HEDAS Analyses of a Rapidly Evolving Eyewall of a Major Hurricane – Fabian (2003)

The tropical cyclone eyewall was assumed to be either circular or elliptical before polygonal eyewall structures were identified from ground-based radar reflectivity. In modeling studies, the polygonal eyewall shapes have been related to possible mesovortices within or near the tropical cyclone eye. The polygon vertices are found to coincide with potential vorticity mixing events at those locations suggesting that these features are related to intensity changes. Definitive observations of vortical circulations in the tropical cyclone eyewall have so far been rare, and none of them have been concurrent with polygonal eyewall events. On 03 September, 2003, the National Oceanic and Atmospheric Administration conducted a two-P3 research mission into Hurricane Fabian just north of Puerto Rico. Due to instrument malfunctions, both aircraft circled in the eye of Fabian for up to 2 h. This provided a serendipitous and unprecedented dual-Doppler radar dataset of the evolution of the core of a major hurricane. During that time, the eye was seen to transform from a circular shape to triangular and back again to circular. In the current study, the HWRF Ensemble Data Assimilation System (HEDAS) is used to provide analyses at 3-km resolution in space every 15 min in time to study the dynamic and thermodynamic evolution of the eyewall, with special emphasis on the period in which the eyewall was seen to be triangular. The presentation will review the environmental conditions in which Fabian was embedded and examine radar reflectivity and other data during the period when Fabian had the triangular-shaped eyewall. HEDAS analyses of the core of Fabian will be shown, with special emphasis on the dynamics and thermodynamics of the evolving eyewall structure.