How surface properties influence mineral dust emissions in the Sahelian region ? A modelling case study during AMMA

Tropical mesoscale convective systems (MCSs) are a prominent feature of the African meteorology. A continuous monitoring of the aeolian activity in an experimental site located in Niger shows that such events are responsible for the major part of the annual local wind erosion, i.e. for most of the Sahelian dust emissions [Rajot, 2001]. However, the net effect of these MCSs on mineral dust budget has to be estimated: on the one hand, these systems produce extremely high surface wind velocities leading to intense dust uptake, but on the other hand, rainfalls associated with these systems can efficiently remove the emitted dust from the atmosphere. High resolution modelling appears as a relevant approach to correctly reproduce the surface meteorology associated with such meteorological systems [Bouet et al., submitted]. The question now arising concerns the reliability of surface characteristics available for the Sahelian region, especially soil texture and surface roughness, which are critical parameters for dust emissions. Contrary to arid regions, which are now well documented, data is still missing to correctly characterize semi-arid regions like the Sahel. This is in particular due to the well pronounced annual cycles of precipitations and vegetation in these regions and to the impact of land-use on surface properties. This study focuses on a case study of dust emission associated with the passage of a MCS observed during one of the Special Observing Periods of the international African Monsoon Multidisciplinary Analysis (AMMA – SOPs 1-2) program. The simulations were made using the Regional Atmospheric Modeling System (RAMS, Cotton et al. [2003]) coupled online with the dust production model developed by Marticorena and Bergametti [1995] and recently improved by Laurent et al. [2008] for Africa. The sensitivity of dust emission associated with the passage of the MCS to surface features is investigated using different data sets of surface properties (Harmonized World Soil Database, HWSD) and land-use (GLOBCOVER). In-situ measurements of dust concentrations (both ground-based and airborne), and of dust emission flux are used to validate the simulations

Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition

International audienceIn the last few years, several ground-based and air-borne field campaigns have allowed the exploration of theproperties and impacts of mineral dust in western Africa,one of the major emission and transport areas worldwide.In this paper, we explore the synthesis of these observationsto provide a large-scale quantitative view of the mineralogi-cal composition and its variability according to source regionand time after transport.This work reveals that mineral dust in western Africa is amixture of clays, quartz, iron and titanium oxides, represent-ing at least 92 % of the dust mass. Calcite ranged between0.3 and 8.4 % of the dust mass, depending on the origin. Ourdata do not show a systematic dependence of the dust min-eralogical composition on origin; this is to be the case as, inmost of the instances, the data represent the composition ofthe atmospheric burden after 1–2 days after emission, whenair masses mix and give rise to a more uniform dust load.This has implications for the representation of the mineraldust composition in regional and global circulation modelsand in satellite retrievals.Iron oxides account for 58±7 % of the mass of elementalFe and for between 2 and 5 % of the dust mass. Most of themare composed of goethite, representing between 52 and 78 %of the iron oxide mass. We estimate that titanium oxides ac-count for 1–2 % of the dust mass, depending on whether thedust is of Saharan or Sahelian origin.The mineralogical composition is a critical parameter forestimating the radiative and biogeochemical impact of min-eral dust. The results regarding dust composition have been used to estimate the optical properties as well as the iron frac-tional solubility of Saharan and Sahelian dust.Data presented in this paper are provided in numeri-cal form upon email request while they are being turnedinto a public database, the Dust-Mapped Archived Proper-ties (DUST-MAP), which is an open repository for compo-sitional data from other source regions in Africa and world-wide

Erosion éolienne dans les régions arides et semi-arides africaines : processus physiques, métrologie et techniques de lutte

