An Estimation of Turbulent Characteristics in the Low-Level Region of Intense Hurricanes Allen (1980) and Hugo (1989)

Abstract

This study analyzes the flight-level data collected by research aircraft that penetrated the eyewalls of category 5 Hurricane Hugo (1989) and category 4 Hurricane Allen (1980) between 1 km and the sea surface. Estimates of turbulent momentum flux, turbulent kinetic energy (TKE), and vertical eddy diffusivity are obtained before and during the eyewall penetrations. Spatial scales of turbulent eddies are determined through a spectral analysis. The turbulence parameters estimated for the eyewall penetration leg are found to be nearly an order of magnitude larger than those for the leg outside the eyewall at similar altitudes. In the low-level intense eyewall region, the horizontal length scale of the dominant turbulent eddies is found to be between 500 and 3000 m, and the corresponding vertical length scale is approximately 100 m. The results suggest also that it is unwise to include eyewall vorticity maxima (EVM) in the turbulence parameter estimation because the EVMs are likely to be quasi-two-dimensional vortex structures that are embedded within the three-dimensional turbulence on the inside edge of the eyewall. This study is a first attempt at estimating the characteristics of turbulent flow in the low-level troposphere of an intense eyewall using in situ aircraft observations. The authors believe that the results can offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity. Because of the small sample size analyzed in this study, further analyses of the turbulent characteristics in the high-wind region of hurricanes are imperative.