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Observational Estimates of the Horizontal Eddy Diffusivity and Mixing Length in the Low-Level Region of Intense Hurricanes

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  • 1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, and NOAA/AOML/Hurricane Research Division, Miami, Florida
  • | 2 NOAA/AOML/Hurricane Research Division, Miami, Florida, and Naval Postgraduate School, Department of Meteorology, Monterey, California
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Abstract

This study examines further the characteristics of turbulent flow in the low-level region of intense hurricanes using in situ aircraft observations. The data analyzed here are the flight-level data collected by research aircraft that penetrated the eyewalls of category-5 Hurricane Hugo (1989), category-4 Hurricane Allen (1980), and category-5 Hurricane David (1979) between 1 km and the sea surface. Estimates of horizontal eddy momentum flux, horizontal eddy diffusivity, and horizontal mixing length are obtained. It is found that the horizontal momentum flux and horizontal diffusivity increase with increasing wind speed. The horizontal mixing length increases slightly with wind speed also, but the mixing length is not significantly dependent on the wind speed. The magnitude of the horizontal momentum flux is found to be comparable to that of the vertical momentum flux, indicating that horizontal mixing by turbulence becomes nonnegligible in the hurricane boundary layer, especially in the eyewall region.

Within the context of simple K theory, the results suggest that the average horizontal eddy diffusivity and mixing length are approximately 1500 m2 s−1 and 750 m, respectively, at about 500 m in the eyewall region corresponding to the mean wind speed of approximately 52 m s−1. It is recalled also that the mixing length is a virtual scale in numerical models and is quantitatively smaller than the energy-containing scale of turbulent eddies. The distinction between these two scales is a useful reminder for the modeling community on the representation of small-scale turbulence in hurricanes.

Corresponding author address: Dr. Jun Zhang, NOAA/AOML/Hurricane Research Division with the University of Miami/CIMAS, 4301 Rickenbacker Causeway, Miami, FL 33149. E-mail: jun.zhang@noaa.gov

Abstract

This study examines further the characteristics of turbulent flow in the low-level region of intense hurricanes using in situ aircraft observations. The data analyzed here are the flight-level data collected by research aircraft that penetrated the eyewalls of category-5 Hurricane Hugo (1989), category-4 Hurricane Allen (1980), and category-5 Hurricane David (1979) between 1 km and the sea surface. Estimates of horizontal eddy momentum flux, horizontal eddy diffusivity, and horizontal mixing length are obtained. It is found that the horizontal momentum flux and horizontal diffusivity increase with increasing wind speed. The horizontal mixing length increases slightly with wind speed also, but the mixing length is not significantly dependent on the wind speed. The magnitude of the horizontal momentum flux is found to be comparable to that of the vertical momentum flux, indicating that horizontal mixing by turbulence becomes nonnegligible in the hurricane boundary layer, especially in the eyewall region.

Within the context of simple K theory, the results suggest that the average horizontal eddy diffusivity and mixing length are approximately 1500 m2 s−1 and 750 m, respectively, at about 500 m in the eyewall region corresponding to the mean wind speed of approximately 52 m s−1. It is recalled also that the mixing length is a virtual scale in numerical models and is quantitatively smaller than the energy-containing scale of turbulent eddies. The distinction between these two scales is a useful reminder for the modeling community on the representation of small-scale turbulence in hurricanes.

Corresponding author address: Dr. Jun Zhang, NOAA/AOML/Hurricane Research Division with the University of Miami/CIMAS, 4301 Rickenbacker Causeway, Miami, FL 33149. E-mail: jun.zhang@noaa.gov
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