Impact of numerical grid spacing and time step on Vortex Rossby-Waves in secondary eyewall formation in hurricane Wilma (2005)

Understanding the dynamics of hurricanes is a key to improve their forecast. Recent studies have shown that the formation of the secondary eyewall (SE) and the eyewall replacement cycle affect hurricane intensity change. Numerical models have suggested that Vortex Rossby waves (VRWs) can cause SE genesis when they propagate to the critical radius to interact with the mean flow. However, it is known that the characteristics of the simulated VRWs can be affected by the grid resolution. To determine the effects of horizontal grid length on simulated VRWs during eyewall replacement cycle, numerical experiments of hurricane Wilma (2005) using the WRF model were carried out using varying grid lengths (2,3,4,6 km) and time steps (10,15,20 seconds). The results were analyzed using the method of Empirical Normal Modes (ENM) and the Eliassen-Palm (EP) fluxes and their divergence. The results indicated that the frequencies and the characteristics of the propagating modes are sensitive to the grid length and to a certain extent the time step. The secondary eyewall became absent in coarse resolution runs. Details of the numerical experiments will be presented.