Abstract. Although the knowledge of the gravity of the Earth has improved considerably
with CHAMP, GRACE, and GOCE (see appendices for a list of abbreviations) satellite missions, the geophysical community has
identified the need for the continued monitoring of the time-variable
component with the purpose of estimating the hydrological and glaciological
yearly cycles and long-term trends. Currently, the GRACE-FO satellites are
the sole dedicated provider of these data, while previously the GRACE
mission fulfilled that role for 15 years. There is a data gap spanning from
July 2017 to May 2018 between the end of the GRACE mission and start the of
GRACE-FO, while the Swarm satellites have collected gravimetric data with
their GPS receivers since December 2013. We present high-quality gravity field models (GFMs) from Swarm data that
constitute an alternative and independent source of gravimetric data, which
could help alleviate the consequences of the 10-month gap between GRACE and
GRACE-FO, as well as the short gaps in the existing GRACE and GRACE-FO
monthly time series. The geodetic community has realized that the combination of different
gravity field solutions is superior to any individual model and set up the
Combination Service of Time-variable Gravity Fields (COST-G) under the
umbrella of the International Gravity Field Service (IGFS), part of the
International Association of Geodesy (IAG). We exploit this fact and deliver the highest-quality monthly GFMs, resulting from the combination of four
different gravity field estimation approaches. All solutions are
unconstrained and estimated independently from month to month. We tested the added value of including kinematic baselines (KBs) in our
estimation of GFMs and conclude that there is no significant improvement.
The non-gravitational accelerations measured by the accelerometer on board
Swarm C were also included in our processing to determine if this would
improve the quality of the GFMs, but we observed that is only the case when the
amplitude of the non-gravitational accelerations is higher than during the
current quiet period in solar activity. Using GRACE data for comparison, we demonstrate that the geophysical signal
in the Swarm GFMs is largely restricted to spherical harmonic degrees below
12. A 750 km smoothing radius is suitable to retrieve the temporal variations
in Earth's gravity field over land areas since mid-2015 with roughly 4 cm
equivalent water height (EWH) agreement with respect to GRACE. Over ocean
areas, we illustrate that a more intense smoothing with 3000 km radius is
necessary to resolve large-scale gravity variations, which agree with GRACE
roughly at the level of 1 cm EWH, while at these spatial scales the GRACE observes variations with amplitudes
between 0.3 and 1 cm EWH. The agreement with GRACE and GRACE-FO over nine
selected large basins under analysis is 0.91 cm, 0.76 cm yr−1, and 0.79 in terms of
temporal mean, trend, and correlation coefficient, respectively. The Swarm monthly models are distributed on a quarterly basis at ESA's Earth
Swarm Data Access (at https://swarm-diss.eo.esa.int/, last access: 5 June 2020, follow
Level2longterm and then EGF) and at the International Centre for Global Earth Models
(http://icgem.gfz-potsdam.de/series/02_COST-G/Swarm, last access: 5 June 2020), as
well as identified with the DOI https://doi.org/10.5880/ICGEM.2019.006 (Encarnacao et al.,
2019).
On the problem of plotting regional analytical approximations of elements of the elements of anomalous gravity field
