Hydrologie et météorologie de méso-échelle dans HAPEX-Sahel : dispositif de mesures au sol et premiers résultats

On présente ici les traitements qui ont été réalisés pour homogénéiser les données des six scènes SPOT dont l'acquisition est nécessaire pour couvrir le degré carré de Niamey. Les données concernées par ces traitements sont celles des deux radiomètres Haute Résolution Visible (HRV1 et HRV2) multispectraux embarqués à bord de SPOT1, en date du 24 octobre 1988. Il s'agissait d'obtenir une spatio-carte préalable à la cartographie des états de surface au 1/200 000 du degré carré et de mettre au point une méthode de traitement réutilisable pour d'autres dates. Les principales étapes de ce traitement sont : la calibration radiométrique, la mosaïque des six images, l'atténuation du gradient bioclimatique, et enfin la correction géométrique. Les spatio-cartes ainsi réalisées sur le degré carré au 1/200 000 et sur le Super-Site central au 1/50 000 sont fournies comme illustration de ce travail. (Résumé d'auteur

Soil moisture active and passive microwave products: intercomparison and evaluation over a Sahelian site

This paper presents a comparison and an evaluation of five soil moisture products based on satellite-based passive and active microwave measurements. Products are evaluated for 2005–2006 against ground measurements obtained from the soil moisture network deployed in Mali (Sahel) in the framework of the African Monsoon Multidisciplinary Analysis project. It is shown that the accuracy of the soil moisture products is sensitive to the retrieval approach as well as to the sensor type (active or passive) and to the signal frequency (from 5.6 GHz to 18.8 GHz). The spatial patterns of surface soil moisture are compared between the different products at meso-scale (14.5° N–17.5° N and 2° W–1° W). A general good consistency between the different satellite soil moisture products is shown in terms of meso-scale spatial distribution, in particular after convective rainfall occurrences. Comparison to ground measurement shows that although soil moisture products obtained from satellite generally over-estimate soil moisture values during the dry season, most of them capture soil moisture temporal variations in good agreement with ground station measurements

Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (“ANPP residuals”). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984–2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has been observed in the area over the same period, pointing towards land degradation. The divergence of these two indicators of ecosystem resilience (stable RUE) and land degradation (increasing run-off coefficient) is referred to as the “second Sahelian paradox”. When shallow soils and deep soils are examined separately, high resilience is diagnosed on the deep soil sites. However, some of the shallow soils show signs of degradation, being characterized by decreasing vegetation cover and increasing run-off coefficient. Such results show that contrasted changes may co-exist within a region where a strong overall re-greening pattern is observed, highlighting that both the scale of observations and the scale of the processes have to be considered when performing assessments of vegetation changes and land degradation

West African Monsoon water cycle: 1. A hybrid water budget data set

International audienceThis study investigates the West African Monsoon water cycle with the help of a new hybrid water budget data set developed within the framework of the African Monsoon Multidisciplinary Analyses. Surface water and energy fluxes are estimated from an ensemble of land surface model simulations forced with elaborate precipitation and radiation products derived from satellite observations, while precipitable water tendencies are estimated from numerical weather prediction analyses. Vertically integrated atmospheric moisture flux convergence is estimated as a residual. This approach provides an advanced, comprehensive atmospheric water budget, including evapotranspiration, rainfall, and atmospheric moisture flux convergence, together with other surface fluxes such as runoff and net radiation. The annual mean and the seasonal cycle of the atmospheric water budget are presented and the couplings between budget terms are discussed for three climatologically distinct latitudinal bands between 6°N and 20°N. West Africa is shown to be alternatively a net source and sink region of atmospheric moisture, depending on the season (a source during the dry season and a sink during the wet season). Several limiting and controlling factors of the regional water cycle are highlighted, suggesting strong sensitivity to atmospheric dynamics and surface radiation. Some insight is also given into the underlying smaller-scale processes. The relationship between evapotranspiration and precipitation is shown to be very different between the Sahel and the regions more to the south and partly controlled by net surface radiation. Strong correlations are found between precipitation and moisture flux convergence over the whole region from daily to interannual time scales. Causality is also established between monthly mean anomalies. Hence, precipitation anomalies are preceded by moisture flux convergence anomalies and followed by moisture flux divergence and evapotranspiration anomalies. The results are discussed in comparison to other studies

