Fleury-la-Vallée, France
Fleury-la-Vallée, France

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Legrand J.,Royal Observatory of Belgium | Bergeot N.,Royal Observatory of Belgium | Bruyninx C.,Royal Observatory of Belgium | Woppelmann G.,University of La Rochelle | And 2 more authors.
Journal of Geodynamics | Year: 2010

Ten years (1997-2006) of weekly GNSS solutions of 205 globally distributed stations have been used to investigate the impact of the reference frame definition on the estimated station velocities. For that purpose, weekly regional solutions (covering the European region) and global solutions have been, respectively, stacked to obtain regional and global velocity fields. In both cases, the estimated long-term solutions (station positions and velocities) were tied to the ITRF2005 under minimal constraints using a selected set of reference stations. Several sets of global and regional reference stations were tested to evaluate first the impact of the reference frame definition on the global and regional velocity fields and later the impact on the derived geodynamic interpretations. Results confirm that the regional velocity fields show systematic effects with respect to the global velocity field with differences reaching up to 1.3 mm/year in the horizontal and 2.9 mm/year in the vertical depending on the geographical extent of the network and the chosen set of regional reference stations. In addition, the estimations of the Euler pole for Western Europe differ significantly when considering a global or a regional strategy. After removing the rigid block rotation, the residual velocity fields show differences which can reach up to 0.8 mm/year in horizontal component. In Northern Europe, the vertical ground motion is dominated by the Glacial Isostatic Adjustment (GIA). A proper modeling of this effect requires sub-mm/year precision for the vertical velocities for latitudes below 56°. We demonstrate that a profile of vertical velocities shows significant discrepancies according to the reference frame definition strategy. In the case of regional solutions, the vertical modeling does not predict any subsidence around 52° as predicted by the global solution and previous studies. In summary, we evidence the limitation of regional networks to reconstruct absolute velocity fields and conclude that when geodynamics require the highest precisions for the GNSS-based velocities, a global reference frame definition is more reliable. © 2009 Elsevier Ltd. All rights reserved.

Nahmani S.,LAREG | Bock O.,LAREG | Santamaria-Gomez A.,Instituto Geografico Nacional | Boy J.-P.,University of Strasbourg | And 8 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

Three-dimensional ground deformation measured with permanent GPS stations in West Africa was used for investigating the hydrological loading deformation associated with Monsoon precipitation. The GPS data were processed within a global network for the 2003-2008 period. Weekly station positions were retrieved with a repeatability (including unmodeled loading effects) of 1-2mm in the horizontal components and between 2.5 and 6mm in the vertical component. The annual signal in the vertical component for sites located between 9.6°N and 16.7°N is in the range 10-15mm. It is consistent at the 3mm-level with the annual regional-scale loading deformations estimated from GRACE satellite products and modeled with a combination of hydrological, atmospheric, and nontidal oceanic models. An additional 6month transient signal was detected in the vertical component of GPS estimates at most of the West African sites. It takes the form of an oscillation occurring between September and March, and reaching a maximum amplitude of 12-16mm at Ouagadougou (12.5°N). The analysis of in situ hydro-geological data revealed a strong coincidence between this transient signal and peak river discharge at three sites located along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar coincidence was found with the seasonal variations of the water table depth. We propose a mechanism to account for this signal that involves a sequence of swelling/shrinking of clays combined with local loading effects associated with flooding of the Niger River. Copyright 2012 by the American Geophysical Union.

Bock O.,LAREG | Bock O.,French National Center for Scientific Research | Guichard F.,Meteo - France | Meynadier R.,French National Center for Scientific Research | And 7 more authors.
Atmospheric Science Letters | Year: 2011

The vertically integrated water budget of West Africa is investigated with a hybrid dataset based on observational and modelling products elaborated by the African Monsoon Multidisciplinary Analyses (AMMA) and with several numerical weather prediction (NWP) products including the European Centre for Medium-Range Weather Forecasts (ECMWF) AMMA reanalysis. Seasonal and intraseasonal variations are quantified over the period 2002-2007. Links between the budget terms are analyzed regionally, from the Guinean coast to the Sahel zone. Water budgets from the NWP systems are intercompared and evaluated against the hybrid dataset. Large deficiencies are evidenced in all the NWP products. Hypotheses are proposed about their origins and several improvements are foreseen. Copyright © 2010 Royal Meteorological Society.

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