How realistic are multi-decadal reconstructions of GRACE-like total
water storage anomalies?
Abstract
The Gravity Recovery and Climate Experiment (GRACE) mission has
monitored total water storage anomalies (TWSA) globally with
unprecedented resolution and accuracy since 2002. However, many
applications require a data-based, multi-decadal extended record of TWSA
prior to the GRACE period and for bridging the eleven-months gap between
GRACE and its successor GRACE Follow-On (GRACE-FO), that does not depend
on hydrological modelling. Statistical and machine-learning
‘reconstruction’ approaches have been developed to this end, mostly via
identifying relations of GRACE-derived TWSA to climate variables, and
some regional or global land data sets are now publicly available.
In this contribution, we compare the two global reconstructions by
Humphrey and Gudmundsson (2019) and Li et al.(2021) mutually and against
output from the water Global Analysis and Prognosis (WaterGAP)
hydrological model from 1979 onwards, against large-scale mass-change
derived from geodetic satellite laser ranging (SLR) from 1992 onwards,
and finally against differing GRACE/-FO solutions from 2002 onwards.
We find that the reconstructions agree surprisingly well in many regions
at seasonal and sub-seasonal timescales, even in the pre-GRACE era. We
find larger differences at inter-annual timescales which we speculate
are in part due to the way reconstructions are trained, and in part on
which specific GRACE solution they are trained as well as the
climatological characteristics of the region. Our comparison against
independent SLR data reveals that reconstructions (only) partially
succeed in representing anomalous TWSA for regions that are influenced
by large climate modes such as El
Ni$\tilde{\text{n}}$o-Southern
Oscillation (ENSO).