Vanya Romanova, Detlef Stammer, and Armin Köhl

The objective of ESA's gravity mission GOCE (Gravity Field and Steady-State Ocean Circulation Explorer, www.esa.int/esaLP/LPgoce.html) is the determination of the Earth's gravitational field with high spatial resolution and accuracy. A first quality assessment of the GOCE gradiometer measurements can be performed at satellite level by comparing the observed gravity gradients with reference gradients. These reference values can be computed from existing geopotential models. The resulting GOCE gravity models will be validated by means of independent gravity field information, i.e. information not already included in the GOCE gravity field determination. Such independent information may be:

1. terrestrial point or mean gravity anomalies (based on gravity measurements and levelling),

2. ocean point, profile or mean gravity anomalies (based on ship gravimetry or satellite altimetry),

3. geoid heights (derived from GPS and levelling),

4. satellite altimetry and dynamic topography (from ocean modelling or ocean levelling),

5. satellite orbits,

6. gravity models (from previous missions such as GRACE, CHAMP and others).

The inclusion of an ocean modelling component into the validation of the GOCE gravity field (Item 4) provides independent insight into the quality of gravity field products. Ocean models provide estimates of the dynamic topography which is part of the total sea surface height (SSH) as it is measured by altimetry. The geoid obtained as the difference between SSH and dynamic topography contains thus uncertainties of the model and altimeter products. Within the ocean state estimation framework, the MIT/ECCO ocean circulation model is combined with most of the available ocean data sources. A time varying circulation is estimated that is consistent with the dynamics and the data. The resulting dynamic topography is thereby expected to be closer to the observations, which are obtained as the difference of SSH and geoid, than a pure simulation. Fig. 4 shows the reduction of the RMS difference after replacing the EGM96 geoid with a more modern geoid based on GRACE measurements (compare left and middle panel). Consistently larger residuals exist for the estimate based on a pure simulation (right panel).

In situ measurements of the ocean bottom pressure for the Indian Ocean, South Atlantic and Weddell Sea are used for model validation and investigation of the short-term variability of the ocean system. The correlation coefficients of the measured data and the model results are higher for the state estimation (of about 10%) compared to the unconstrained run. The coherence spectra showed the maximum correlation with the measured bottom pressure at 15 days period. The use of gravity field products as part of the data constraints used in the state estimation provides additionally also an assessment of the impact of a more accurate gravity field on the ocean circulation.

Vanya Romanova