The collapse of the eyewall of Hurricane Dolly (2008) into multiple mesovortices

Airborne and ground-based radar observations have been used to identify polygonal eyewall structure in tropical cyclones (TC). Doppler wind data and other wind observations have revealed mesovortices, typically associated with vortex Rossby waves at the vertices of the polygonal structure. These mesovortices can mix air between the eye and eyewall and are believed to be important for intensity changes in TCs. On 23 July 2008, as Hurricane Dolly was approaching the Texas coast, reflectivity imagery from the high-resolution WSR-88D radar at Brownsville revealed a remarkable transformation of the structure of Dolly's eyewall over a period of several hours. This transformation consisted of a complete breakdown of an asymmetric but continuous eyewall into a high-wave number eyewall with as many as six discrete reflectivity maxima. Doppler wind observations from the Brownsville radar indicated that each of the reflectivity maxima was associated with a mesocylone signature, i. e., a velocity couplet imbedded within the mean cyclonic flow. This transition or breakdown of the eyewall into discrete vortical structures occurred over a six-hour period with rapid evolution of the structure and number of the mesovortices, which ranged from 2 to 6. After the breakdown, the mesovortices either weakened or merged to form a more continuous eyewall up to the point of landfall. A combination of reflectivity data and Doppler wind observations will be used to present this remarkable transformation of Dolly's eyewall from being nearly continuous in structure to a discrete set of mesovortices. Data from reconnaissance flights will be used to supplement the radar observations. A discussion relating the intensity changes of Dolly to the eyewall transformation will be given.