticity (PV) anomalies, it is suggested that barotropic instability is most likely the driving mechanism for these asymmetries. The dominant modes for m = 4, and 3 asymmetries obtained from the ENM analysis are found to be vortex Rossby waves (VRWs) that possess characteristics of unstable modes, further supporting the importance of barotropic instability. The EP calculations associated with these modes indicate that the VRWs act to decelerate the flow at the initial radius of maximum wind, while they act to accelerate the flow radially inside and outside of this location, suggesting that VRWs may have a positive feedback on the intensification of the vortex. The present results complements the previous findings of theoretical and highly idealized numerical studies in that polygonal eyewalls and mesovortices are the result of barotropic instability, and therefore it can be considered to be a bridge between idealized studies and observations. It also provides a new inside on the role of asymmetries and VRWs in the intensification of hurricanes.