Transcript Improvements in Global Geoid Models by Including Ocean

The Potential to Estimate Ocean
Thermal Expansion by Combining
GRACE and Satellite Altimetry
Don P. Chambers
Center for Space Research
The University of Texas at Austin
Understanding Sea-Level Rise and Variability
6-9 June, 2006
Paris, France
GOALS
• Computing mean ocean mass component of sea level
from GRACE
• Potential for combining with altimetry to determine longterm trend in steric sea level
» Steric SL = Altimeter SL - GRACE SL
• Sources of uncertainty in rate estimate for GRACE
» Glacial Isostatic Adjustment (GIA) correction
» Degree 1 gravity coefficients (geocenter)
» Interannual variations in ocean mass and a short record
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Gravity Recovery & Climate Experiment
(GRACE)
Science Goals
Measure time variable gravity field
to detect changes in the water
storage and movement from
reservoir to another (e.g., from ice
sheets to ocean)
Mission
Joint NASA/German mission
implemented by NASA and DLR
(Deutschen Zentrum für Luft-und
Raumfahrt) under the NASA Earth
System Science Pathfinder
Program.
Science data processing by
University of Texas Center for Space
Research (UTCSR) and
GeoForschungsZentrum (GFZ)
Orbit
Launched: March 17, 2002
Regular Science Data: August, 2002
Original Lifetime: 5 years
Recently NASA/DLR extended
mission through 2009
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GRACE Errors
long
wavelength
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short
• GRACE project produces a
set of global gravity
coefficients (DClm, DSlm)
every month
• Can convert these to a
time-series of monthly
average water level (sea
level) over a basin by
Dba sin
Ocean kernel
Ql
C
S

W
DC

W

lm
lm
lmDSlm 
l,m ba sin
aE 2l  1
Ql 
3W 1 kl 
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• Ocean kernel
designed to minimize
error from GRACE
noise AND aliasing of
hydrology signals
[Swenson and Wahr,
JGR, 2002]
• From CSR_RL01 GRACE coefficients
» Replacing DC20 with values from SLR analysis and using
seasonal model of DC10, DC11, and DS11 terms (Chambers et al.,
GRL, 2004)
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Glacial Isostatic Adjustment
• GRACE will measure:
» The long-term gravitational change due to glacial
isostatic adjustment (GIA)
» Gravitational changes due to water mass transfer from
melting of ice sheets
» Shorter period exchanges of water mass with
continents
• Can we model GIA adequately over the ocean to
remove this signal?
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GIA in GRACE Observation
• GRACE will observe a fall in SL related to GIA
• Part of drop in sea level measured by GRACE
since 2002 is this GIA signal
• M. Tamisiea has calculated that the GIA signal in
the GRACE observations ranges from -0.6 to -2
mm/year
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• Adding maximum GIA correction to GRACE changes
interpretation of trend significantly
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Degree 1 Gravity Variations
• GRACE satellites orbit instantaneous mass center of
Earth
• Degree 1 gravity coefficients are zero in a reference
frame that is centered on the instantaneous mass center
• Terrestrial reference frame has a fixed center not at
instantaneous mass center
• Water mass flux in a terrestrial reference frame will have
variations in degree 1 terms
• To use GRACE to measure mass flux in an Earth-fixed
frame, we have to model/measure these degree 1
variations
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GRACE w/o degree 1 coefficients
GRACE w/ degree 1 coefficients
• Previously demonstrated importance of modeling degree 1
variations for seasonal ocean mass studies [Chambers et al., GRL,
2004; Chambers, JGR, 2006].
• Seasonal models of degree 1 variations have some level of
consistency
• Trends are completely unknown
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Greenland: 22.0 cm/m2 water mass
lost per year (0.75 mm SL)
Oceans: 1.5 mm/year increase in SL
Antarctica: 4.1 cm/m2 water mass lost
per year (0.75 mm SL)
Land: No change
• Convert simulated water level changes into
gravity field coefficients (to degree/order 180)
• Compute ocean mass with and without degree 1
terms
• Result: trend is 0.1 mm/year lower if degree 1
not used.
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Trend removed from Altimeter - TSL
With 1-year smoothing
• We have limited knowledge of interannual
variations in ocean mass
• Some evidence of ± 4-5 mm variations at ENSO
periods
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• Simulate interannual ocean mass by scaling SOI to
estimate from J. Willis in 1997-1998
» 55 yr. trend set to zero
• Estimate 95% confidence interval based on standard
deviation of trends over 3-15 yr intervals
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Rate Uncertainty for Ocean Mass from
GRACE with 3-years of Observations
Source
Uncertainty
(mm/year)
Formal
0.3
Knowledge of GIA correction
0.71
Knowledge of degree 1 rates
0.22
3-year period & ENSO-like variability
2.8
RSS (3-year rate)3
0.8
RSS (long-term)4
2.9
1 - from range of GIA corrections
2 - doubled simulation estimate to be conservative; systematic!
3 - without interannual uncertainty
4 - all sources of trend uncertainty
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Yearly averages, maximum GIA correction added to GRACE
• Why the big difference between in situ TSL and spacebased estimates?
» Unknown error in one or more of the systems?
» Changes in deep ocean heat storage not measured by Argo floats?
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