The significance of secular trends of mass variations derived from GRACE monthly solutions

Since 2002, the Earth's gravity field is globally observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission. The GRACE monthly gravity field solutions reflect mass variations in the atmosphere, hydrosphere and geosphere. The GRACE data are applied to determine variations with different periodic signatures (e.g., seasonal, short and medium-term), but also long-periodic mass variations and secular trends are derived. As the GRACE monthly solutions always show the integral effect of all mass variations, various analysis methods, filtering techniques and reduction models are used to separate single processes such as glacial isostatic adjustment and hydrological changes. On the one hand this requires, that the reduction models (e.g. from hydrology) have the same quality and resolution as the processes to be addressed. On the other hand, the restricted time span of the measurements may limit the separation of long-periodic and secular signals. Therefore, one has to be cautious with the final interpretation of the results. The filtering of the monthly gravity fields is another candidate that affects the interpretation, as the amplitudes of the signals and their spatial resolution are influenced. We show this on examples like the North American and Fennoscandian uplift areas and selected river basins, where the use of different reduction models from hydrology (such as WGHM, LaDWorld, GLDAS), the selection of the filters or a change of the analyzed time span can lead to misinterpretation of geophysical processes to be considered. It also becomes evident that the determination of trends should be performed together with the determination of periodic components. However, it is only appropriate to include such periodic terms which are really contained in the data, and consequently, which can be detected as significant.