Convectively-induced PV and Vortex Rossby Waves in Hurricanes Katrina and Rita (2005)

Radar observations in hurricanes reveal inner spiraling rainbands emanating from the eyewall and propagating outward. Theoretical analysis indicated that these inner bands are azimuthally and radially propagating vortex Rossby waves (VRW). The outward propagating waves convey potential vorticity (PV) from the inner core to outer regions and thus lead to PV redistribution within a hurricane. It has also been hypothesized that the outward propogating VRWs may interact with an existing secondary PV ring in the outer region of a hurricane, which could lead to a development of concentric eyewalls. However, the lack of simultaneous observations over the inner-core and rainband regions is a major difficulty in our understanding of the complex interaction. The importance of VRW in hurricane intensity change remains to be a question. This study aims to address the question using high-resolution model (MM5) forecasts of Hurricanes Katrina and Rita during the Hurricane Rainbands and Intensity Change Experiment (RAINEX) in 2005. The two major hurricanes went through a similar rapid intensification over the Gulf of Mexico. Both RAINEX observations and model forecast fields showed that Rita developed a secondary eyewall and went through an eyewall replacement before landfall, whereas Katrina did not. We analyze the model output at 1.67 km grid-resolution with 12-min time intervals. Azimuthally and radially propagating VRWs were found in the PV, rainrate, and vertical velocity fields in both storms. However, the PV distribution and wave propagations differ significantly in the two storms. In Katrina, the VRWs are generated in the eyewall region and propagate outward smoothly over a relatively long distance. No VRW activity is detectable beyond 80-100 km radius because of the fading PV gradient that cannot support VRW at greater radii. This result indicates that interactions between the VRWs and outer PV disturbance must take place within this region, which was absent in Katrina. In contrast, Rita had a secondary ring of enhanced PV, evidently associated with the diabatic heating in the outer rainband region at a radius > 100 km. The analysis shows that, between the eyewall and this outer ring of enhanced PV, VRW activity is barely detectible in the early stage. It indicates that the enhanced PV in the rainband region are most likely independent of the VRWs. A secondary wind maximum is observed in both the model output and airborne measurements. The secondary PV and wind maxima contracts with time and eventually becomes the new eyewall. Only when the developing secondary eyewall is moving inward and close to the primary eyewall, VRWs are detected in the model output. A PV budget analysis is currently under way, which will help to quantify the PV source and a collocated secondary wind maximum in the rainband region. It may shed some light on how rainbands interact with the inner core and formation of concentric eyewalls in hurricanes.