Abstract
Observing global terrestrial water storage changes (TWSCs) from (inter-)seasonal to (multi-)decade time-scales is very important to understand the Earth as a system under natural and anthropogenic climate change. The primary goal of the Gravity Recovery And Climate Experiment (GRACE) satellite mission (2002–2017) and its follow-on mission (GRACE-FO, 2018–onward) is to provide time-variable gravity fields, which can be converted to TWSCs with ∼ 300 km spatial resolution; however, the one year data gap between GRACE and GRACE-FO represents a critical discontinuity, which cannot be replaced by alternative data or model with the same quality. To fill this gap, we applied time-variable gravity fields (2013–onward) from the Swarm Earth explorer mission with low spatial resolution of ∼ 1500 km. A novel iterative reconstruction approach was formulated based on the independent component analysis (ICA) that combines the GRACE and Swarm fields. The reconstructed TWSC fields of 2003–2018 were compared with a commonly applied reconstruction technique and GRACE-FO TWSC fields, whose results indicate a considerable noise reduction and long-term consistency improvement of the iterative ICA reconstruction technique. They were applied to evaluate trends and seasonal mass changes (of 2003–2018) within the world’s 33 largest river basins.
Highlights
The Gravity Recovery And Climate Experiment (GRACE, 2002–2017) satellite mission [1] was a joint United States (National Aeronautics and Space Administration, NASA) and German (Deutsche Zentrum für Luft- und Raumfahrt, DLR) space mission, which provided estimates of variations in the gravity field arising from mass movements within the Earth system
We show that the iterative reconstruction yields terrestrial water storage changes (TWSCs) fields that contain a lower level of noise, and in Section 4.2, these fields were used to assess water mass changes within the world’s 33 largest river basins
The positive–negative anomalies of the northern and southern hemispheres are well reflected in the Swarm TWSC fields; even by a simple visual comparison of the Swarm and GRACE fields, it is clear that mass changes from Swarm data are over-smoothed, i.e., overall a damping of ∼ 1.5 to 2 in terms of standard deviation (StD) was estimated to match Swarm TWSC fields over land to that of GRACE
Summary
The Gravity Recovery And Climate Experiment (GRACE, 2002–2017) satellite mission [1] was a joint United States (National Aeronautics and Space Administration, NASA) and German (Deutsche Zentrum für Luft- und Raumfahrt, DLR) space mission, which provided estimates of variations in the gravity field arising from mass movements within the Earth system. Over any typical 30-day span (the nominal data accumulation interval of GRACE gravity field solutions) of the non-repeating orbit configurations, a dense ground track grid can be obtained (see e.g., [2]); usually (almost) one month of the pre-processed GRACE along-track KBR data were used to compute time-variable gravity field solutions that are known as the GRACE level 2 data (GRACE L2, [3]) These L2 data were used to provide satellite-based estimates of global and regional terrestrial water storage change (TWSC, a vertical summation of water storage changes) with a few hundred kilometers spatial resolution, e.g., over Asia (e.g., [4,5,6]), Africa (e.g., [7,8]), and Australia (e.g., [9,10,11]). A review of GRACE applications in climate studies is provided in [17]
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