Abstract
In order to effectively recover surface mass or geoid height changes from the gravity recovery and climate experiment (GRACE) time-variable gravity models, spatial smoothing is required to minimize errors from noise. Spatial smoothing, such as Gaussian smoothing, not only reduces the noise but also attenuates the real signals. Here we investigate possible amplitude attenuations and phase changes of seasonal water storage variations in four drainage basins (Amazon, Mississippi, Ganges and Zambezi) using an advanced global land data assimilation system. It appears that Gaussian smoothing significantly affects GRACE-estimated basin-scale seasonal water storage changes, e.g., in the case of 800 km smoothing, annual amplitudes are reduced by about 25–40%, while annual phases are shifted by up to 10°. With these effects restored, GRACE-estimated water storage changes are consistently larger than model estimates, indicating that the land surface model appears to underestimate terrestrial water storage change. Our analysis based on simulation suggests that normalized attenuation effects (from Gaussian smoothing) on seasonal water storage change are relatively insensitive to the magnitude of the true signal. This study provides a numerical approach that can be used to restore seasonal water storage change in the basins from spatially smoothed GRACE data.
Highlights
The primary goal of the gravity recovery and climate experiment (GRACE) twin-satellite gravity mission is to produce measurements of the Earth’s time-variable gravity field at approximately 30-day intervals with unprecedented accuracy based on precise measurements of the distance between two satellites orbiting in tandem, as well as data from on-board accelerometers and global positioning system (GPS) receivers (Tapley et al 2004a)
Annual and semiannual amplitudes and phases are estimated using least-squares fit of global land data assimilation system (GLDAS)-estimated terrestrial water storage changes for each of the four river basins when different spatial radii are used in the Gaussian smoothing
On the basis of the GLDAS-model-estimated water storage changes, we investigate possible amplitude attenuations and phase changes in GRACE data as a result of Gaussian spatial smoothing
Summary
The primary goal of the gravity recovery and climate experiment (GRACE) twin-satellite gravity mission is to produce measurements of the Earth’s time-variable gravity field at approximately 30-day intervals with unprecedented accuracy based on precise measurements of the distance between two satellites orbiting in tandem, as well as data from on-board accelerometers and global positioning system (GPS) receivers (Tapley et al 2004a). We assume that when the spatial radius is appropriately chosen (or large enough), at least for these selected major basins with strong seasonal variability, the errors from spatial noise do not significantly affect the estimated seasonal water storage changes Another motivation behind this study is the fact that some ‘standard’ surface mass change products derived from GRACE time-variable gravity solutions are based on the commonly used Gaussian smoothing, e.g., the online interactive data archive at the GRACETellus Information Website at http://grace.jpl.nasa.gov (Zlotnicki et al 2005), and GRACE observed terrestrial water storage change products provided by the Global Geophysical Fluids Center’s Special Bureau for Hydrology (http://www.csr.utexas.edu/research/ggfc) (Chen and Wilson 2005). A quantitative assessment of potential attenuation effects from Gaussian smoothing at different spatial scales is of great interest to the general geoscience community, and provides a numerical (and independent) approach to restore GRACE-observed seasonal terrestrial water storage changes in selected river basins (when Gaussian smoothing is applied)
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