Weighing Ice Sheets from Space

Pavel Ditmar
Department of Geoscience and Remote Sensing
Delft University of Technology

C...
How to weigh up an
ice sheet?

Challenge the future

3
Measurement principle of satellite
gravimetry

M
Newton's law of universal
gravitation:

F

Newton's 2nd law:

a

Equation...
GRACE Satellite
Mission

• Launch date: March 17, 2002
• Altitude: 450 – 500 km

(C) http://www.csr.utexas.edu

• Expected...
Mass variations in Greenland and
Antarctica observed with GRACE
(Delft Mass Transport model DMT-2)

Meters of equivalent w...
Satellite altimetry

h = Alt - d

d

Elevation

Satellite altitude

ΔV = Δh * S
Volume
change

Elevation
change

Area

Cha...
Satellite altimetry missions
used in ice sheet studies
Mission Operational Agency
period

Primary instrument

ERS-1

1991 ...
Mass balance estimates: intercomparison of different methods and
final numbers
Mass balance in 2003 – 2008

Challenge the ...
Temporal variability of mass changes

(Gravimetry and
radar altimetry
time-series are
smoothed with
a 13-month
moving aver...
Features and limitations of
satellite altimetry
Conventional
radar

SIRAL

Laser

Spatial resolution

~ 10 km

~ 10 km cro...
Features and limitations of satellite gravimetry:
Sensitivity to Glacial Isostatic Adjustment (GIA)

Figure from http://ww...
Features and limitations of satellite gravimetry:
Sensitivity to GIA (cont’d)
GIA-related mass change rate
(ice model: ICE...
Mass change rates in Antarctica from
GRACE and ICESat data (2003 – 2009)
observed

M
M
VV

Total mass
change (GRACE)

EWH ...
Further limitation of GRACE:
anisotropic sensitivity
GRACE Ground-tracks
(a day in July 2006)
GRACE-1
GRACE-2
(not to scal...
Mitigation of GRACE limitations:
application of “intelligent” constraints
Unconstrained
GRACE solution

GRACE solution
con...
Limitations of satellite gravimetry: attenuation
of signal with altitude
Gravitational attraction of two point masses
(1 G...
Future of satellite gravimetry
• Better geometry of satellite formations, usage of
several satellite pairs simultaneously
...
Impact of noise in satellite attitudes onto
the quality of mass transport solutions

Unconstrained GRACE solution,
total n...
More distant future of “gravimetric
remote sensing”
Questions to be
answered: how to
ensure a suitable
•

•
•

•

... accu...
Conclusions
• Current technology level allows the rate of total ice mass loss
in Greenland and Antarctic to be reliably es...
Acknowledgements
The author thanks colleagues from the Dept. GRS of
TU Delft for providing contributions to the presentati...
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2013.10.17 ice sheet-symposium_ditmar

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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/

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  • Add how many Gt/yr!!! And equivalent sea level rise!!!The figures show the result of the GRACE-ICESat combination for the period 03-2003 till 03-2007. The first two plots show trends from 1) mass change measured by GRACE, 2) surface elevation change measured by ICESat. The combination of the two data sets allows the isolation of trends of surface elevation changes due to GIA and ice mass changes. Figure c) shows for the first time ever the result of the first direct observation of the Antarctic GIA over the whole continent. Noteworthy, the result of this direct observation shows some common features with IJ05 model results. The presence of significant differences over the two main ice shelves and the Antarctic Peninsula, however,, suggest the potential of those results to considerably improve existing models of Antarctic glacial history.Compared with other techniques for mass balance estimation (e.g. budget method), the combination approach has the advantage to allow an accurate localization of the areas of mass change. This is important for ice streams discharging into the Amundsen Sea Embayment (between 230-270 deg longtiude), which appear to be currently out of blaance and are therefore raising concerns about the stability of the WAIS.
  • 2013.10.17 ice sheet-symposium_ditmar

    1. 1. Weighing Ice Sheets from Space Pavel Ditmar Department of Geoscience and Remote Sensing Delft University of Technology Challenge the future 1
    2. 2. How to weigh up an ice sheet? Challenge the future 3
    3. 3. Measurement principle of satellite gravimetry M Newton's law of universal gravitation: F Newton's 2nd law: a Equation of motion: x Challenge the future 4
    4. 4. GRACE Satellite Mission • Launch date: March 17, 2002 • Altitude: 450 – 500 km (C) http://www.csr.utexas.edu • 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 Challenge the future 5
    5. 5. Mass variations in Greenland and Antarctica observed with GRACE (Delft Mass Transport model DMT-2) Meters of equivalent water height Challenge the future 6
    6. 6. Satellite altimetry h = Alt - d d Elevation Satellite altitude ΔV = Δh * S Volume change Elevation change Area Challenge the future 7
    7. 7. Satellite altimetry missions used in ice sheet studies Mission Operational Agency period 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) Challenge the future 8
    8. 8. Mass balance estimates: intercomparison of different methods and final numbers Mass balance in 2003 – 2008 Challenge the future 9 Shepherd et al, Science, 2012
    9. 9. Temporal variability of mass changes (Gravimetry and radar altimetry time-series are smoothed with a 13-month moving average) Challenge the future 10 Shepherd et al, Science, 2012
    10. 10. 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 presence of cloud cover Yes Yes No Senses mass changes directly No No No Challenge the future 11
    11. 11. Features and limitations of satellite gravimetry: Sensitivity to Glacial Isostatic Adjustment (GIA) Figure from http://www.physicalgeography.net/ Challenge the future 12
    12. 12. 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 Challenge the future 13
    13. 13. Mass change rates in Antarctica from GRACE and ICESat data (2003 – 2009) observed M M VV Total mass change (GRACE) EWH (cm/yr) (CSR RL04 DDK3) iceice VV iceice snow snow GIA VV GIA ice snow V ice V V snow V Total height change (ICESat) (cm/yr) snow snow GIA pre-defined GIA mass change EWH (mm/yr) Ice mass change EWH (mm/yr) Didova et al, 2013 Challenge the future 14
    14. 14. Further limitation of GRACE: anisotropic sensitivity GRACE Ground-tracks (a day in July 2006) GRACE-1 GRACE-2 (not to scale) Let f =: f(y) => Challenge the future =0 15
    15. 15. Mitigation of GRACE limitations: application of “intelligent” constraints Unconstrained GRACE solution GRACE solution constrained over the ocean Challenge the future 16
    16. 16. Limitations of satellite gravimetry: attenuation of signal with altitude Gravitational attraction of two point masses (1 Gt each) at the altitude h = 500 km Challenge the future 17
    17. 17. 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 Challenge the future 18
    18. 18. Impact of noise in satellite attitudes onto the quality of mass transport solutions Unconstrained GRACE solution, total noise RMS = 20 m Unconstrained GRACE solution, noise in satellite attitudes only RMS = 5 m Challenge the future 19
    19. 19. More distant future of “gravimetric remote sensing” Questions to be answered: how to ensure a suitable • • • • ... accuracy of gravimetric measurements? ... accuracy of positioning? ... spatial coverage and observation repeat period? ... Airships? “Conventional” airplanes? HALE (High Attitude Long Endurance) Unmanned Vehicles ? Challenge the future 20
    20. 20. 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. Challenge the future 21
    21. 21. 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 Challenge the future 22

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