On 12 Sep 2003, a remarkable pentagonal pattern resembling a starfish was clearly evident in the clouds in the eye of Hurricane Isabel. The pattern was due to the presence of six distinct mesovortices -- one in the center and five others arranged fairly symmetrically around the center -- and remained fairly steady for a few hours while rotating cyclonically within the eye. One of the exciting aspects of the appearance of the ``starfish'' pattern in Isabel's eye is that this pattern was previously found within a theoretical framework of maximum simplicity -- unforced two-dimensional barotropic flow. In this framework, the mesovortices are a product of horizontal mixing resulting from barotropic instability.

The mesovortices that form in hurricanes likely play an essential role in hurricane intensification. Previous studies have shown that axisymmetric models - which filter barotropic instability-induced mixing mechanisms - can not produce a strong hurricane vortex without introducing an artificial mixing parameterization. The parameterization is usually in the form of eddy diffusivity. However, it can be shown that diffusion is a particularly poor parameterization when mixing is accomplished via mesovortices. The reason for this is the mesovortices can protect their inner-cores from further mixing and are thus able to transport very high angular momentum from the eyewall directly into the low angular momentum environment of the eye.

Hurricane Isabel was very well observed by a number of observing platforms during its lifetime. We will exploit these rich data sources to make comparisons between the two-dimensional barotropic framework and observed behaviors.