Effects of boundary layer vertical diffusion on forecasts of tropical cyclone rapid intensification

This talk evaluates the impact of boundary layer vertical diffusion on forecasts of tropical cyclone (TC) rapid intensification (RI) using the operational Hurricane Weather Research and Forecasting (HWRF) model. Emphases are given to the performance of HWRF for forecasting TCs with RI using retrospective forecasts. We found substantial improvements in the number of RI events with excellent forecasts (Hits) in response to the improvement of the vertical diffusion, although the number of false alarm forecasts is also larger. Our composite analyses show that the simulated TCs at the RI onset have shallower boundary layer, stronger inflow in the boundary layer, stronger outflow above the boundary layer, stronger updrafts in regions close to the radius of maximum wind speed, stronger boundary layer convergence with smaller vertical diffusion, although the mean storm intensity is similar for the two different setups of vertical diffusion. We also found that the departure of the maximum tangential wind from the gradient wind balance is much larger in the forecasts with smaller vertical diffusion. In addition, a detailed case study suggests that the vertical diffusion modulates the vortex tilt in response to environmental vertical wind shear and the location of convective bursts relative to the radius of the maximum wind speed.