2013.10.17 ice sheet-symposium_ditmar
DESCRIPTION
On 17/10/2013 TU Delft Climate Institute organised the symposium The Greenland and Antarctic ice sheets: present, future, and unknowns. This is one of the four presentations given there. http://www.tudelft.nl/nl/actueel/agenda/event/detail/symposium-tu-delft-climate-institute-17th-october-2013/TRANSCRIPT
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Weighing Ice Sheets from Space
Pavel Ditmar
Department of Geoscience and Remote SensingDelft University of Technology
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How to weigh up an ice sheet?
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Measurement principle of satellite gravimetry
M
F
a
x
Newton's law of universal gravitation:
Newton's 2nd law:
Equation of motion:
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GRACE SatelliteMission
(C) http://www.csr.utexas.edu
• Launch date: March 17, 2002
• Altitude: 450 – 500 km
• Expected operation period: until 2017
• Satellite-to-satellite distance: ≈ 200 km
• Primary sensor: K-Band Ranging (KBR)
system
• Inter-satellite ranging accuracy: ≈ 10-6 m
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Mass variations in Greenland and Antarctica observed with GRACE(Delft Mass Transport model DMT-2)
Meters of equivalent water height
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Satellite altimetry
d
h = Alt - d
Elevation Satellite altitude
ΔV = Δh * S
Volumechange
Elevationchange
Area
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Satellite altimetry missions used in ice sheet studies
Mission
Operational period
Agency Primary instrument
ERS-1 1991 – 2000
ESA Radar
ERS-2 1995 – 2003
ESA Radar
Envisat 2002 – 2012
ESA Radar
ICESat 2003 – 2009
NASA Laser
CryoSat 2010 – now ESA SIRAL (high-resolution radar)
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Mass balance estimates: inter-comparison of different methods and final numbers
Shepherd et al, Science, 2012
Mass balance in 2003 – 2008
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Temporal variability of mass changes
(Gravimetry and radar altimetry time-series are smoothed with a 13-month moving average)
Shepherd et al, Science, 2012
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Features and limitations of satellite altimetry
Conventional radar
SIRAL Laser
Spatial resolution ~ 10 km ~ 10 km cross-track~ 0.3 km along-
track
~ 0.1 km
Senses fresh snow No No Yes
Number of suitable satellite missions
at least, 3 1 1
Temporal coverage
1991-2012 2010-now 2003-2009
Temporal resolution
~ 1 month ~ 1 month ~ 1 year
Operates in the pre-sence of cloud cover
Yes Yes No
Senses mass changes directly
No No No
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Figure from http://www.physicalgeography.net/
Features and limitations of satellite gravimetry: Sensitivity to Glacial Isostatic Adjustment (GIA)
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Features and limitations of satellite gravimetry: Sensitivity to GIA (cont’d)
GIA-related mass change rate(ice model: ICE-5G; Earth’s viscosity model: VM2)
Liu et al,GJI, 2010
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snowice
snowsnowiceice
VVV
VVM
Mass change rates in Antarctica from GRACE and ICESat data (2003 – 2009)
Total mass change (GRACE)
Total height change (ICESat) GIA mass change Ice mass change
snowice
snowsnowiceice
VVV
VVM
observed
pre-defined GIA
GIA GIA
Didova et al, 2013
EWH (cm/yr)(CSR RL04 DDK3)
(cm/yr) EWH (mm/yr) EWH (mm/yr)
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Further limitation of GRACE: anisotropic sensitivity
Let f =: f(y) => = 0
GRACE Ground-tracks (a day in July 2006)
GRACE-1
GRACE-2
(not to scale)
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Mitigation of GRACE limitations:application of “intelligent” constraints
Unconstrained GRACE solution
GRACE solution constrained over the ocean
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Limitations of satellite gravimetry: attenuation of signal with altitude
Gravitational attraction of two point masses (1 Gt each) at the altitude h = 500 km
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Future of satellite gravimetry
• Better geometry of satellite formations,
usage of several satellite pairs
simultaneously
• Lower satellite altitude (250-300 km)
• Usage of more accurate onboard
instruments
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Impact of noise in satellite attitudes onto the quality of mass transport solutions
RMS = 20 mRMS = 5 m
Unconstrained GRACE solution,total noise
Unconstrained GRACE solution,noise in satellite attitudes only
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“Conventional”
airplanes?
Airships?
HALE (High Attitude Long Endurance) Unmanned
Vehicles ?
Questions to be answered: how to ensure a suitable
• ... accuracy of gravimetric measurements?
• ... accuracy of positioning?
• ... spatial coverage and observation repeat period?
• ...
More distant future of “gravimetric remote sensing”
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Conclusions
• Current technology level allows the rate of total ice mass loss in Greenland and Antarctic to be reliably estimated. Ice sheets in Greenland and Antarctic steadily loose mass in the last 20 year: the average rate of mass loss is 142 Gt/yr and 71 Gt/yr, respectively (Shepherd et al, 2012).
• Further improvements in data processing and observational concepts are needed to monitor ice sheet changes with a high temporal and spatial resolution.
• Gravimetry is the only observational concept that can sense total mass changes. Further development of this technique will have a large impact on ice sheet studies.
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Acknowledgements
The author thanks colleagues from the Dept. GRS of TU Delft for providing contributions to the presentation:
• Sun Yu• Jiangjun Ran• Olga Didova• Pedro Inacio• Hassan Hashemi Farahani• Riccardo Riva