Tropical Cyclones (TC) are physical phenomena with fundamental interconnections to the oceanic, atmospheric and wave environments in which they exist.  While these environments are drastically modified by the existence of the TC and vice versa, most current TC prediction numerical models do not consider the dynamic interactions between these environments.  We aim to improve solutions provided by individual numerical models representing the atmosphere, ocean, and waves through coupling of these models together allowing exchange of dynamically evolving model fields.  The models used to represent these environments are the Weather Research and Forecasting Model (WRF), Regional Ocean Modeling System (ROMS), and Simulating WAves Nearshore (SWAN) model.  These models are coupled together using the Model Coupling Toolkit (MCT), creating the Coupled Ocean-Atmosphere-Wave Sediment Transport (COAWST) modeling system (Warner et al. 2010).  The coupling scheme exchanges the following fields between the three numerical models: sea surface temperature (SST), ocean surface currents, wave heights, lengths, periods, bottom orbital velocities, and atmospheric momentum and radiation fluxes.

We utilize the COAWST system to understand the impact of dynamic couplings of the ocean, atmosphere, and waves during Hurricane Ivan (2004). The translation of Hurricane Ivan (2004) across the Gulf of Mexico provides a realistic scenario in which a number of important dynamic interactions between the Tropical Cyclone (TC) and ocean are present during an extreme hurricane.  These interactions are examined by using model sensitivity experiments, represented by enabling or disabling features of both uncoupled and coupled models and then comparing with in-situ and remotely sensed data.  Data comparisons to in-situ data include temperature and wave height measurements from NDBC buoys in the Gulf of Mexico, TC sea level pressure, winds, and best track from P-3 "Hurricane Hunter" flights.  Remotely sensed data includes SST derived from satellite infrared and microwave data, Sea Surface Height (SSH) available from satellite radiometer data, and 3-dimensional currents provided by data from several Acoustic Doppler Current Profilers (ADCPs) positioned along the shelf (Teague et al., 2007).

Several configurations of the uncoupled and coupled model have been examined.  Three configurations of uncoupled, WRF-only, simulations show small changes to the atmospheric solution based on different representations of SST.  The first WRF-only solution, Static SST, featured an unchanging sea state based on the SST at initialization.  Due to the unchanging sea-state, this simulation provided the strongest and most intense hurricane, which was not realistc due to the presence of multiple cold-water eddies in the Gulf of Mexico.  The second WRF-only solution, Dynamic SST, changed the SST state based on 24-hourly input Real-Time Global (RTG) SST (Gemmill et al. 2007) which was interpolated to the WRF temporal and spatial domain at 6-hour intervals. Despite the changing SST condition, this product also produced significant TC overintensification.  The final WRF-only solution, OML SST, utilized a 1-dimensional Ocean Mixed Layer (OML) model which was integrated into WRF (Davis et al. 2007).  This case showed slightly less overintensification than the Static and Dynamic SST cases, and provided the best uncoupled simulation upon which to compare results from the fully-coupled COAWST model.

Using the COAWST model in a 2-way, Atmosphere-Ocean, coupling configuration resulted in drastic changes to Hurricane Ivan's intensity and strength with little change to its track.  The comparison of the modeled atmospheric state to verification showed great improvement in predicting Ivan as the storm had been greatly over-intensified in the uncoupled (WRF-only) model simulations.  Comparison of the modeled ocean to verification showed good agreement with in-situ and remote observations.  In several discrete locations, the 3-dimensional model output of currents is compared with measurements provided by ADCP data.  Across the entire Gulf of Mexico domain, remote sensed observations of SST and SSH demonstrate good agreement of the verified sea-state with the model and demonstrate the strong right-side cooling bias of passing tropical cyclones.

Running the COAWST model in a 3-way, Atmosphere-Ocean-Wave, coupling configuration resulted in further changes to Hurricane Ivan's intensity and strength (track remained largely unchanged), as well as the sea state.  Coupling with the wave model created an enhanced surface momentum and heat flux to the atmosphere decreasing the intensity of the TC. The effect on the ocean was to create increased surface mixing and this reduced the SST slightly, thereby decreasing the storm intensity and strength.  As in the 2-way COWAST case, comparison of the modeled ocean to in-situ and remote sensing data showed good agreement.  In addition to the comparisons outlined above, comparison to wave data provided at several NDBC buoys along Hurricane Ivan's track demonstrated excellent results from the wave model.

Use of the COAWST model demonstrates improvement to predicting TC intensity and provides a complete 3-dimensional view of the geophysical environment from the ocean bottom to the stratosphere through coupling the individual numerical models WRF, ROMS, and SWAN representing the atmospheric, ocean, and wave environments.  Hurricane Ivan's simulated track and intensity, its effect on the 3-dimensional atmosphere and ocean states, along with comparisons with remote and in-situ observations will be presented.  We find that representing a realistic, 3-dimensional ocean through the model coupling is critical for TC hindcast and forecast applications.

References:

Davis, C., W. Wang, S. S. Chen, Y. Chen, K. Corbosiero, M. DeMaria, J. Dudhia, G. Holland, J. Klemp, J. Michalakes, H. Reeves, R. Rotunno, C. Snyder, Q. Xiao, 2007: Prediction of landfalling hurricanes with the Advanced Hurricane WRF model.  Mon. Wea. Rev, 136 (6), 1990-2005.

Gemmill, W., B. Katz and X. Li, 2007: Daily Real-Time Global Sea Surface Temperature - High Resolution Analysis at NOAA/NCEP. NOAA / NWS / NCEP / MMAB Office Note Nr. 260, 39 pp.

Teague, W. J., E. Jarosz, D. W. Wang, and D. A. Mitchell, 2007: Observed oceanic response over the upper continental slope and outer shelf during Hurricane Ivan.  J. Phys. Oceanogr., 37, 2181–2206.

Warner, J. C., B. Armstrong, R. He, J. B. Zambon, 2010: Development of a Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System.  Ocean Model., 35 (3), 230–244.