Improving the ocean model response to tropical cyclones

The ocean response to tropical cyclones is studied using the Hybrid Coordinate Ocean Model (HYCOM), which is being developed by NOAA/NCEP as the next ocean component of the coupled Hurricane Weather Research and Forecasting model, in an effort to improve the ocean model response. The overarching goal is to improve the capability of coupled TC forecast models to forecast intensity change, which depends in part on the temperature and thickness of the upper ocean warm layer as represented by the ocean heat content (OHC). Combined model-observational studies are critically important for evaluating and improving ocean model performance, particularly in regards to the magnitude and pattern of SST cooling driven by tropical cyclones. Simulations of the ocean response to hurricane Ivan (2004) in the northwest Caribbean and Gulf of Mexico have (1) demonstrated the importance of accurately initializing ocean eddies and boundary currents in the ocean model; (2) revealed sensitivity of the current and temperature response to the model vertical mixing parameterizations; and (3) emphasized the importance of ocean observations for both initializing and evaluating the ocean model. The ocean response was evaluated against microwave satellite SST measurements and moored ocean current observations. During the period of deployment, Hurricane Ivan passed directly over 14 Acoustic Doppler Current Profiler (ADCP) moorings that were deployed as part of the Navy Research Laboratory Slope to Shelf Energetics and Exchange Dynamics (SEED) project from May through Nov 2004. These observations enable the simulated ocean current response to a hurricane in a continental shelf/slope region to be evaluated with unprecedented detail. We will present the results of additional Ivan response simulations designed to identify the optimal choice of horizontal and vertical resolution for operational TC forecasting; i.e., the coarsest resolution that can be used without significantly degrading the simulated response. With these choices identified, additional simulations of Ivan along with simulations of Katrina and Rita (2005) in the Gulf of Mexico will be performed. In addition to ocean model improvement, goals include (1) understanding the physical processes responsible for mixed layer deepening and cooling while devising strategies for improving the mixed layer response and (2) understanding how the pre-existing quasi-geostrophic flow field modifies the ocean current response forced by storm winds and how this response in conjunction with the initial OHC pattern impacts the magnitude and pattern of SST cooling.