4. Summary and conclusions
Using a feature-based modeling approach that assimilates satellite-derived SSH, SST, and in situ data in the GoM, a new ocean initialization has been developed for the GFDL/URI coupled hurricane-ocean model. This new procedure is designed to account for spatial and temporal variability of mesoscale oceanic features in the Gulf of Mexico, including the LC, LCEs (WCRs), and CCRs. Using near real-time satellite altimetry and in situ temperature profiles, these features are assimilated into the original climatology to produce a more realistic three-dimensional temperature field valid at the model initialization time. Vertical profiles from the resulting data-assimilated temperature field are compared to 18 AXBT temperature profiles on 15 September 2005, the ocean climatology, and an alternative data assimilation technique to determine the relative accuracy of the initialization procedure presented here. Also, the TCHP from each of these profiles is calculated.
The feature-based ocean initialization creates a significantly improved three-dimensional temperature field over climatology. Also, when evaluating OML temperature and depth and upper thermocline temperature against 18 GoM AXBT profiles, this initialization technique is more accurate than a best estimate of the three-dimensional ocean temperature obtained using an alternative data assimilation technique (i.e. RSMAS HYCOM). Calculation of the TCHP and the TCHP rmse supports this conclusion and suggests that the initialization technique presented here is particularly superior to the RSMAS HYCOM technique in the northern GoM and where a LCE may be separating from (or reattaching to) the LC. In the latter case, the ability to manually adjust the position of the features based on near real-time AXBT profiles rather than relying exclusively on satellite altimetry is particularly advantageous. It is hoped that these new improvements to the three-dimensional temperature of the LC and associated rings will improve hurricane intensity prediction in the GoM.
Unfortunately, not all of the initialization improvements discussed here were ready before the deadline for incorporation into the 2006 operational GFDL model. Some of the improvements were incorporated though, including the ability to define multiple points along the LC path and to define a single elliptical WCR. During the 2006 hurricane season, forecasters at NHC used d26 maps derived from satellite altimetry (see section 3) to define LC and WCR parameters for initialization of the GFDL model operational forecasts. At present, the ocean component of the GFDL/URI coupled model, the Princeton Ocean Model, is being transitioned to NCEP’s Hurricane WRF model. All of the initialization improvements discussed here will likely be included. It is important to note, however, that this initialization procedure can be easily adapted to work with the ocean component of other coupled models. Also, the initialization procedure is currently in the process of being partially automated so that, for example, a specified contour line on a GoM d26 map or SSH map can be used to automatically provide a first guess of the LC and ring positions (e.g. Leben 2005), after which the positions can be automatically adjusted based on any available in situ ocean temperature profiles with minimal human input.