Effects of Horizontal Diffusion on Tropical Cyclone Intensity Change and Structure in Idealized Three-Dimensional Numerical Simulations Monthly Weather Review

This study examines the effects of horizontal diffusion on tropical cyclone (TC) intensity change and structure using idealized simulations of the Hurricane Weather Research and Forecasting Model (HWRF). A series of sensitivity experiments were conducted with varying horizontal mixing lengths (L-h), but kept the vertical diffusion coefficient and other physical parameterizations unchanged. The results show that both simulated maximum intensity and intensity change are sensitive to the L-h used in the parameterization of the horizontal turbulent flux, in particular, for L-h less than the model's horizontal resolution. The results also show that simulated storm structures such as storm size, kinematic boundary layer height, and eyewall slope are sensitive to L-h as well. However, L-h has little impact on the magnitude of the surface inflow angle and thermodynamic mixed layer height. Angular momentum budget analyses indicate that the effect of L-h is to mainly spin down a TC vortex. Both mean and eddy advection terms in the angular momentum budget are affected by the magnitude of L-h. For smaller L-h, the convergence of angular momentum is larger in the boundary layer, which leads to a faster spinup of the vortex. The resolved eddy advection of angular momentum plays an important role in the spinup of the low-level vortex inward from the radius of the maximum wind speed when L-h is small. Grant no. NA14NWS4680028