Jean-Paul Boy

Global atmospheric loading and gravity

Abstract The purpose of this paper is to model atmospheric pressure effects on surface gravity. Pressure corrections are usually done with the help of an empirical trans fer function called barometric admittance between pressure and gravity both measured locally. We show the validity of this local approach with the help of spectral coherency between air pressure and gravity residuals after subtraction from the superconducting gravimeter observations in Strasbourg (France) of luni-solar tides, polar motion and instrumental drift contributions. We compute the atmospheric surface loading effects using a Greens function formalism and global surface pressure data provided by the European Centre for Medium Range Weather Forecasts (ECMWF). We model a specific oceanic response to atmospheric loading which incorporates interactions between ocean, atmosphere and solid Earth and differs from the classical inverted or non-inverted barometer hypotheses. We investigate the contribution of this global pressure correction in a 3000 day (1987–1996) tidal analysis of the French superconducting gravimeter.

Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models

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 1–2 mm in the horizontal components and between 2.5 and 6 mm in the vertical component. The annual signal in the vertical component for sites located between 9.6N and 16.7N is in the range 10–15 mm. It is consistent at the 3 mm-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 6 month 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 12–16 mm at Ouagadougou (12.5N). 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.

Land Water Storage Changes from Ground and Space Geodesy: First Results from the GHYRAF (Gravity and Hydrology in Africa) Experiment

This paper is devoted to the first results from the GHYRAF (Gravity and Hydrology in Africa) experiment conducted since 2008 in West Africa and is aimed at investigating the changes in water storage in different regions sampling a strong rainfall gradient from the Sahara to the monsoon zone. The analysis of GPS vertical displacement in Niamey (Niger) and Djougou (Benin) shows that there is a clear annual signature of the hydrological load in agreement with global hydrology models like GLDAS. The comparison of GRACE solutions in West Africa, and more specifically in the Niger and Lake Chad basins, reveals a good agreement for the large scale annual water storage changes between global hydrology models and space gravity observations. Ground gravity observations done with an FG5 absolute gravimeter also show signals which can be well related to measured changes in soil and ground water. We present the first results for two sites in the Sahelian band (Wankama and Diffa in Niger) and one (Djougou in Benin) in the Sudanian monsoon region related to the recharge–discharge processes due to the monsoonal event in summer 2008 and the following dry season. It is confirmed that ground gravimetry is a useful tool to constrain local water storage changes when associated to hydrological and subsurface geophysical in situ measurements.

Are the free core nutation parameters variable in time

Abstract The Free Core Nutation (FCN) is a normal mode of the Earth which induces resonance in the diurnal tides as well as in the nutations. This is particularly true for frequencies near the resonance frequency, i.e., for the retrograde annual nutation and the ψ1 tide. The paper aims at analyzing Very Long Baseline Interferometry (VLBI) data and superconducting gravimeter (SG) data in order to gain more information on the FCN period, damping, resonance strength and free mode amplitude. Section 2 deals with the variability as seen in precise tidal gravimetric observations from consecutive subsets of an SG located near Strasbourg (France). We will show that the apparent variations appearing in the eigenperiod, quality factor and strength are related to a variable noise level in the tidal gravimetric data. Section 3 is devoted to the analysis of the time variability of the VLBI series. In particular we show that the amplitude variations found in the forced nutational response of the Earth are not real but rather induced by a variable free mode excitation. We show that the eigenperiod is stable within a range of 3 days.

Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry

In basement catchments of subhumid West Africa, base flow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of base flow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. WSC behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (<2.5 lGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis, make it possible to derive a conceptual model for the catchment hydrology.

Steric and mass‐induced sea level variations in the Mediterranean Sea revisited

This work was elaborated during the stay of the first author at the National Central University of Taiwan, thanks to a grant from the Generalitat Valenciana, Spain. Jean-Paul Boy is currently visiting NASA Goddard Space Flight Center, with a Marie Curie International Outgoing Fellowship (PIOF-GA-2008-221753). This work was partly funded by two Spanish projects from MICIN, ESP2006-11357, and AYA2009-07981 and one from Generalitat Valenciana (ACOMP2009/031).

Geodetic effects of the ocean response to atmospheric forcing in an ocean general circulation model

[1] We have adapted the Coupled Large-scale Ice-Ocean (CLIO) model in order to include atmospheric pressure forcing in addition to the classical atmospheric forcing ( heat and freshwater fluxes and wind stress). We have compared the modeled bottom pressure to in situ bottom pressure measurements and also to another ocean model. We have computed the induced effects on Earth orientation parameters ( polar motion and nutations) and also on Earth time-variable gravity field. We show that the response of the ocean to atmospheric pressure forcing has geodetic consequences well above the precision level and that the response of CLIO seems reasonable with respect to the few data sets we can compare with. We also show that the results obtained from the model improve the modeling of polar motion.

Modeling and Observation of Loading Contribution to Time-Variable GPS Sites Positions

We investigate loading consequences on the time-variable GPS station positions of one hundred stations around the world during the 2001–2006 time period. We model the three dimensional site displacements using a Love number formalism to describe the elastic deformation of a spherical Earth model submitted to atmospheric, oceanic and hydrological loadings.

Surface atmospheric pressure excitation of the translational mode of the inner core

Abstract Using hourly atmospheric surface pressure field from ECMWF (European Centre for Medium-Range Weather Forecasts) and from NCEP (National Centers for Environmental Prediction) Climate Forecast System Reanalysis (CFSR) models, we show that atmospheric pressure fluctuations excite the translational oscillation of the inner core, the so-called Slichter mode, to the sub-nanogal level at the Earth surface. The computation is performed using a normal-mode formalism for a spherical, self-gravitating anelastic PREM-like Earth model. We determine the statistical response in the form of power spectral densities of the degree-one spherical harmonic components of the observed pressure field. Both hypotheses of inverted and non-inverted barometer for the ocean response to pressure forcing are considered. Based on previously computed noise levels, we show that the surface excitation amplitude is below the limit of detection of the superconducting gravimeters, making the Slichter mode detection a challenging instrumental task for the near future.

Preliminary Results from the Superconducting Gravimeter SG-060 Installed in West Africa (Djougou, Benin)

A GWR superconducting gravimeter of the new generation (OSG-60) has been installed in July 2010 in sub-humid West Africa, at the Djougou station in Benin. This station is located in the AMMA-CATCH long term hydrological observing system. We present the first results in terms of instrumental drift as well as the calibration results using FG5 absolute gravity measurements. We show that geophysical contributions due to hydrological load can bias the initial drift estimate. The noise level is compared to the Strasbourg SG as well as to the reference New Low Noise Model (NLNM) used in seismology. We also investigate the gravity response to atmospheric pressure changes and show that, because of the presence of large thermal tides, the gravity response to mass changes in the atmosphere is more complex than in the simple case of a constant barometric admittance.