Two-way coupled mesoscale air-sea interaction in hurricane Frances (2004) and Katrina (2005)

Strong near-surface winds associated with mesoscale structures embedded within tropical cyclones can have a profound impact on air-sea interaction processes. It has been hypothesized that these air-sea exchange processes can act in a synergistic manner. For example, strong surface stresses can impact the near-surface ocean circulation, increase the ocean boundary layer mixing, and change the sea surface temperature distribution, which then modifies the surface flux distributions and further impacts the structure and intensity of the tropical cyclone. Only recently have cloud-scale nonhydrostatic models been able to properly represent the mesoscale air-sea interaction processes through the inclusion of two-way atmosphere-ocean coupling. In this study, we use an air-ocean coupled model to investigate how the mutual ocean and atmosphere respond to the two hurricanes; Frances (2004) and Katrina (2005). Our simulations make use of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^®1) and the NRL Coastal Ocean Model (NCOM). In these novel simulations, three nested grids (27 km, 9 km, and 3 km) are used in the COAMPS atmospheric model to simulate Hurricane Frances and Katrina, with the inner-most mesh following the tropical cyclone center. The NCOM uses a single mesh with a 3 km grid spacing. A flux coupler is used to handle the data transfers from the atmosphere to the ocean, and vice versa between the ocean domain and three nested atmosphere domains. The results show that the simulated SST cooling in the wake of these two hurricanes are consistent with AMSR/TMI satellite SST imagery, lending some confidence in the capability of the coupled modeling system to realistically represent the air-sea interaction processes. The simulations suggest that the strong surface stress gradients, forced in part by the localized mesoscale structures within the tropical cyclone boundary layer, enhance the oceanic mixing and SST cooling. As a result, the SST and surface currents are found to be very sensitive to the resolution of the atmospheric forcing. The oceanic response from the varying resolution of the atmospheric forcing will be compared and contrasted for these two tropical cyclones.

¹ COAMPS^® is a registered trademark of the Naval Research Laboratory.