Vertical Eddy Diffusivity in the Tropical Cyclone Boundary Layer during Landfall

Abstract

This study investigated surface layer turbulence characteristics and parameters using 20 Hz eddy covariance data collected from five heights with winds up to 42.27 m s–1 when Super Typhoon Maria (2018) made landfall. The dependence of these parameters including eddy diffusivities for momentum (Km) and heat (Kt), vertical mixing length (Lm), and strain rate (S) on wind speed (un), height, and radii was examined. The results show that momentum fluxes (τ), turbulent kinetic energy (TKE), and Km had a parabolic dependence on un at all five heights outside three times the RMW, the maximum of Km and S increased from the surface to a maximum value at a height of 50 m, and then decreased with greater heights. However, Km and S were nearly constant with wind and height within two to three times the RMW from the TC center before landfall. Our results also found the |τ|, TKE, and Km were larger than over oceanic areas at any given wind, and Km was about one to two orders of magnitude bigger than Kt. The turbulence characteristic and parameters’ change with height and radii from the TC center should be accounted for in sub-grid scale physical processes of momentum fluxes in numerical TC models.

Atmospheric Boundary Layer Processes, Characteristics and Parameterization

“…One of the four papers looked into the vertical eddy diffusivity in the atmospheric boundary layer during landfall of a tropical cyclone, which was observed by three-dimensional ultrasonic anemometers. An exceptional finding in this paper was the variation in the turbulent parameters with regard to the distance to the tropical cyclone center: outside three times of the radius of maximum wind (RMW) from the tropical cyclone center, the turbulent kinetic energy and eddy diffusivity values increased with increasing wind speed; however, in the area that was within one to three times the RMW from the tropical cyclone center, these values decreased slowly with increasing wind speed [1]. The other three papers investigated the turbulent fluxes exchanged over the ocean under tropical cyclone conditions through aircraft eddycovariance measurements [2], dropsondes observations [3], and numerical simulations [4].…”