Les régions arides du sud de la Tunisie sont des zones naturellement très sensibles à l'érosion éolienne. Non seulement les précipitations dans ces régions sont faibles (inférieures à 200 mm), mais les sols sont fins, sableux et peu profonds, c'est-à-dire facilement érodables par le vent. L'utilisation de nouvelles techniques agricoles à la place des techniques traditionnelles a conduit à une augmentation de l'érosion éolienne dans ces régions. Par exemple, l'augmentation croissante de l'utilisation de la déchaumeuse à disques a eu d'importantes conséquences sur la dégradation des champs en modifiant la structure des sols et les caractéristiques de leur surface. Le présent travail de modélisation est centré sur la quantification de la déflation éolienne à l'échelle du sud tunisien en vue de déterminer en particulier les zones les plus sensibles à ce phénomène pour des objectifs de diagnostic et de stratégie de lutte efficace contre l'érosion éolienne. Les flux d'érosion éolienne sur le sud de la Tunisie ont été simulés pour l'année 2008 à une résolution de 10 km x 10 km en prenant en compte le type d'usage des sols et les pratiques agricoles associées à l'aide du modèle d'érosion éolienne Dust Production Model (DPM, Marticorena et Bergametti [1995] ; Alfaro et al. [1998]). Afin de prendre en compte les différents types d'outils agricoles utilisés sur le domaine étudié, les paramétrisations du seuil et du flux d'érosion éolienne en fonction des caractéristiques des billons de labour (hauteur et espacement) développées par Kardous et al. [2005a ; b] ont été intégrées au DPM. [...

Evolution des paysages Sahélines au cours des six dernières décennies dans la région de Niamey : de la disparition de la brousse tigrée à l'encroutement de surface des sols

International audienceIn the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an

Evolution des paysages Sahélines au cours des six dernières décennies dans la région de Niamey : de la disparition de la brousse tigrée à l'encroutement de surface des sols

International audienceIn the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an

Temporal variability of mineral dust in southern Tunisia : analysis of 2 years of PM10 concentration, aerosol optical depth, and meteorology monitoring

International audienceThe south of Tunisia is a region very prone to wind erosion. During the last decades, changes in soil management have led to an increase in wind erosion. In February 2013, a ground-based station dedicated to the monitoring of mineral dust (that can be seen in this region as a proxy of the erosion of soils by wind) was installed at the Institut des Régions Arides (IRA) of Médenine (Tunisia) to document the temporal variability of mineral dust concentrations. This station allows continuous measurements of surface PM10 concentration (TEOM™), aerosol optical depth (CIMEL sunphotometer), and total atmospheric deposition of insoluble dust (CARAGA automatic sampler). The simultaneous monitoring of meteorological parameters (wind speed and direction, relative humidity, air temperature, atmospheric pressure, and precipitations) allows to analyse the factors controlling the variations of mineral dust concentration from the sub-daily to the annual scale. The results from the two first years of measurements of PM10 concentration are presented and discussed. In average on year 2014, PM10 concentration is 56 µg/m3. However, mineral dust concentration highly varies throughout the year: very high PM10 concentrations (up to 1,000 µg/m3 in daily mean) are frequently observed during wintertime and springtime, hardly ever in summer. These episodes of high PM10 concentration (when daily average PM10 concentration is higher than 240 µg/m3) sometimes last several days. By combining local meteorological data, air-masses trajectories, sunphotometer measurements, and satellite imagery, the part of the high PM10 concentration due to local emissions and those linked to an advection of dusty air masses by medium and long range transport from the Sahara desert is quantified

Gully erosion prediction in a Sahelian context

In Sahelian region, concentrated overland flow often leads to the formation of gullies. Although this phenomenon is widespread in those regions, research efforts are still needed to be able to model their spatial distribution and the role of the different parameters involved in this process. In this context, the objectives of this study are twofold. The first step is to investigate to what extent the role of Sahelian soil surface crusts (biological and/or physical) on soil surface infiltrability and detachment affect the formation and development of gullies. The second step is to integrate the results of these investigations in a simple geomorphological model to predict gully location at the watershed scale. The evaluation of the resulting model on two test catchments demonstrated that the integration of soil crusting is a key parameter to insure the quality and relevance of gully prediction. The model is able to distinguish between two types of gullies, those whose width range between 0.5m and 4m and those whose width exceeds 4m. The application of the model at the regional scale is however limited by the resolution of available regional digital elevation model (i.e. the 90m resolution SRTM DEM) which only permits the prediction of large gullies

The AMMA mulid network for aerosol characterization in West Africa

Three ground based portable low power consumption microlidars (MULID) have been built and deployed at three remote sites in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) project for the characterization of aerosols optical properties. A description of the instrument and a discussion of the data inversion method, including a careful analysis of measurement uncertainties (systematic and statistical errors) are presented. Some case studies of typical lidar profiles observed over the Banizoumbou site during 2006 are shown and discussed with respect to the AERONET 7-day back-trajectories and the biomass burning emissions from the Combustion Emission database for the AMMA campaign