Temporal and spatial evolution of mesovortices in the eye and eyewall of Hurricane Isabel (2003)

Recent analytical, theoretical, and observational studies have demonstrated that mesovortices that form near the eye/eyewall interface of hurricanes can effectively mix mass and momentum between the eye and eyewall. This mixing can lead to significant changes in storm intensity. Observations of sharply-peaked wind profiles in strong hurricanes have shown that the eyewall may become barotropically unstable and lead to the growth of vortex Rossby waves which, in turn, may lead to the formation of mesovortices along the eye/eyewall interface. On the one hand, the mesovortices that form along the interface and migrate into the eye may mix the high momentum air of the eyewall into the relatively quiescent air inside of the eye and thereby reduce the overall strength of the cyclone. Some analytical and observational studies have shown, however, that these mesovortices may also tap into the high entropy air of the lower part of the eye, mixing it with lower equivalent potential temperature air of the eyewall. This can act as a boost of energy for convective growth in the eyewall and lead to vorticity production and an overall increase in hurricane intensity.

During 12-14 September 2003 while Hurricane Isabel maintained its intensity at or near Category 5, the NOAA P3 aircraft probed the storm and conducted multiple orbits within the large eye. Airborne radar imagery and photographs taken from the aircraft confirmed the existence of low-level mesovortices along the eyewall and within the eye of Isabel. The eye orbits performed by the P3 aircraft provided a unique opportunity to track and study the evolution and interaction of these mesovortices. The mesovortices were only observed at altitudes below about 2 km, rotated cyclonically and, at times, moved either radially away from or toward the eyewall. Individual mesovortices would sometimes merge with one another, creating a larger vortex. Others would merge into the eye where they could not be tracked any further with the radars. Finer-scale structures, that resemble Kelvin-Helmholtz billows were also observed at the base of the eyewall and which rotated at nearly the speed (~70 m/s) of the low-level wind. At least one these billows contained horizontal and vertical winds > 100 and 25 m/s, respectively.

Data from the horizontally- and vertically-scanning radars on the P3 aircraft are used to document the 3-dimensional structure and evolution of some of mesovortices observed in the eye and eyewall of Hurricane Isabel. The analyses provides a synthesis for other observational work and a framework for future high-resolution modeling of these features.