Aerosol retrieval experiments in the ESA Aerosol_cci project

Within the ESA Climate Change Initiative (CCI) project Aerosol_cci (2010–2013), algorithms for the production of long-term total column aerosol optical depth (AOD) datasets from European Earth Observation sensors are developed. Starting with eight existing pre-cursor algorithms three analysis steps are conducted to improve and qualify the algorithms: (1) a series of experiments applied to one month of global data to understand several major sensitivities to assumptions needed due to the ill-posed nature of the underlying inversion problem, (2) a round robin exercise of "best" versions of each of these algorithms (defined using the step 1 outcome) applied to four months of global data to identify mature algorithms, and (3) a comprehensive validation exercise applied to one complete year of global data produced by the algorithms selected as mature based on the round robin exercise. The algorithms tested included four using AATSR, three using MERIS and one using PARASOL. This paper summarizes the first step. Three experiments were conducted to assess the potential impact of major assumptions in the various aerosol retrieval algorithms. In the first experiment a common set of four aerosol components was used to provide all algorithms with the same assumptions. The second experiment introduced an aerosol property climatology, derived from a combination of model and sun photometer observations, as a priori information in the retrievals on the occurrence of the common aerosol components. The third experiment assessed the impact of using a common nadir cloud mask for AATSR and MERIS algorithms in order to characterize the sensitivity to remaining cloud contamination in the retrievals against the baseline dataset versions. The impact of the algorithm changes was assessed for one month (September 2008) of data: qualitatively by inspection of monthly mean AOD maps and quantitatively by comparing daily gridded satellite data against daily averaged AERONET sun photometer observations for the different versions of each algorithm globally (land and coastal) and for three regions with different aerosol regimes. The analysis allowed for an assessment of sensitivities of all algorithms, which helped define the best algorithm versions for the subsequent round robin exercise; all algorithms (except for MERIS) showed some, in parts significant, improvement. In particular, using common aerosol components and partly also a priori aerosol-type climatology is beneficial. On the other hand the use of an AATSR-based common cloud mask meant a clear improvement (though with significant reduction of coverage) for the MERIS standard product, but not for the algorithms using AATSR. It is noted that all these observations are mostly consistent for all five analyses (global land, global coastal, three regional), which can be understood well, since the set of aerosol components defined in Sect. 3.1 was explicitly designed to cover different global aerosol regimes (with low and high absorption fine mode, sea salt and dust)

Evidence and implications of solar eclipses in short wavelength global remotely sensed data

International audienc

GEOFCAN : actes du 2ème colloque de géophysique des sols et des formations superficielles : résumés étendus

A geophysical survey has been conducted in Siem Reap province of Cambodia. The aim was to test DC, TDEM and PMR methods to improve the borehole success rate. The preliminary results lead to the proposition of an implementation methodology which can be described as follows : 1) to use direct current methods (1D and 2D) in any case, as standard methods ; 2) to use jointly PMR, DC and TDEM methods, when the borehole success rate is less than 30% ; 3) to use jointly direct current and TDEM methods, when the success rate ranges 30 to 50% ; 4) to use electrical well logging (normal probes) to implement the borehole screens. (Résumé d'auteur

Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin)

Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from a permanent and deep groundwater table to the atmosphere even during the dry season. It should be the case for riparian forests too, as these streambank forests are key landscape elements in Sudanian West Africa. The interplay of riparian forest and groundwater in the local hydrodynamics was investigated, by quantifying their contribution to the water balance. Field observations from a comprehensively instrumented hillslope in northern Benin were used. Particular attention was paid to measurements of actual evapotranspiration, soil water and deep groundwater levels. A vertical 2-D hydrological modelling approach using the Hydrus software was used as a testing tool to understand the interactions between the riparian area and the groundwater. The model was calibrated and evaluated using a multi-criteria approach (reference simulation). A virtual experiment, including three other simulations, was designed (no forest, no groundwater, neither forest nor groundwater). The model correctly simulated the hydrodynamics of the hillslope regarding vadose zone dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics. The virtual experiment showed that the riparian forest transpiration depleted the deep groundwater table level and disconnected it from the river, which is consistent with the observations. The riparian forest and the deep groundwater table actually form an interacting transpiration system: the high transpiration rate in the riparian area was shown to be due to the existence of the water table, supplied by downslope lateral water flows within the hillslope soil layer. The simulated riparian transpiration rate was practically steady all year long, around 7.6 mm d(-1). This rate lies within high-end values of similar study results. The riparian forest as simulated here contributes to 37% of the annual hillslope transpiration, and reaches 57% in the dry season, whereas it only covers 5% of the hillslope area

Riparian forest and permanent groundwater: a key coupling for balancing the hillslope water budget in Sudanian West Africa

International audience(Com., 7 juin 2011, n° 10-18.108

Characterization of volcanic material from combined IR-lidar observations in the frame of the CALIPSO mission

