This paper presents a study using in situ measurements (AERONET data) to characterize the intra-seasonal variability of aerosols and their shortwave radiative impacts on Sahel (11N - 18N and 20W - 15E) climate using a long time series (2000 to 2010) from AERONET data. We first used the MISR aerosol optical depth (AOD) averaged between 2000 and 2010 to identify dust maxima regions. They are mainly located over Eastern Mauritania-Northern Mali and Eastern Niger-Western Tchad (near the Bodele depression). Moreover the aerosol loading is stronger during the spring (MAM) and particularly during the summer (JJA). The analysis of AERONET data at 4 Sahelian stations (Dakar, Agoufou, Banizoumbou and Ouagadougou) shows that Sahel is under dust influence all year long. The dust season is mainly located between March and October, with two peaks of AOD recorded in March and June; while the maximum of biomass burning activity is recorded in winter (December to February). The radiative forcing of aerosols on the Sahelian climate results mainly in a cooling as well as at the surface than at the top of the atmosphere (TOA) but the forcing is stronger at the surface and particularly during the summer (JJA) and spring (MAM) periods. Although there is not a sharp difference between DJF and SON periods from a station to another one when considering the aerosol loading (AOD), the radiative forcing is stronger at the surface and weaker at the TOA during DJF for most stations. The analysis of the force effectiveness (ratio between AOD and the radiative forcing) suggests that the BOA radiative forcing is mainly influenced by the nature of aerosols.
West Africa is the biggest dust and biomass burning smoke aerosol source of the world [1,2]. Massive dust plumes regularly propagate from the continent to the ocean and as far as Florida particularly during the summer in the so-called Saharan air Layer [3,4]. These mineral particles exert a strong influence on the radiative balance and the climate [5,6]. The years of rainfall deficits over West Africa are marked by anomalies in the dynamics of the monsoon system and mineral dust has kept increasing during drought years [7,8]. But the cause of this continuous dust increase is still unknown because it may be linked to the increase in wind intensity during dry years and/or a decrease in vegetation [9,10]. Several studies using short time and spatially limited observed data have been conducted to characterize these particles and their impacts on the regional climate. Mineral dust particles were investigated during the SHADE campaign over the West Africa, while their radiative effects were measured in the solar spectrum by ground-based measurements [11,12]). The particles found in [
The modeling approach has been used by some authors [16-18] to study aerosols spatio-temporal distribution and their impacts on the climate.
Despite these studies, the spatio-temporal distribution of aerosols and their radiative impacts on the climate is still an investigation topic.
A long time-series (2000-2010) of in-situ measurements (AERONET data) is used in this study to characterize and evaluate the aerosols properties and their impacts on climate over the Sahel considering 4 stations: Dakar (Senegal), Agoufou (Mali), Ouagadougou (Burkina-Faso) and Banizoumbou (Niger). These stations (particularly Dakar) are ideally placed to study the spatiotemporal distribution of coarse (mineral dust) and fine (biomass burning) particles over the Sahel. We present first the data and the methodology used. A seasonal characterization of aerosols properties at each station is performed considering the same period (2000-2010) in the second part. Finally, the shortwave radiative impact of aerosols is assessed.
The AErosol RObotic NETwork (AERONET) program is a federation of ground-based remote sensing aerosol networks established by the NASA and PHOTONS (University of Lille 1, CNES, and CNRS-INSU) and is greatly expanded by collaborators from national agencies, institutes, universities, individual scientists, and partners [
The second dataset used is the Multi-Angle Imaging Spectro Radiometer (MISR) sensors aeronet optical depth [
Data used in this work are AERONET aerosol optical depth and radiative properties products (level 2) available on http://aeronet.gsfc.nasa.gov/file_help.html. The AOD is available at 532 nm which corresponds to the maximum of solar radiation. Other parameters such as the angstrom coefficient and single scattering albedo are derived from the spectral AOD. The aerosol optical properties retrieval algorithm is extensively described in [
After diagnosing the aerosols maxima, we used the AERONET data to characterize their optical properties and radiative impacts over the Sahel.