Multilevel Tower Observations Of Vertical Eddy Diffusivity And Mixing Length In The Tropical Cyclone Boundary Layer During Landfalls

This study analyzes the fast-response (20 Hz) wind data collected by a multilevel tower during the landfalls of Tropical Storm Lionrock (1006), Typhoon Fanapi (1011), and Typhoon Megi (1015) in 2010. Turbulent momentum fluxes are calculated using the standard eddy-correlation method. Vertical eddy diffusivity K-m and mixing length are estimated using the directly measured momentum fluxes and mean-wind profiles. It is found that the momentum flux increases with wind speed at all four levels. The eddy diffusivity calculated using the direct-flux method is compared to that using a theoretical method in which the vertical eddy diffusivity is formulated as a linear function of the friction velocity and height. It is found that below similar to 60 m, K-m can be approximately parameterized using this theoretical method, though this method overestimates K-m for higher altitude, indicating that the surface-layer depth is close to 60 m in the tropical cyclones studied here. It is also found that K-m at each level varies with wind direction during landfalls: K-m estimated based on observations with landward fetch is significantly larger than that estimated using data with seaward fetch. This result suggests that different parameterizations of K-m should be used in the boundary layer schemes of numerical models forecasting tropical cyclones over land versus over the ocean.