On the use of terrestrial data gravity, GPS, hydrology to validate GRACE

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On the use of terrestrial data (gravity, GPS,
hydrology) to validate GRACE

• Caroline de Linage, Jacques Hinderer,
• Jean-Paul Boy Pascal Gégout
• EOST-Institut de Physique du Globe de Strasbourg

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Introduction

• Comparison GRACE/global hydrology models
• large variability between models
• lack of well sampled hydrology data
• Need to have other independent data sets for
  comparison to GRACE
• Comparison GRACE/GGP in Europe (Neumeyer,
  Crossley)
• small amplitude of the signal (a few µGal)

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Outline

• Main issues on the validation of
  satellite-derived gravity observations with
  ground gravity measurements
• A new validation experiment in western Africa

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Comparison between ground and satellite
measurements

• need to have GRACE solutions with the best
  spatial and temporal resolution (mascons)
• calibration is not possible
• rather intercomparison between different data
  sets

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• low-degree effect in ground gravity due to the
  free-air effect (elastic part of the loading)
• estimating the hydrology signal in the GPS time
  series of vertical displacement and removing the
  free-air effect from the ground gravity residual
  signal (after the same corrections as for the
  GRACE observations)

Degree-power spectra (µGal) of gravity
predictions from the global hydrology model
LaDWorld for April 2002
Ground gravity
Satellite gravity
90
90
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Validation experiment in Africa
absolute gravity measurements (FG5) every 2
months at 4(5) stations (2008-2010)
complementary measurements (A10) relative
gravity measurements (field SG) at Djougou

• Tamanrasset 20 mm/yr (Sahara) -gt null test
• Agadez 100 mm/yr (Sahara-Sahel transition)
• Diffa 300 mm/yr (Sahara-Sahel transition, lake
  Chad)
• Niamey 560 mm/yr (Sahel, Niger basin)
• Djougou 1200 mm/yr (monsoon)

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Monthly predictions from the GLDAS model
(2002-2005)
Free-air gravity effect µGal
-2.3
2.3
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Predictions at each station from GLDAS

• Gravity changes (µGal) Vertical displacement (mm)

• Peak-to-peak amplitudes
• Djougou 15 µGal
• 9.5 mm
• Niamey 10 µGal
• 5 mm
• Diffa 10 µGal
• 4.5 mm
• Tamanrasset 1-2 µGal
• 3.5 mm

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GPS station array

• 6 stations from the AMMA experiment maintained
  by EOST from 2009
• TOMB, OUAG, TAMA, DJOU, NIAM, GAO
• 2 EOST stations
• Diffa, Agadez
• 3 IGS stations
• Dakar, Libreville, Franceville
• 1 CRAAG (Algeria) station
• Tamanrasset

Agadez
Diffa
From Olivier Bock
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Estimation of the local hydrological gravity
changes

• Niamey, Diffa and Djougou have been studied since
  the 90s by hydrologists and hydrogeologists
• dense network of hydrological, geophysical and
  meteorological in situ observations at Djougou
  and Niamey (AMMA-CATCH 2003-2010)
• local ground water budget estimate
• topography
• precise modelling of the local Newtonian
  attraction term in gravity

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Conclusion

• Need to have independent in-situ measurements in
  addition to GRACE observations in order to
  improve the quality of hydrology models
• Continuous GPS measurements to assess the
  free-air effect not seen by GRACE
• In-situ measurements of hydrological parameters
  at each station to assist us in modelling the
  local gravity effect