Title: On the use of terrestrial data gravity, GPS, hydrology to validate GRACE
1On 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
2Introduction
- 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)
3Outline
- Main issues on the validation of
satellite-derived gravity observations with
ground gravity measurements - A new validation experiment in western Africa
4Comparison 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
5- 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
6Validation 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)
7Monthly predictions from the GLDAS model
(2002-2005)
Free-air gravity effect µGal
-2.3
2.3
8Predictions 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
9GPS 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
10Estimation 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
11(No Transcript)
12Conclusion
- 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