The sensitivity of nocturnal low-level jets and near-surface winds over the Sahel to model resolution, initial conditions and boundary-layer set-up

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Couplings between the seasonal cycles of surface thermodynamics and radiative fluxes in the semi-arid Sahel

International audienceA good knowledge of surface fluxes and atmospheric low levels is central to improving our understanding of the West African monsoon. This study provides a quantitative analysis of the peculiar seasonal and diurnal cycles of surface thermodynamics and radiative fluxes encountered in Central Sahel. It is based on a multi-year dataset collected in the Malian Gourma over a sandy soil at 1.5°W-15.3°N (a site referred to as Agoufou) with an automated weather station and a sunphotometer (AERONET), complemented by observations from the AMMA field campaign. The seasonal cycle of this Tropical region is characterized by a broad maximum of temperature in May, following the first minimum of the solar zenith angle by a few weeks, when Agoufou lies within the West African Heat-Low, and a late summer maximum of equivalent potential temperature within the core of the monsoon season, around the second yearly maximum of solar zenith angle, as the temperature reaches its Summer minimum. More broadly, subtle balances between surface air temperature and moisture fields are found on a range of scales. For instance, during the monsoon, apart from August, their opposite daytime fluctuations (warming, drying) lead to an almost flat diurnal cycle of the equivalent potential temperature at the surface. This feature stands out in contrast to other more humid continental regions. Here, the strong dynamics associated with the transition from a drier hot Spring to a brief cooler wet tropical Summer climate involves very large transformations of the diurnal cycles. The Summer increase of surface net radiation, Rnet, is also strong; typically 10-day mean Rnet reaches about 5 times its Winter minimum (~30 W.m-2) in August (~150 W.m-2). A major feature revealed by observations is that this increase is mostly driven by modifications of the surface upwelling fluxes shaped by rainfall events and vegetation phenology (surface cooling and darkening), while the direct impact of atmospheric changes on the total incoming radiation is limited to shorter time scales in Summer over this Central Sahelian location. However, observations also reveal astonishing radiative signatures of the monsoon on the surface incoming radiative flux. The incoming longwave flux does not reach its maximum during the monsoon season when the atmosphere is the most cloudy and humid, but earlier, prior to the onset of rainfall, as the dry and warmer atmosphere suddenly becomes moist. This feature points to the significance of the atmospheric cooling during the monsoon season and of the aerosol amounts in Spring. It also reveals that prior to the rainfall onset, the monsoon flow plays a major role on the diurnal cycle of the low-level temperature, due to its radiative properties. Conversely, the incoming solar radiation at the surface increases slightly from late Spring to the core monsoon season even though the atmosphere becomes moister and cloudier; this again involves the high aerosol optical thickness prevailing in late Spring and early Summer against a weaker shortwave forcing by monsoon clouds. The climatological combination of thermodynamic and radiative variations taking place during the monsoon eventually leads to a positive correlation between the equivalent potential temperature and Rnet. This correlation is, in turn, broadly consistent with an overall positive soil moisture rainfall feedback at this scale. Beyond these Sahelian-specific features, and in agreement with some previous studies, strong links are found between the atmospheric humidity and the net longwave flux, LWnet at the surface all year long, even across the much lower humidity ranges encountered in this region. They point to, and locally quantify the major control of water vapour and water-related processes on the surface-atmosphere thermal coupling as measured by LWnet. Namely, they are found to be more tightly coupled (LWnet closer to 0) when the atmosphere is moister and cloudier. Observational results such as presented here provide valuable ground truth for assessing models over a continental area displaying a challenging variety of surface-atmosphere regimes throughout the year, from a desert-like to a rainy tropical-like climate during the core of the monsoon. Indeed, the mechanisms emphasized by these data do not all comply to existing conceptual schemes

