Eyewall Mesovortices in Hurricane Fabian (2003) using the HWRF Ensemble Data Assimilation System (HEDAS)

Two NOAA-P3 Coupled Boundary Layers Air-Sea Transfer missions observed a triangular eye from 1813-1821 UTC while circling within the eye of Hurricane Fabian on 02 September 2003. That observation motivated this research where the Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) was employed to analyze the dynamic and thermodynamic state of Fabian. This research marks the first time that HEDAS has been used for high-resolution analyses of small-scale features in a tropical cyclone core. A 24-h forecast from an 11-member, 50-km-resolution NOAA Global Ensemble Forecast System Reforecast, Version 2, was used as a basis for the regional-model ensemble. HWRF was used to spin up the storm from 1200-1700 UTC using three different model configurations. High-resolution flight-level, stepped frequency microwave radiometer, and tail Doppler radar data from the two missions were assimilated to produce analyses every 15 min from 1730-1900 UTC. Three maximum wind-speed features which persisted for about 1.5 h were identified. Each feature held similar humidity, vorticity, temperature, potential temperature, radial wind, and geopotential height properties throughout the entire period. All three features had contact with the surface during at least one of their respective 15-min analyses. Features had a range of depths, anywhere from a shallow depth of 75 hPa to a deep depth of 600 hPa. Two of the features had shallower average depths of 157 hPa and 275 hPa and rotated approximately 180-degrees counter-clockwise within the eye of Fabian over the time period while the third feature had a deeper average depth of 485 hPa and remained stationary due to the asymmetry forced by the northward propagating storm and the southwesterly shear causing convection downshear and downshear left. Fourier analysis of vorticity and tangential velocity showed wavenumber 2 and 3 dominance during 1800 UTC and near-zero wavenumber 1 contribution accompanied by wavenumber 2 and 3 dominance at 1815 UTC respectively.