Comparison of 4DVAR Assimilation of Altimeter and Underwater Glider Observations in the Ligurian Sea, May–June 2016

We perform ocean data assimilation experiments using satellite altimeter observations and observations from subsurface ocean gliders piloted along the satellite altimeter tracks. Gliders were deployed by the NATO Center for Marine Research and Experimentation (CMRE) in the 2016 Long-Term Glider Mission for Environmental Characterization (LOGMEC) field experiment in the Ligurian Sea (Western Mediterranean Sea) during May and June 2016. Gliders followed Jason-2 and SARAL satellite tracks and observations between the two platforms were typically collocated within 5 km in space and 5 days in time.

Glider observations are assimilated in the Relocatable Navy Ocean Coastal Model (RELO NCOM) with four dimensional variational assimilation (NCOM-4DVAR). RELO NCOM is used by the US Navy to predict the ocean. RELO NCOM is a baroclinic, Boussinesq, free-surface, and hydrostatic ocean model with a flexible sigma-z vertical coordinate.

Several experiments are conducted, using glider observations, altimeter observations, or both data sets. Data sets are further varied by either limiting data to a one-to-one match between satellite observations and glider profiles (typically 5-10 altimeter observations/glider profiles per analysis cycle) or by using all available data (20+ altimeter observations/50+ glider profiles). Further, in the latter experiment, spatially dense glider profiles are assimilated using a new multi-scale data processing and assimilation approach, allowing for many more profiles than standard NCOM-4DVAR, improving the analysis and forecast. Glider observations have the potential to improve model forecasts, particularly for temperature and salinity.