Surface soil moisture estimation over the AMMA Sahelian site in Mali using ENVISAT/ASAR data

This paper focuses on different methods for estimating soil moisture in a Sahelian environment by comparing ENVISAT/ASAR and ground data at the same spatial scale. The analysis is restricted to Wide Swath data in order to take advantage of their high temporal repetitivity (about 3-4 days) corresponding to a moderate spatial resolution (150 m). On the one hand, emphasis is put on the characterization of Surface Soil Moisture (SSM) at a spatial scale compatible with the derivation of the backscattering coefficients, and a transfer function is developed for up-scaling local measurements to the 1 km scale. On the other hand, three different approaches are used to normalize the angular variation of the observed backscattering coefficients. The results show a strong linear relationship between the HH normalized backscattering coefficients and SSM. The best result is obtained when restricting the ASAR data to low incidence angles and by taking into account vegetation effects using multi-angular radar data. For this case, the rms error of the SSM retrieval is 2.8%. These results highlight the capabilities of the ASAR instrument to monitor SSM in a semiarid environment

Surface soil moisture estimation over the AMMA Sahelian site in Mali using ENVISAT/ASAR data

This paper focuses on different methods for estimating soil moisture in a Sahelian environment by comparing ENVISAT/ASAR and ground data 10 at the same spatial scale. The analysis is restricted toWide Swath data in order to take advantage of their high temporal repetitivity (about 3–4 days) 11 corresponding to a moderate spatial resolution (150 m). On the one hand, emphasis is put on the characterization of Surface Soil Moisture (SSM) at a 12 spatial scale compatible with the derivation of the backscattering coefficients, and a transfer function is developed for up-scaling local measurements 13 to the 1 kmscale. On the other hand, three different approaches are used to normalize the angular variation of the observed backscattering coefficients. 14 The results show a strong linear relationship between the HH normalized backscattering coefficients and SSM. The best result is obtained when 15 restricting the ASAR data to low incidence angles and by taking into account vegetation effects using multi-angular radar data. For this case, the rms 16 error of the SSM retrieval is 2.8%. These results highlight the capabilities of the ASAR instrument to monitor SSM in a semiarid environment

Analysis of the in situ and MODIS albedo variability at multiple time scales in the Sahel - art. no. D14119

The variability of the Sahelian albedo is investigated through the combined analysis of 5 years of in situ radiation data from the African Monsoon Multidisciplinary Analysis northernmost sites and remotely sensed albedo from 7 years of Moderate Resolution Imaging Spectroradiometer data. Both data sets are found to be in good agreement in terms of correlation and bias. The drivers of albedo variability are identified by means of in situ measurements of biological and physical properties of the land surface collected over a network of 29 long-term survey sites. Short-term variability is dominated by changes in the spectral composition of incident radiation, which reflects aerosol optical depth and integrated water content, and changes in soil moisture, which have a short-lived effect (1 d). Bush fires cause a marked decrease of albedo of the order of 10 d, whereas a dry season storm event is suspected to have increased albedo through litter and soil surface abrasion. Seasonal plant growth causes the largest changes in rainy season albedo, and displays a large interannual variability: Because of the 2004 drought, albedo increases steadily from late 2003 to early 2005 at latitude 15 degrees N. Grazing pressure is found to impact albedo mostly in the dry season. Dry season albedo is controlled by the amount of litter and standing dead phytomass hiding the bright soils. Thus rainfall anomalies have a direct effect on albedo through plant growth but also a lagged effect caused by above normal amounts of dry phytomass that can persist until the arrival of the next monsoon. EOF analysis and Hovmuller diagrams show these effects to be present on a large scale