Real-case simulations of hurricane-ocean interaction using a high-resolution coupled model: Effects on hurricane intensity

In order to investigate the effect of tropical cyclone-ocean interaction on the intensity of observed hurricanes, the GFDL movable triply nested mesh hurricane model was coupled with a high-resolution version of the Princeton Ocean Model. The ocean model had 1/6 degrees uniform resolution, which mat ched the horizontal resolution of the hurricane model in its innermost grid . Experiments were run with and without inclusion of the coupling for two c ases of Hurricane Opal (1995) and one case of Hurricane Gilbert (1988) in t he Gulf of Mexico and two cases each of Hurricanes Felix (1995) and Fran (1 996) in the western Atlantic. The results confirmed the conclusions suggest ed by the earlier idealized studies that the cooling of the sea surface ind uced by the tropical cyclone will have a significant impact on the intensit y of observed storms, particularly for slow moving storms where the SST dec rease is greater. In each of the seven forecasts, the ocean coupling led to substantial improvements in the prediction of storm intensity measured by the storm's minimum sea level pressure. Without the effect of coupling the GFDL model incorrectly forecasted 25-hPa deepening of Gilbert as it moved across the Gulf of Mexico. With the coupl ing included, the model storm deepened only 10 hPa, which was much closer t o the observed amount of 4 hPa. Similarly, during the period that Opal move d very slowly in the southern Gulf of Mexico, the coupled model produced a large SST decrease northwest of the Yucatan and slow deepening consistent w ith the observations. The uncoupled model using the initial NCEP SSTs predi cted rapid deepening of 58 hPa during the same period. Improved intensity prediction was achieved both for Hurricanes Felix and Fr an in the western Atlantic. For the case of Hurricane Fran, the coarse reso lution of the NCEP SST analysis could not resolve Hurricane Edouard's wake, which was produced when Edouard moved in nearly an identical path to Fran four days earlier. As a result, the operational GFDL forecast using the ope rational SSTs and without coupling incorrectly forecasted 40-hPa deepening while Fran remained at nearly constant intensity as it crossed the wake. Wh en the coupled model was run with Edouard's cold wake generated by imposing hurricane wind forcing during the ocean initialization, the intensity pred iction was significantly improved. The model also correctly predicted the r apid deepening that occurred as Fran began to move away from the cold wake. These results suggest the importance of an accurate initial SST analysis a s well as the inclusion of the ocean coupling, for accurate hurricane inten sity prediction with a dynamical model. Recently, the GFDL hurricane-ocean coupled model used in these case studies was run on 163 forecasts during the 1995-98 seasons. Improved intensity fo recasts were again achieved with the mean absolute error in the forecast of central pressure reduced by about 268 compared to the operational GFDL mod el. During the 1998 season, when the system was run in near-real time, the coupled model improved the intensity forecasts for all storms with central pressure higher than 940 hPa although the most significant improvement (sim ilar to 60%) occurred in the intensity range of 960-970 hPa. These much lar ger sample sets confirmed the conclusion from the case studies, that the hu rricane-ocean interaction is an important physical mechanism in the intensi ty of observed tropical cyclones.