Impacts of Ocean Coupling on HWRF Forecasts for Tropical Cyclones over the Western North Pacific Basin in 2015

The Hurricane Weather Research and Forecasting (HWRF) modeling system became operational at National Centers for Environmental Prediction (NCEP) in 2007, originally providing numerical model guidance for tropical cyclones (TCs) in the North Atlantic (NATL) and Eastern North Pacific (EPAC) basins. After a few years of experimental real-time forecasts for the Western North Pacific (WPAC) and North Indian Ocean basins, the NCEP operational HWRF model guidance was extended to cover all global TC basins in 2015, but with different configurations for different basins. Originally the atmospheric component of the HWRF system is coupled to the Princeton Ocean Model (POM), and it can also be coupled to the HYbrid Coordinate Ocean Model (HYCOM) with recent developments. However, a configuration without ocean coupling is currently utilized in the 2015 operational HWRF for the WPAC basin. With the newly enabled ocean coupling capability using HYCOM and POM, coupled model experiments are conducted for the WPAC storms in 2015. We present comparisons of three sets of TC forecasts with and without ocean coupling. The performance of the 2015 operational HWRF forecasts for the WPAC storms is first summarized, followed by the analyses of the impacts of coupling to different ocean models on HWRF TC forecasts. Results show that the oceanic impacts on HWRF TC track and intensity forecasts are complicated, varying for different storms in different ways. The impacts primarily depend on the upper ocean structure (SST, OHC, mixed layer depth, ocean eddies, etc.) as well as the TC system itself (translation speed, storm intensity, etc.).