Geocenter Motion from a Combination of GRACE Mascon and SLR Data
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena (CA), United States of America (claudio.abbondanza@jpl.caltech.edu)
GRACE and GRACE Follow-On (FO) Level 2 data provide quasi-monthly, band-limited estimates of Stokes (geopotential, spherical harmonic) coefficients mostly reflecting surface mass variability due to non-tidal atmosphere, ocean, and continental hydrology.
Although space gravimetry does not directly provide CM-related degree-1 Stokes coefficients, GRACE data have been successfully used over the years to complement time series of station positions from global space-geodetic (SG) network when inverting for Center-of-Mass to Center-of-Network (CM-CN) displacements (Wu et al, 2006).
Surficial mass variability observed through GRACE/GRACE-FO can be conveniently converted into load-induced (ENU) deformations at SG observing sites by adopting a spectral (i.e. load Love-number based) formalism and assuming Earth’s response is fully elastic and isotropic. GRACE-derived elastic displacements at observing sites would represent, if accurate, band-limited (degree 2 to 96, or higher if Mascon solutions are adopted) load-induced deformations that can be removed from SG-derived station displacements in order to more accurately recover degree-1 surface deformation signature (and therefore geocenter motion).
In this study, we adopt GRACE JPL Mascon RL06 data in conjunction with Preliminary Reference Earth Model-derived load Love numbers to infer elastic displacement at SG sites and remove them from SLR inherently geocentric time series of station positions.
In so doing, the residual SLR station displacements, consistently expressed in a geocentric frame, would in principle reflect a degree-1 deformation signature that can be recovered via either surface deformation (Chanard et al, 2018) or translational approach.
We will compare the SLR/GRACE (CM-CN) determined in this study to standard estimates of geocenter motion such as ILRS’s and JTRF2014’s estimated via translational approach and spectrally inverted solutions (CM-CF).
References
Chanard K et al, (2018). JGR-Sol Ea doi:10.1002/2017JB015245
Wu X et al, (2006). JGR-Sol Ea doi:10.1029/2005JB004100.
How to cite: Abbondanza, C., Chin, T. M., Gross, R. S., Heflin, M. B., Parker, J. W., Soja, B. S., Wiese, D. N., and Wu, X.: Geocenter Motion from a Combination of GRACE Mascon and SLR Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10369, https://doi.org/10.5194/egusphere-egu2020-10369, 2020
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Hi, Claudio, The amplitudes in the Tx and Ty are obviously smaller than those of Net Shift, particularly, 2mm smaller in the Ty,
do you think the geocenter motion in the Y is underestimated by using Degree 1 method?
Hello Hongjuan,
Thanks for your comment.
Yes, you're absolutely right in pointing
out the much smaller amplitude of the
seasonal geocentre motion signal in
Ty.
Methods computing geocentre motion (GM)
based on spectral inversions
(in the spherical harmonic domain)
tend to recover amplitudes of the
annual GM that are consistently smaller
than GM derived from pure LAGEOS-1/2
satellites.
In this exercize, the annual amplitude
of the signal is even smaller than, for
instance, that reported by Xiaoping Wu's
inversions in many of his papers.
Keep in mind that we are using an experimental
solution for SLR in this case. One in which
station-to-satellite range biases
(for each station of the SLR network) are being
fixed, during the data reduction, to some
pre-determined values.
This ILRS analysis scheme
seems to have an impact on geocentre motion in
that it produces an attenuation of the GM annual
signal on all of the three components.
The "attenuation" has to be interpreted as
"decrease" of the amplitude of GM signal
compared to standard-pre-ITRF2020 ILRS analyses (where
only a smaller number of range biases
were fixed/estimated).
Since we are considering an SLR data set
that has "smaller" GM oscillations to
begin with, when we remove GRACE-derived elastic
displacements, we end up having smaller amplitudes
of GM (compared to what we would get if we were to
adopt standard-pre-ITRF2020 ILRS solutions).
GRACE-derived displacements are here reconstructed
by removing GIA signals and by restoring the
de-aliasing products (GFZ analysis).