International audienceImportant volcanic material injections in the troposphere can induce significant environmental and meteorological perturbations. Several volcanic eruptions have been recently observed before the eruption of Eyjafjallajökull in Iceland, but this one was an important source of problems to European air transport, as this volcano was directly injecting material over northern Europe. It has thus become of major importance to precisely characterize transported material (identify silicate and sulfate aerosol and their properties) from observations and provide such information as inputs and control to transport models. IR observations have long been used to characterize volcanic emitted material. It is one of the advantages of the CALIPSO mission to combine IR and lidar instruments to provide co-located observations directly exploitable to improve the characterization of aerosol and clouds. CALIPSO, as part of the A-Train, further benefits from an unprecedented observational environment, which enables combined analyses with CloudSat and AQUA. Furthermore, the overpasses of the A-Train provide a snapshot into observations from geostationary platforms, such as MSG over Europe and Africa, which can be of valuable interest to follow the dispersion and modifications of plumes on a regular basis. In this presentation, we will focus on observations made after the eruption of the Chaiten volcano in Chile in 2008, and of the Icelandic volcano in 2010. Emitted plumes were transported over areas possibly covered with low clouds, so that potential observations from the surface need to be complemented, and that radiometric observations in the visible are largely perturbed. We show that the complementary observations of A-Train and geostationary sensors allow to better identify and characterize volcanic ash properties, and to follow the evolution of the plumes. More specifically, we focus on IR observations to characterize the size of silicate particles, and on combined CALIPSO IIR and lidar observations, involving newly developed research products, to analyze aerosol optical depths over clouds and ocean

Characterization of volcanic material from combined IR-lidar observations in the frame of the CALIPSO mission

International audienceImportant volcanic material injections in the troposphere can induce significant environmental and meteorological perturbations. Several volcanic eruptions have been recently observed before the eruption of Eyjafjallajökull in Iceland, but this one was an important source of problems to European air transport, as this volcano was directly injecting material over northern Europe. It has thus become of major importance to precisely characterize transported material (identify silicate and sulfate aerosol and their properties) from observations and provide such information as inputs and control to transport models. IR observations have long been used to characterize volcanic emitted material. It is one of the advantages of the CALIPSO mission to combine IR and lidar instruments to provide co-located observations directly exploitable to improve the characterization of aerosol and clouds. CALIPSO, as part of the A-Train, further benefits from an unprecedented observational environment, which enables combined analyses with CloudSat and AQUA. Furthermore, the overpasses of the A-Train provide a snapshot into observations from geostationary platforms, such as MSG over Europe and Africa, which can be of valuable interest to follow the dispersion and modifications of plumes on a regular basis. In this presentation, we will focus on observations made after the eruption of the Chaiten volcano in Chile in 2008, and of the Icelandic volcano in 2010. Emitted plumes were transported over areas possibly covered with low clouds, so that potential observations from the surface need to be complemented, and that radiometric observations in the visible are largely perturbed. We show that the complementary observations of A-Train and geostationary sensors allow to better identify and characterize volcanic ash properties, and to follow the evolution of the plumes. More specifically, we focus on IR observations to characterize the size of silicate particles, and on combined CALIPSO IIR and lidar observations, involving newly developed research products, to analyze aerosol optical depths over clouds and ocean

Hydro-gravimetry in West-Africa : first results from the Djougou (Benin) superconducting gravimeter

The increasing number of hydro-gravimetry studies proves the rising interest of the hydrology community toward this monitoring method. The accuracy of superconducting gravimeters (SG) potentially allows the retrieval of small water storage changes (WSC) down to a few millimeters of equivalent water thickness. However, the importance of corrections applied to SG data to achieve such a precision in gravity residuals should be recalled. The Djougou permanent gravity station presented in this paper and located in northern Benin, West-Africa, provides a good opportunity to review these considerations. This station is equipped since July 2010 with the superconducting gravimeter SG-060 aimed at deriving WSC at different time-scales, daily to inter-annual. In this area, WSC are (1) part of the control system for evapotranspiration (ET) process, a key variable of the West-African monsoon cycle and (2) the state variable for resource management, a critical issue in storage-poor hard rock basement contexts such as in northern Benin. The potential for deriving WSC from time-lapse gravity data partly depends on environmental features such as topography and the instrument shelter. Therefore, this issue is addressed first, with the background idea that such sensitivity analysis should be undertaken before setting up any new instrument. In Djougou, local topography is quite flat leading to a theoretical straightforward relationship between gravity changes and WSC, close to the standard Bouguer value. However, the shelter plays a significant masking role, which is the principal limitation to the retrieval of fast hydrological processes such as ET following a rain event. Several issues concerning classical gravity corrections are also addressed in the paper. These include gap-filling procedures during rain-events and drift estimates for short time series. Special attention is provided to atmospheric corrections, and different approaches are tested: a simple scalar admittance, a filtered scalar admittance, a frequency-dependent admittance and direct atmospheric loading calculations. It is shown that the physically based approach of direct loading calculations performs better in both residual minimization and ET retrieval. Moreover, non-local hydrological effects are investigated and account for about 20% of the gravity residuals. Finally, gravity residuals are briefly analyzed at two distinct time scales: rapid (up to a few days) and seasonal. At the rapid time-scale, it is shown that ET retrieval is hardly achievable given shelter size and state-of-the-art atmospheric corrections. Still, mean values retrieved from this study are in accordance with known values of potential ET and lateral flow. Direct comparison of gravity changes with hydrological data (neutron probe monitoring and water table levels) show some discrepancies, particularly for the hydrological year of 2011, for which all hydrological data show a deficit, but SG and FG5 data do not. This preliminary analysis both provides a basis and call for further hydro-gravity modeling, to comprehensively investigate the water-cycle at the Djougou station

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