Sensitivity of the Tropical Cyclone Boundary Layer to Vertical Diffusion in a Turbulent Kinetic Energy‐Based Boundary Layer Parameterization Scheme at Gray‐Zone Resolution

Abstract

AbstractThe impact of eddy diffusivities in the Mellor–Yamada–Nakanishi–Niino (MYNN) boundary layer parameterization on the fine‐scale turbulence and intensity of tropical cyclone (TC) simulations at gray‐zone resolution was investigated through comparison with large‐eddy simulation (LES) and observations. A series of idealized experiments were performed with varying vertical mixing lengths using the Advanced Research version of the Weather Research and Forecasting model. For enhanced eddy diffusivity cases, the storm intensity was sensitive to the eddy diffusivities and reducing the overestimation of the eddy diffusivities caused a significant increase in intensity. For the reduced eddy diffusivity cases, the TC intensity was marginally influenced by the eddy diffusivities. However, the reduction in eddy diffusivities effectively reproduced the fine‐scale turbulence. The stronger the resolved fine‐scale turbulence that could be established, the more frictional tendencies near the location of maximum wind were provided to the TC, the closer it was to the reference LES and observations. The resolved turbulence also had consequences in terms of preventing the wind gradient from increasing, which was one of the primary reasons for the discrepancies in the nonlinear intensity and fine‐scale features. The results of this study demonstrate that the modification of eddy diffusivities in the MYNN scheme can measure or guide improvements in the simulation of TC boundary layer fine‐scale features and intensity when operating at the gray‐zone resolution.