Sensitivity of the NOAA Hurricane Research and Forecasting Model (HWRF) to Horizontal and Vertical Diffusion

A series of idealized experiments with the NOAA Hurricane Research and Forecasting Model (HWRF) are performed to investigate the impact of the model's horizontal and vertical diffusion on an idealized bottom-up tropical cyclone (TC) intensification process. Comparisons of these experiments indicate that (1) the divergence damping term and the strong horizontal diffusion in the model slow down the convergence of the solution to the quasi-steady state, particularly with respect to the radius of maximum wind (RMW) near the surface, and (2) both the vertical and horizontal diffusion have a great impact on the structure of the simulated TC. The results from comparing these experiments show that the combination of the GFS planetary boundary layer scheme with the GFDL surface layer scheme, as originally implemented in the HWRF model, has an undesirable property of overestimating the sub-grid vertical turbulent diffusivity. They also indicate that there is a need to reevaluate the merit of including the divergence damping term in the HWRF model.