[1] We use gravity implied by the Earth's rock-equivalent topography (RET) and modeled isostatic compensation masses to evaluate the new global gravity field models (GGMs) from European Space Agency (ESA)'s Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite gravimetry mission. The topography is now reasonably well-known over most of the Earth's landmasses, and also where conventional GGM evaluation is prohibitive due to the lack (or unavailability) of ground-truth gravity data. We construct a spherical harmonic representation of Earth's RET to derive band-limited topography-implied gravity, and test the somewhat simplistic Airy/Heiskanen and Pratt/Hayford hypotheses of isostatic compensation, but which did not improve the agreement between gravity from the uncompensated RET and GOCE. The third-generation GOCE GGMs (based on 12 months of space gravimetry) resolve the Earth's gravity field effectively up to spherical harmonic degree $200-220 ($90-100 km resolution). Such scales could not be resolved from satellites before GOCE. From the three different GOCE processing philosophies currently in use by ESA, the time-wise and direct approaches exhibit the highest sensitivity to short-scale gravity recovery, being better than the space-wise approach. Our topography-implied gravity comparisons bring evidence of improvements from GOCE to gravity field knowledge over the Himalayas, Africa, the Andes, Papua New Guinea and Antarctic regions. In attenuated form, GOCE captures topography-implied gravity signals up to degree $250 ($80 km resolution), suggesting that other signals (originating, e.g., from the crust-mantle boundary and buried loads) are captured as well, which might now improve our knowledge on the Earth's lithosphere structure at previously unresolved spatial scales.
The new Release-06 (RL06) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions are evaluated by converting them into equatorial effective angular momentum functions (so-called excitation functions) for polar motion and comparing these to respective time series based on space-geodetic observations (geodetic excitation). The same is performed for the older RL05 solutions using identical processing. Maps of equivalent water heights derived from both releases show that the signal-to-noise ratio is significantly improved in RL06. The derived polar motion excitation functions from RL05 and RL06 differ by about 15%. An analysis of the contributions of different Earth subsystems revealed that the release update mainly influenced the hydrological (12%) and oceanic excitations (17%), but it has a relatively small impact on the cryospheric excitations related to Antarctica (4%) and Greenland (1%). The RL06 data from different GRACE processing centers are more consistent among each other than the previous RL05 data. Comparisons of the GRACE-based excitation functions with the geodetic and model-based oceanic excitations show that the latest release update improved the agreement by about 2 to 15 percentage points.
Abstract:In this study, reliable water levels for four lakes are estimated based on an innovative processing strategy using a semi-automatic CryoSat-2 Synthetic Aperture Radar (SAR) multi-looked waveform classification. The selection of valid water returns is an essential step in inland altimetry applications. In order to identify reliable observations allowing for an accurate retracking, an unsupervised classification method for CryoSat-2 SAR multi-looked waveforms has been developed based on the k-mean algorithm. With this approach, changes in the water surface extent or surrounding inundation areas can be taken into account. In addition, a modified version of the Improved Threshold Retracker is developed in order to obtain optimal results for the lake heights. The used method is based on the identification of the optimal sub-waveform by employing height thresholds. The validation of the derived CryoSat-2 SAR time series with in-situ gauging data yields root mean square (RMS) differences between 3 and 90 cm for the different lakes. Compared to modeled CryoSat-2 water heights derived according to the approach used in the AltWater database our water level time series are slightly improved in terms of RMS accuracy but they contain more gaps due to the lack of reliable observations. In comparison with classical radar altimeter missions such as Envisat or Jason-2, the SAR-based time series show smaller RMS differences for the small lakes but larger RMS differences for the large lakes covered by multiple repeat missions. The presented innovative processing strategy can be easily adopted to other satellite altimetry SAR data such as from the new Sentinel-3 mission.
Before the background of more accurate and denser gravity data it is worthwhile to reassess geodetic isostasy. Currently, in geodesy isostatic models are primarily applied to gravity reduction as needed by geoid and gravity modeling. The selection of the isostatic model is based on four criteria: Isostatically reduced gravity anomalies should be (1) geophysically meaningful, (2) easy to compute, (3) small, smooth and therefore easy to interpolate and (4) the indirect effect, i.e. the change of potential and gravity due to isostatic mass replacement, should be small. In this study we analyze free air anomalies as well as isostatic anomalies based on the Airy-Heiskanen model and on the Pratt-Hayford model in regard to these criteria. Several facts suggest that free air anomalies are the most realistic type of isostatic anomalies. They reflect the actual isostatic compensation, are easy to compute and their indirect effect is negligibly small. However, they are not smooth due to the fact that local topographic loads are only partially compensated. Smoothness can be achieved by introducing either a mathematical low-pass filter or a hydrostatic isostatic model, such as the Airy-Heiskanen or the Pratt-Hayford model. In both cases the resulting isostatically reduced gravity anomalies fulfill all requirements. In order to improve the numerical efficiency, a new mathematical description of the Pratt-Hayford model is formulated. The level of smoothing with respect to free air anomalies is analyzed in global and regional contexts. It turns out that the mechanism of mass compensation in regions of large topographic loads is better described by the Airy-Heiskanen model, whereas the Pratt-Hayford model is more suitable for regions of deep ocean trenches.
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