Secondary Eyewall Formation and Convectively-Generated Potential Vorticity in Rainbands in Hurricane Rita

Formation of concentric eyewalls and eyewall replacements in mature tropical cyclones (TCs) often result in intensity fluctuations and are difficult to predict. Many previous studies have focused on dynamic processes in the inner core of a TC and effects of certain specified potential vorticity (PV) distributions that would lead to the formation of a secondary eyewall. However, these studies cannot explain why some intense TCs develop secondary eyewalls and others do not. Furthermore, they cannot address how a particular PV distribution specificed in these studies could develop in real TCs. This study aims to address these questions using high-resolution, full physics model forecast fields of Hurricanes Katrina and Rita (2005) to better understand convectively-generated PV in the outer rainband region and its role in the formation of a secondary eyewall in Hurricane Rita. Katrina and Rita were extensively observed by three research aircraft during the Hurricane Rainband and Intensity Change Experiment (RAINEX) that was designed to study the interaction of the rainbands and the inner eye/eyewall of TCs. Rita developed a secondary eyewall that eventually replaced the primary eyewall and went through a nearly complete eyewall replace cycle, while Katrina maintained a primary eyewall during the RAINEX observation period before landfall. These distinct features observed in RAINEX provide a unique opportunity to examine the physical and dynamic processes affecting the formation of concentric eyewalls. Specifically, this study investigates the convectively-generated PV in rainbands and how it differs in Rita and Katrina, which leads to the formation of the secondary eyewall in Rita but not Katrina. A triply nested MM5 with 1.67km resolution in the inner-most domain was used for model forecasts in realtime during RAINEX. The model was initialized with global model forecast fields. The model output was used to examine the PV generation and vorticity characteristics in Rita and Katrina. Furthermore, dynamical processes such as vortex Rossby wave (VRW) activity in the inner core region were analyzed. The rainrate, PV and vertical velocity fields have been decomposed into wavenumber components. RAINEX aircraft observations, such as the wind and vorticity fields derived from the Doppler radar data are used to evaluate and validate the model results. The vorticity field is closely related to PV and thus a manifestation of the PV generation process in the rainbands. It is shown that Rita developed a principal rainband with higher PV generation rates at radii beyond 80 km than that of Katrina, which led to a secondary PV/vorticity maximum in the rainband region of Rita. A strong “moat” area between the primary eyewall and the secondary PV ring becomes a region of minimum VRW activity. In contrast, the VRW were quite active in the inner core of Katrina. The VRW were not a contributing factor to the initial formation of the secondary eyewall as the PV gradient and the moat region in Rita did not allow for radial VRW propagation. The high values of convectively-generated PV in the rainband region is a key in the formation of the secondary eyewall in Rita.