Evaluation and Improvement of a TKE-Based Eddy-Diffusivity Mass-Flux (EDMF) Planetary Boundary Layer Scheme in Hurricane Conditions

Accurately representing boundary layer turbulent processes in numerical models is critical to improve tropical cyclone forecasts. A new turbulence kinetic energy (TKE)<based moist eddy<diffusivity mass<flux (EDMF<TKE) planetary boundary layer scheme has been implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS). This study evaluates EDMF<TKE in hurricane conditions based on a recently developed framework using large<eddy simulation (LES). Single<column modeling tests indicate that EDMF<TKE produces much greater TKE values below 500<m height than LES benchmark runs in different high<wind conditions. To improve these results, two parameters in the TKE scheme were modified to ensure a match between the PBL and surface<layer parameterizations. Additional improvements were made by reducing the maximum allowable mixing length to 40 m based on LES and observations, by adopting a different definition of boundary layer height, and by reducing nonlocal mass fluxes in high<wind conditions. With these modifications, the profiles of TKE, eddy viscosity, and winds compare much better with LES results. Three<dimensional idealized simulations and an ensemble of HAFS forecasts of Hurricane Michael (2018) consistently show that the modified EDMF<TKE tends to produce a stronger vortex with a smaller radius of maximum wind than the original EDMF<TKE, while the radius of gale<force wind is unaffected. The modified EDMF<TKE code produces smaller eddy viscosity within the boundary layer compared to the original code, which contributes to stronger inflow, especially within the annulus of 1–3 timestheradiusofmaximumwind. The modified EDMF<TKE shows promise to improve forecast skill of rapid intensification in sheared environments.