Impact of Assimilating Surface Velocity Observations on the Model Sea Surface Height using the NCOM-4DVAR

The assimilation of surface velocity observations and their impact on the model sea surface height (SSH) is examined using an operational regional ocean model and its four-dimensional variational (4D-Var) analysis component. In this work, drifter-derived surface velocity observations are assimilated into the Navy's Coastal Ocean Model (NCOM) 4D-Var in weak-constraint mode for a Gulf of Mexico (GoM) experiment during August-September, 2012. During this period the model is trained by assimilating surface velocity observations (in a series of 96-hr assimilation windows), which is followed by a 30-day forecast through the month of October, 2012. A free-run model and a model that assimilates along-track SSH observations are also run as baseline experiments to which the other experiments are compared. It is shown here that the assimilation of surface velocity measurements has a substantial impact on improving the model representation of the SSH on par with the experiment that assimilates along-track SSH observations directly. Finally, an assimilation experiment is done where both along-track SSH and velocity observations are utilized in an attempt to determine if the observation types are redundant or complementary. It is found that the combination of observations provides the best SSH forecast, in terms of the fit to observations, when compared to the previous experiments.