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

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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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

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

Is water availability really the main environmental factor controlling the phenology of woody vegetation in the central Sahel ?

Rainfall distribution and the soil moisture regime have been recognized to be the key drivers of the phenological rhythms in Sahelian woody plants, although different climate triggers have been assumed to be involved in determining the date of the onset of the phenophase. However, almost no comparisons have been made of the actual relative predictive power of these environmental factors. The aim of our study was to quantify the ability of several factors to predict phenophase occurrence in the dominant woody populations of northern Mali. Canopy leafing, flowering and fruiting were monitored from May 2005 to July 2007. Multiple logistic regressions were used to test the predictive power of cumulative rainfall, soil moisture, air temperature, air humidity and day length, with time lags of up to 2 months. Artificial variables derived from time lags observed in phenophases were included as predictors to account for possible auto-correlation and cross-correlation among phenophases. Surprisingly, a decrease in temperature associated with different time lags was most often found to be the strongest predictor of both leafing and reproductive phenophases. In Sahelian shrubs, morphological and physiological adaptations strongly contribute to the relative independence of their activity from water availability, leaf phenology being a way to adjust the plant water balance to current water availability and atmospheric water content. This study provides insight towards the development of a mechanistic understanding of phenological control in the Sahel, which is becoming increasingly important in the context of expected climate changes

Analysis of the in-situ and MODIS albedo variability at multiple time scales in the Sahel.

The variability of the Sahelian albedo is investigated through the combined analysis of five years of in situ radiation data from the AMMA northernmost sites (Agoufou 1°28'W, 15°20'N and Bamba 1°24'W, 17°06'N Mali) and remotely sensed albedo from seven years of MODIS data. Both datasets 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 25 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 day). Bush fires cause a marked decrease of albedo of the order of 10 days, 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 toearly 2005 at latitude 15°N. Dry season albedo is controlled by the amount of litter and standing dead phytomass hiding the bright soils. Grazing pressure is found to impact albedo mostly in the dry season, through the pace of dry phytomass decay. EOF analysis and Hovmüller diagrams (10°W to 0°, 10°N to 20°N) show these effects to be present on a large scale. Above average rainfall thus 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. This provides positive feedbacks at both the intra-seasonal and inter seasonal scales. Surface albedo in the Sahel exhibits a strong variability at different timescales caused by external forcing and internal dynamics of land surface, which should be included in climate models

Surface thermodynamics and radiative budget in the Sahelian Gourma : seasonal and diurnal cycles

Our understanding of the role of surface-atmosphere interactions in the West African monsoon has been particularly limited by the scarcity of measurements. The present study provides a quantitative analysis of the very pronounced seasonal and diurnal cycles of surface thermodynamics and radiative fluxes in the Central Sahel. It makes use of data collected from 2002 to 2007 in the Malian Gourma, close to Agoufou, at 1.5 degrees W-15.3 degrees N and sounding data collected during the AMMA field campaign. The seasonal cycle is characterized by a broad maximum of temperature in May, following the first minimum of the solar zenith angle (SZA) by a few weeks, when Agoufou lies within the West African Heat Low, and a late summer maximum of equivalent potential temperature (0e) within the core of the monsoon season, around the second yearly maximum of SZA. Distinct temperature and moisture seasonal and diurnal dynamics lead to a sharpening of the early (late) monsoon increase (decrease), more steadiness of 0e and larger changes of relative humidity in between. Rainfall starts after the establishment of the monsoon flow, once temperature already started to decrease slowly, typically during June. Specific humidity increases progressively from May until August, while the monsoon flow weakens during the same period. Surface net radiation (R-net) increases from around 10-day mean values of 20 W m(-2) in Winter to 120-160 W m(-2) in late Summer, The increase is sharper during the monsoon than before, and the decrease fast. The seasonal cycle of R-net arises from distinct shortwave and longwave fluctuations that are both strongly shaped by modifications of surface properties related to rainfall events and vegetation phenology (with a decrease of both surface longwave emission and albedo). During the monsoon, clouds and aerosols reduce the incoming solar radiation by 20-25% (about 70 W m(-2)). They also significantly enhance the day-to-day variability of R-net. Nevertheless, the surface incoming longwave radiative flux (LWin) is observed to decrease from June to September. As higher cloud covers and larger precipitable water amounts are typically expected to enhance LWin, this feature points to the significance of changes in atmospheric temperature and aerosols during the monsoon season. The strong dynamics associated with the transition from a drier hot Spring to a brief cooler moist tropical Summer climate involves large transformations of the diurnal cycle, even within the monsoon season, which significantly affect both thermodynamical, dynamical and radiative fields (and low-level dynamics). In particular, for all moist Summer months except August, specific humidity decreases in such a way during daytime that it prevents an afternoon increase of 0e. In agreement with some previous studies, strong links are found between moisture and LWnet all year long and a positive correlation is identified between R-net and 0e during the monsoon. The observational results presented in this study further provide valuable ground truth for assessing models over an area displaying a rich variety of surface-atmosphere regimes