Using 3-degree mascons as a basis function acts as an inherent smoother on the data. Therefore, the GRACE/Tellus Project has derived a set of scale factors (what are also referred to as global gain factors) to aid in the interpretation of signals at sub-mascon resolution. This approach is similar to the derivation of the gain factors for the spherical harmonic solutions (see Landerer and Swenson, 2012). The approach utilized is to “mascon-average” the Community Land Model (CLM) hydrology model (for more details, see: http://www.cesm.ucar.edu/models/clm/hydrologic.html), and compute a least-squares fit to the “mascon-averaged” hydrology model and the original data. This result is a set of 0.5 degree gain factors which can be used over land areas for terrestrial hydrology applications; they are not intended for use in mountain glacier regions or ice sheets, as these are not modeled in CLM. Users should note that the these gain factors are much closer to 1 than for the spherical harmonic gravity solutions (Landerer et al., 2012). Furthermore, the gain factors are suitable for application to the CRI-filtered version of the data only, but are not suitable for users who prefer to use the standard Mascon solutions that have not been CRI-filtered. For more information on scaling the Mascon solutions, please see Watkins et al. (2015). For general information about the Mascon datasets, please visit: https://podaac.jpl.nasa.gov/datasetlist?search=mascon.
Summarily, the gain factor is the multiplicative scaling coefficient that minimizes the difference between the model's smoothed and unfiltered monthly terrestrial water storage variations at any geographic location influenced by land hydrology. These gain factor coefficients are independent of the GRACE data, hence they are provided as a separate file. Furthermore, the gain factors tend to be dominated by the annual cycles of land water storage variations, and may thus not be suitable for trend analysis, which have their own corresponding gain factors.
For more detailed information about the processing of the Mascon solutions, please refer to: https://grace.jpl.nasa.gov/data/get-dat ... l_mascons/.
1. Landerer, F. W., and S. C. Swenson (2012), Accuracy of scaled GRACE terrestrial water storage estimates, Water Resources Research, 48(4), W04531, doi: http://dx.doi.org/10.1029/2011WR011453
2. Watkins, M. M., D. N. Wiese, D.-N. Yuan, C. Boening, and F. W. Landerer (2015), Improved methods for observing Earth’s time variable mass distribution with GRACE using spherical cap mascons, J. Geophys. Res. Solid Earth, 120, doi:10.1002/2014JB011547.