Evaluation of the Simulated Oceanic Response to Hurricane Ivan in Comparison to High- Quality Ocean Observations
George R. Halliwell1, Lynn K. Shay1, William J. Teague2
2 Division of Meteorology and Physical Oceanography, RSMAS, University of Miami US Naval Research Laboratory, Oceanography Division, Stennis Space Center
The ocean response to hurricane Ivan (Sept 2004) was simulated within the Gulf of Mexico using the Hybrid Coordinate Ocean Model (HYCOM), which has been selected by NOAA/NCEP as the ocean component of the coupled Hurricane Weather Research and Forecasting model. These simulations (1) demonstrate the importance of accurately initializing the ocean model; (2) reveal sensitivity of the current and temperature response to the model vertical mixing parameterizations; and (3) emphasize the importance of ocean observations for both initializing and evaluating the ocean model. 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. Improving ocean model performance in coupled hurricane forecast models thus has the potential to significantly improve intensity prediction. In this study, the ocean model response to hurricane Ivan is 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.
The Ivan simulations were initialized by oceanic fields provided by the latest generation of the U. S. Navy ocean nowcast-forecast system being developed at the Naval Research Laboratory. This product provided a good initialization of the ocean heat content (OHC) distribution associated with the Loop Current, a recently-detached warm eddy, and two cold eddies adjacent to the warm eddy. This had a large impact on the simulated SST cooling pattern, with the largest cooling (> 5°C) occurring within the cold eddies. The simulated cooling pattern was in reasonable agreement with microwave satellite measurements. The combination of ocean models and observations is very important for providing accurate ocean model initializations. Although results for Ivan were good, larger initial OHC errors are observed for other storms. For example, OHC was underestimated by up to 30% prior to Katrina within the LC and warm ring that provided the energy for rapid intensification. Observational coverage prior to hurricanes should be improved to insure the most accurate possible initial ocean fields. The simulated current response to Ivan in the SEED domain was dominated by vigorous near-inertial currents that contained both barotropic and baroclinic motions, in reasonable qualitative agreement with observations. Simulations performed using three vertical mixing parameterizations reveal substantial differences in the vertical penetration of the currents forced during the storm, and also with respect to the temporal decay rate of the near-inertial oscillations following the storm. Detailed comparisons between model and observations will be performed to evaluate the vertical mixing parameterizations and devise strategies for improving ocean model performance in the HWRF prediction model.