A modified vertical diffusion scheme and its impacts on numerical prediction of landfalling hurricanes with HWRF

A vertical diffusion scheme is modified in the NCEP Hurricane Weather Research and Forecasting (HWRF) model. The impact of this new scheme on numerical predictions of track, intensity and structures of landfalling hurricanes is evaluated. In particular, a series of sensitivity experiments with different eddy diffusivity for both momentum (Km) and heat (Kh) were conducted for hurricane Dennis, Katrina and Rita. Results show that: compared with the observations, the original Km in HWRF model that considers surface stability only, tends to make the HWRF forecasts overestimate the hurricanes evolution over land with lower surface pressure, stronger surface wind, larger track error and overestimated precipitation. In contrast, the modified Km, which considers both surface stability and fluxes at the same time, has helped HWRF model to achieve profound improvements on hurricane's evolution over land with more reasonable track, intensity and Quantitative Precipitation Forecasting (QPFs). In addition, the results also demonstrate substantial improvements on hurricane structures with this new scheme, compared with the forecasts using the original scheme. Specifically, with the new scheme, the adjustment of vertical mixing effect is more efficient to adapt the environment over land, leading to the reasonable evolutions of hurricane environment and rainbands. However, with the original scheme, the landfalling hurricanes tends to inherit their characteristics over ocean with overestimated spiral structures all the time, resulting in less efficient environment intrusion effects for hurricanes' decay over land. Details results will be presented.