Are there benefits of ocean coupling for tropical cyclone forecasts?

A next-generation operational Hurricane Weather Research Forecast (HWRF) system has been developed at NCEP/EMC by replacing the eddy-permitting 3D Princeton Ocean Model (POM) with eddy-resolving 3D hydrostatic, primitive variable HYbrid Coordinate Ocean Model (HYCOM). Extensive evaluation of the system maturity has been conducted in real-time mode for each season since 2009 and 2012 for the North Atlantic hurricanes and the Western North Pacific typhoons, respectively. Comparisons with operational HWRF simulations consistently show that the coupled HWRF-HYCOM has a similar skill for track forecasts, and it has a tendency of shifting intensity bias, while reducing absolute intensity error. This implies that HYCOM coupling may play a role in altering the gradient wind balance of operational HWRF. The operational system employs the ½-degree Real-Time Global (RTG) Sea Surface Temperature (SST) daily product for initializing HWRF from Global Forecast System (GFS) as well as for data assimilation using the GSI (Grid point Statistical-Interpolation) system. The same RTG SST field is also used by the coupled POM for oceanic state initialization by assimilating into and updating the ½-degree Generalized Digital Environment Model (GDEM) climatology.

This study addresses the following questions: 1) are the unbalanced gradient winds due to air-sea temperature differences in the initial conditions? ; and 2) if not, what causes these unbalanced gradients during HYCOM coupling? Some plausible answers to these questions will be presented.