2013.10.17 ice sheet-symposium_ditmar

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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