Aircraft observations of concentric eyewall formation and evolution in Hurricane Rita on 22 September 2005

Early in the morning on 22 September 2005, Hurricane Rita completed a period of explosive deepening, reaching Category 5 intensity with a measured minimum pressure of 897 mb and maximum surface winds of 150 kt. Later in the day, as 4 aircraft probed the core of Rita, the storm weakened and aircraft observations showed the formation and evolution of an outer eyewall. The aircraft included both of the NOAA P3s, a USAF C-130, and a Naval Research Laboratory P3, flying in conjunction with the NOAA aircraft in support of the NSF-sponsored RAINEX program. One of the planes, NOAA42, spent nearly 5 hours within 50 nmi of Rita's center and witnessed a remarkable evolution of both the inner and outer eyewalls.

When NOAA42 first arrived near Rita's center, the outer eyewall, while prominent on the airborne radar display, contained only a weak wind maxima. At this time, the inner eyewall was still closed, contained large areas of deep convection and was located at about 12 nmi radius. The outer eyewall was at about 25 nmi radius, relatively close to the center, at least compared to other storms with concentric eyewalls in the formative stage. During the course of the flight, the outer eyewall contracted slightly to a radius of about 20 nmi and became the dominant wind and radar reflectivity feature. Winds in the outer eyewall increased and the wind profile sharpened near the inner edge of the outer wall associated with a strong reflectivity gradient. Concurrently, the winds became weaker in the inner eyewall, yet the wind profile remained sharply peaked, and the radar pattern became more asymmetric with high reflectivity appearing only in the western semicircle.

A remarkable aspect of the evolution of Rita's eyewalls was the appearance and propagation of thin rainbands or filaments of radar reflectivity that emanated from the outer edge of the inner eyewall, spiraled outward towards the outer one, and coalesced there. It is postulated that these filaments are associated with vortex Rossby waves (VRW) that originate from the outside edge of the inner eyewall, propagate radially outward and pool their vorticity at a stagnation radius that coincides with the incipient outer eyewall, thereby strengthening the winds locally. Another intriguing aspect of Rita during the flight was the appearance, at times, of polygonal structure of both eyewalls, indicating the presence of VRW activity. Time sequences of radar reflectivity and flight-level wind (vorticity) profiles are used to describe and discuss the structure and evolution of Rita's eyewalls.