De-aliasing of Short-term Atmospheric and Oceanic Mass Variations for GRACE
GFZ is responsible for routine calculation of atmospheric and oceanic mass variations which have to be considered during GRACE precise orbit determination and calculation of gravity field partial derivatives. This Level-1B Atmosphere and Ocean De-aliasing product (AOD1B) is made available to the GRACE Science Data System and user community in terms of spherical harmonic coefficients with a maximum time delay of about 3–4 days dependent on the availability of required ECMWF meteorological fields. The spatial and time-variable vertical structure of the atmosphere is taken into account by vertical integration of the atmospheric masses. Oceanic mass variations are derived from a barotropic ocean model (PPHA) which was provided by JPL. The individual atmospheric and oceanic contributions as well as the processing strategy to derive the combined AOD1B product are described in the first part of this paper.
The PPHA model has some deficiencies such as the exclusion of the Arctic Ocean or reduced level of energy compared to in-situ ocean bottom pressure data. Thus, the influence of different non-tidal ocean models on GRACE gravity field solutions has been investigated for a seasonal cycle. It turned out that the barotropic MOG2D and the baroclinic OMCT models, both providing global output and based on more complex algorithms and parameterization, produce slightly better agreement when compared to NIMA gravity anomalies or to an altimeter-derived geoid. Similar results are obtained when comparing daily times series of 10×10 degrees gravity field models, which have been derived without correcting short-term mass variations, with the candidate non-tidal ocean models.
These tests indicate that the PPHA model shall be substituted by OMCT or MOG2D. Nevertheless, a dramatic improvement of the monthly gravity field solutions towards the pre-launch simulated baseline accuracy will not be reached. Instead, future work should primarily concentrate on the improvement of the temporal resolution and the inclusion of short-term (daily) hydrological mass variations.
Key wordsDe-aliasing Mass Variation Ocean Model
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