Multilevel Tower Observations of Vertical Eddy Diffusivity and Mixing Length in the Tropical Cyclone Boundary Layer during Landfalls

Abstract

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 Km 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 ~60 m, Km can be approximately parameterized using this theoretical method, though this method overestimates Km 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 Km at each level varies with wind direction during landfalls: Km 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 Km should be used in the boundary layer schemes of numerical models forecasting tropical cyclones over land versus over the ocean.

Vertical Eddy Diffusivity in the Tropical Cyclone Boundary Layer during Landfall

“…The spatial patterns and magnitude of the coastal variations in K m values with u n are similar to the aircraft observation results of Zhao et al [22]. The slope of increasing K m with u n at <30 m s −1 is similar to the results of Zhang and Drenman [5], Tang et al [24], and Zhao et al [25]. The decreasing K m at wind speeds of >30 m s −1 is different from the observational results of hurricane eyewalls at 500 m altitude from Zhang et al [19], which shows that K m consistently increases with increasing u n to >40 m s −1 .…”

“…Katz and Zhu [23] used observational data to evaluate surface layer flux parameterizations. Based on flux tower observations, Tang et al [24] concluded that K m values in a hurricane PBL are greater when the wind is blowing from inland than the ocean at u n of up to ~30 m s −1 during typhoon landfall. Zhao et al [25] analyzed mean wind profiles by a 350-m height flux tower during a typhoon landfall with u n up to ~33 m s -1 .…”

“…However, due to the difficulties in obtaining turbulence data during extreme typhoon conditions [24], previous studies have been mostly limited to low and moderate wind conditions, or in the PBL at >100 m height over the ocean [21][22][23]. Moreover, changes in surface conditions induce the boundary layer of TCs to differ significantly moving from the oceans to land [11,26].…”

Improving Hurricane Boundary Layer Parameterization Scheme Based on Observations

“…Using the methodology discussed in Tang et al. (2018) it was found that the maximum value of l 2 did not exceed 100 m, whereas the original default was 300 m in the J. Han and Bretherton (2019) formulation.…”

Vertical Eddy Diffusivity Parameterization Based on a Large‐Eddy Simulation and Its Impact on Prediction of Hurricane Landfall

“…Based on fast‐response wind data collected during typhoons in 2010, Tang et al. (2018) concluded that the VED in a hurricane PBL is greater for wind blowing from land than from the ocean. F. Zhang and Pu (2017) and F. Zhang et al.…”