Evaluating and Improving HYCOM Performance for Simulating the Ocean Response to Tropical Cyclones

George Halliwell
RSMAS, University of Miami


The ocean response to hurricanes is simulated using the Hybrid Coordinate Ocean Model (HYCOM) in an effort to improve model performance in strong forcing regimes and to improve model efficiency for potential operational use as the ocean component of the HWRF hurricane forecast 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, which potentially influences intensity evolution. In this presentation, initial results are presented from a JHT project performed with colleague Nick Shay to evaluate HYCOM against high-quality ocean observations. The first round of ocean simulations is underway, focusing on the ocean response to hurricane Ivan (2004) in the northwest Caribbean and Gulf of Mexico. Analyses are being performed to (1) identify the horizontal and vertical resolution that maximizes efficiency to the greatest extent possible without significantly compromising the accuracy of the simulation; (2) demonstrate the importance of accurately initializing ocean eddies and boundary currents in the ocean model; (3) quantify sensitivity of the current and temperature response to the model vertical mixing parameterizations and identify strategies to improve the representation of vertical mixing; (4) quantify sensitivity of the current and temperature response to air-sea flux parameterizations and identify strategies to improve these parameterizations; and (5) identify other methods to speed up the model for operational purposes. The latest results from the model evaluation conducted by comparison to satellite SST measurements and moored ocean current observations that were directly hit by Ivan will be presented. The realistic Ivan simulations are supplemented by simulating the response of an initially quiescent ocean to an idealized hurricane to demonstrate that HYCOM simulates the theoretically expected dynamical and thermodynamical response patterns.