Tuesday, 11 February 2003: 11:15 AM
Estimation of the surface stress in the eye wall of hurricanes using WSR-88D radar data
Analysis of Doppler velocity data from the WSR-88D radar during hurricane landfall reveals evidence of organized secondary circulations in the hurricane eye wall at low elevations. A Fourier analysis of the Velocity-Azimuthal Display (VAD) provides estimates of divergence (0th harmonic), wind speed and direction (1st harmonic), and deformation (2nd harmonic). A residual velocity field is obtained by subtracting the mean VAD velocity from the radial Doppler velocity for elevation angles between 0.5 and 5.5 degrees. The wavelength, length, depth, magnitude, and motion of velocity anomalies were then compiled from the residual velocity displays. The resulting statistics suggest the presence of organized secondary circulations or boundary layer (BL) rolls in the marine boundary layer of the hurricanes. To date, three storms have been examined: Fran (1996), Bonnie (1998), and Georges (1998) using WSR-88D data from Wilmington, N.C.; Morehead City, N.C.; and Key West, FL, respectively. The analysis focuses on the period between the time the first BL roll is identified and hurricane landfall. Fran was a strong category 2 hurricane as it passed within 28 km of Wilmington, and the first BL rolls were identified 105 km from the eye. Bonnie was a weak category 2 hurricane during the sampling period and passed within 30 km of the radar at Morehead City. The first BL rolls were identified when Bonnie passed within 120 km of Morehead City. Georges passed 25 km from Key West as a category 2 storm, and BL rolls were identified within 80 km from the eye. The number of BL rolls tracked in Bonnie, Fran, and Georges was 44, 56, and 24, respectively. BL rolls were less frequent in Georges, and the magnitude of the velocity anomalies was less than those in Fran and Bonnie. The average low-level (800 m - 50 m) shear in Georges was substantially less than in the other storms, likely contributing to the fewer number of rolls identified and a lower intensity of the rolls. The wavelength of a BL roll is twice the horizontal distance between adjacent positive and negative velocity anomalies. Georges had the largest average wavelength (~1400 m), followed by Fran (~1320 m) and Bonnie (~1200 m). The gradient between adjacent positive and negative anomalies corresponds to a horizontal wind shear of ~14 m s-1 over 660 m, and a vertical shear component of vorticity of 2.0x10-2 s-1. Momentum fluxes associated with the secondary circulations are estimated with reference to mixing length theory. Estimates of the surface stress are obtained from the radar derived wind profiles using a modified momentum budget approach. The impact of secondary circulations on the magnitude of the surface stress in the hurricane eye wall will be discussed and contrasted with other approaches for estimating the stress.