Sensitivity of hurricane intensity and structure to horizontal diffusion in idealized HWRF simulations

Monday, 15 December 2014: 11:50 AM
Jun Zhang and Frank Marks, NOAA/AOML, Key Biscayne, FL, United States
This talk presents results of sensitivity tests on the effects of horizontal diffusion on simulated hurricane intensity and structure using idealized Hurricane Weather and Research Forecast (HWRF) simulations. In the sensitivity experiments, simulations were run using different horizontal mixing length (Lh) ranging from 0 to ~5000 m while keeping the vertical diffusion coefficient and other physical parameterizations unchanged as in the operational HWRF model. The results show that the simulated intensity is sensitive to Lh used in the parameterization of horizontal turbulent fluxes, in particular for Lh<2000m. The sensitivity experiments also demonstrate that the simulated storm structures such as the size of the storm, the kinematic boundary layer height, and eyewall slope are sensitive to Lh. But Lh has little impact on the magnitude of surface inflow angle and thermodynamic mixed layer height. Consistent with previous studies, the results indicate that the main effect of horizontal diffusion is to reduce the radial gradient of the important dynamical and thermodynamical parameters. When Lh is larger, the radial gradient of angular momentum is smaller both within and above the boundary layer, which leads to a weaker hurricane vortex.