An Observational Study of Vertical Eddy Diffusivity in the Hurricane Boundary Layer

Although vertical eddy diffusivity or viscosity has been extensively used in theoretical and numerical models simulating tropical cyclones, little observational study has documented the magnitude of the eddy diffusivity in high-wind conditions (>20 m s(-1)) until now. Through analyzing in situ aircraft data that were collected in the atmospheric boundary layer of four intense hurricanes, this study provides the first estimates of vertical distributions of the vertical eddy diffusivities for momentum, sensible heat, and latent heat fluxes in the surface wind speed range between 18 and 30 m s(-1). In this work, eddy diffusivity is determined from directly measured turbulent fluxes and vertical gradients of the mean variable, such as wind speed, temperature, and humidity. The analyses show that the magnitudes of vertical eddy diffusivities for momentum and latent heat fluxes are comparable to each other, but the eddy diffusivity for sensible heat flux is much smaller than that for the latent heat flux. The vertical distributions of the eddy diffusivities are generally alike, increasing from the surface to a maximum value within the thermodynamic mixed layer and then deceasing with height. The results indicate also that momentum and latent heat are mainly transferred downgradient of the mean flow and that countergradient transport of the sensible heat may exist. The observational estimates are compared with the eddy diffusivities derived from different methods as used in planetary boundary layer (PBL) parameterization schemes in numerical models as well as ones used in previous observational studies.