This exercize will be repeated (and the results
will be carefully double-checked) as soon as
ILRS will release its entirely reprocessed
data set (for ITRF2020).
Also, JPL Mascons have been recently
updated with a longer data set including
GRACE-FO fields.
Whether or not spectral inversions are downestimating
GM signal is hard to tell.
Some scholars have opposing views claiming that SLR
(LAGEOS1-2) actually tends to overestimate
GM signal and the ~7-mm annual oscillation we see on
SLR (LAGEOS1-2)-derived axial geocentre is somehow
"contaminated" by noise.
And so your question in the end boils down to the issue of
accuracy of one methodology (this exercize) versus
others (pure SLR solutions, spectral inversions).
And frankly I do not have an answer to that effect.
List of Acronyms
---------------
(*) GM stands for Geocentre Motion. Not to be confused with the
product of the Gravitational Constant and mass of a "heavenly body".
(*) SLR stands for Satellite Laser Ranging. Not to be confused with
Sea-Level-Rate.
(*) ILRS is the International Laser Ranging Service.
(*) MASCON stands for Mass Concentration.
(*) GIA is Glacial Isostatic Adjustment.
Hi Claudio,
Very nice work again. Saddly, we did not have the time to discuss during the session.
So, would you like to comment on the sensitiviy of the geocenter series you got with different GRACE and SLR solutions? It may be significant or maybe not. In case it is, how would you suggest tackling the problem? Maybe with a combined GRACE and SLR solutions?
Thanks.
Hello Alvaro,
Thanks for your interest in the EGU display
and I am sorry I was not able to answer properly your
question in the chat-room.
To answer your question on the sensitivity of
these results on the GRACE/SLR solutions
adopted, I would say it is significant,
in the sense that if one were to change
the datasets, either GRACE
or SLR, one would see changes in the
seasonal geocentre motion estimates
in the order of a few mm.
GRACE Mascon solutions are less (if not at
all) affected by North-South spatially
correlated errors and generally
characterized by higher spatial
resolution.
Standard Spherical Harmonic solutions
require smoothing/destriping be applied
and this produces an attenuation of the
geophysical signal.
So to use GRACE standard Spherical
Harmonics Solution (e.g. RL06) vs
MASCON would produce differences in geocentre
motion. I do not have numbers to
share as to the differences
GRACE MASCON vs GRACE RL06 for this
test as of now. But I am positive there
might be.
Likewise, the SLR solution adopted
in this exercize has an impact on the
final geocentre.
In the current SLR reprocessing, ILRS
has introduced fixed
station-to-satellite range biases for all
of the stations. This is something new that
was never done before by ILRS.
And the impact of this processing strategy
on seasonal geocentre
motion could be significant: we're talking
about attenuations of the annual signal
amplitude in the order of 2-mm for
axial geocentre.
If I were to repeat this exercize
with old-style SLR solutions, I would
probably get larger geocentre
oscillations.
As to combining GRACE/SLR at the observation
level, not sure about the potential improvement.
In gravity literature, GRACE and SLR are
generally viewed as "complementary"
missions: SLR, on the hand, is
good at constraining the lower degrees
of the gravity field.
GRACE, on the other, does a good job with
the higher degrees.
Zonal coefficients from degree 2 up to degree 5/6
determined via GRACE (and in particular GRACE-FO) are
known for being rather noisy, if not inaccurate
at all. So my gut feeling in this respect would
be that mixing together normal equations
from SLR and GRACE/GRACE-FO could degrade
the quality of C_{l0}, unless the weighting
strategy adopted when building the NEQs
is such to favour SLR. And if we did
that, then we would resort to a situation
where SLR constraints the lower-degree gravity
whereas GRACE takes care of the rest.
But I am not expert on inversion of
SLR/GRACE NEQs. And these
considerations are purely speculative
and not grounded on actual analyses.
Thanks again for your interest,
-claudio
List of Acronyms
----------------
RL06 - Release 06 (for GRACE/GRACE-FO and de-aliasing products)
NEQ - Normal Equations
Mascon - Mass Concentratios
SLR - Satellite Laser Ranging
ILRS - International Laser Ranging Service.