Observation of vertical eddy diffusivity and mixing length during landfalling Super Typhoons

Understanding the vertical turbulent mixing processes in the planetary boundary layer (PBL) of tropical cyclones (TCs, or typhoons, hurricanes) and improving the PBL parameterization in numerical models of TCs is of paramount importance for TC modeling and forecasting. Based on the multi-level, high-resolution observations from Asia's tallest meteorological tower, the 356-m-high Shenzhen Meteorological Gradient Tower, this paper investigates the characteristics and vertical profiles of vertical eddy diffusivity and mixing length during two landfalling typhoons, namely Super Typhoons Mangkhut and Hato. The observed vertical eddy diffusivity and mixing length are compared with several widely-used parametrization schemes to assess the performance of the schemes. The results demonstrate that the K-profile parameterization scheme with the coefficient α = 1.5 satisfactorily portrays the magnitude and vertical distribution of the vertical eddy diffusivity. Moreover, a novel parametrization for mixing length is proposed, which is shown effective in reproducing the vertical mixing when used in conjunction with the Mellor–Yamada scheme. This study aims to enhance the understanding of vertical turbulent mixing in TC boundary layer and facilitate the numerical simulation of TCs over land.