Transport and Deposition of Saharan Dust Observed from Satellite Images and Ground Measurements

Haboob occurrence strongly impacts the annual variability of airborne desert dust in North Africa. In fact, more dust is raised from erodible surfaces in the early summer (monsoon) season when deep convective storms are common but soil moisture and vegetation cover are low. On 27 June 2018, a large dust storm is initiated over North Africa associated with an intensive westward dust transport. Far away from emission sources, dust is transported over the Atlantic for the long distance. Dust plume is emitted by a strong surface wind and further becomes a type of haboob when it merges with the southwestward deep convective system in central Mali at 0200 UTC (27 June). We use satellite observations to describe and estimate the dust mass concentration during the event. Approximately 93% of emitted dust is removed the dry deposition from the atmosphere between sources (10°N–25°N; 1°W–8°E) and the African coast (6°N–21°N; 16°W–10°W). The convective cold pool has induced large economic and healthy damages, and death of animals in the northeastern side of Senegal. ERA5 reanalysis has shown that the convective mesoscale impacts strongly the climatological location of the Saharan heat low (SHL)

Seasonal cycle of desert aerosols in western Africa: analysis of the coastal transition with passive and active sensors

The impact of desert aerosols on climate, atmospheric processes, and the environment is still debated in the scientific community. The extent of their influence remains to be determined and particularly requires a better understanding of the variability of their distribution. In this work, we studied the variability of these aerosols in western Africa using different types of satellite observations. Sea-WiFS (Sea-Viewing Wide Field-of-View Sensor) and OMI (Ozone Monitoring Instrument) data have been used to characterize the spatial distribution of mineral aerosols from their optical and physical properties over the period 2005-2010. In particular, we focused on the variability of the transition between continental western African and the eastern Atlantic Ocean. Data provided by the lidar scrolling CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) onboard the satellite CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) for the period 2007-2013 were then used to assess the seasonal variability of the vertical distribution of desert aerosols. We first obtained a good representation of aerosol optical depth (AOD) and singlescattering albedo (SSA) from the satellites SeaWiFS and OMI, respectively, in comparison with AERONET estimates, both above the continent and the ocean. Dust occurrence frequency is higher in spring and boreal summer. In spring, the highest occurrences are located between the surface and 3 km above sea level, while in summer the highest occurrences are between 2 and 5 km altitude. The vertical distribution given by CALIOP also highlights an abrupt change at the coast from spring to fall with a layer of desert aerosols confined in an atmospheric layer uplifted from the surface of the ocean. This uplift of the aerosol layer above the ocean contrasts with the winter season during which mineral aerosols are confined in the atmospheric boundary layer. Radiosondes at Dakar Weather Station (17.5 degrees W, 14.74 degrees N) provide basic thermodynamic variables which partially give a causal relationship between the layering of the atmospheric circulation over western Africa and their aerosol contents throughout the year. A SSA increase is observed in winter and spring at the transition between the continent and the ocean. The analysis of mean NCEP (National Centers for Environmental Prediction) winds at 925 hPa between 2000 and 2012 suggest a significant contribution of coastal sand sources from Mauritania in winter which would increase SSA over the ocean

Potential contribution of climate conditions on COVID-19 pandemic transmission over West and North African countries

COVID-19, caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), is a very contagious disease that has killed many people worldwide. According to data from the World Health Organization (WHO), the spread of the disease appears to be slower in Africa. Although several studies have been published on the relationship between meteorological parameters and COVID-19 transmission, the effects of climate conditions on COVID-19 remain largely unexplored and without consensus. However, the transmission of COVID-19 and sensitivity to climate conditions are also not fully understood in Africa. Here, using available epidemiological data over 275 days (i.e., from 1 March to 30 November 2020) taken from the European Center for Disease Prevention and Control of the European Union database and daily data of surface air temperature specific humidity and water vapor from the National Center for Environmental Prediction (NCEP), this paper investigates the potential contribution of climate conditions on COVID-19 transmission over 16 selected countries throughout three climatic regions of Africa (i.e., Sahel, Maghreb, and Gulf of Guinea). The results highlight statistically significant inverse correlations between COVID-19 cases and temperature over the Maghreb and the Gulf of Guinea regions. In contrast, positive correlations are found over the Sahel area, especially in the central part, including Niger and Mali. Correlations with specific humidity and water vapor parameters display significant and positive values over the Sahelian and the Gulf of Guinea countries and negative values over the Maghreb countries. Then, the COVID-19 pandemic transmission is influenced differently across the three climatic regions: (i) cold and dry environmental conditions over the Maghreb; (ii) warm and humid conditions over the Sahel; and (iii) cold and humid conditions over the Gulf of Guinea. In addition, for all three climatic regions, even though the climate impact has been found to be significant, its effect appears to display a secondary role based on the explanatory power variance compared to non-climatic factors assumed to be dominated by socio-economic factors and early strong public health measures

Le bas delta du fleuve Sénégal face aux risques de dégradation et conflits d’usage dans un contexte de restauration des écosystèmes et des activités. Approche par les outils d’analyse spatiale

International audienceLes barrages de Manantali et de Diama ont été mis en place sur le fleuve Sénégal pour favoriser le développement de l’agriculture irriguée, l’électrification et la navigation, et répondre ainsi aux graves conséquences environnementales et socio-économiques des sécheresses des dernières décennies. Mais dans le bas delta du fleuve, les dysfonctionnements hydrologiques qui en ont découlés, y ont engendré non seulement la dégradation du milieu biophysique, mais également la disparition de la plupart des activités socio-économiques liées. C’est pour remédier à ces conséquences dramatiques qu’a été crée le Parc National du Diawling (PND) en 1991 du côté mauritanien. Sa vocation était de concilier la protection de l’environnement et le développement socio-économique des collectivités locales. Des diagnostics réalisés par des analyses diachroniques d’images satellites et de photos aériennes, vérifiés et complétés sur le terrain par des enquêtes auprès des populations et des gestionnaires du Parc ont montré que la mise en place du PND a permis une réelle réhabilitation du Bas Delta ; sur le plan environnemental avec la remise en eau des plaines inondables qui a favorisé une bonne régénération de la végétation sur les dunes et dans les bassins d’inondation, ainsi qu’un bon potentiel halieutique et le retour d’une riche faune ornithologique ; sur le plan socio-économique avec la restauration des activités de pêche, d’élevage, de cueillette et le développement du maraîchage. Toutefois de nombreux problèmes environnementaux subsistent et les conditions de vie des populations restent encore difficiles malgré une amélioration sensible. Le grand défi du parc aujourd’hui est de concilier les activités d’exploitation des ressources naturelles, base de l’économie locale, et la conservation de la biodiversité, une de ses vocations majeures.C’est dans ce contexte que nous avons mis en œuvre un SIG intégrant des données issues de traitements d’images satellites, de terrain et de cartes mentales des ressources et de calendrier d’usages pour identifier les risques de dégradation des ressources et de conflits d’usage liés aux différentes activités pratiquées par les populations du bas delta mauritanien du fleuve Sénégal

Formation and Transport of a Saharan Dust Plume in Early Summer

This research studies the capability of the Weather Research and Forecasting model coupled with the Chemistry/Aerosol module (WRF-Chem) with and without parametrization to reproduce a dust storm, which was held on 27th June 2018 over Sahara region. The authors use satellite observations and ground-based measurements to evaluate the WRF-Chem simulations. The sensitivities of WRF-Chem Model are tested on the replication of haboob features with a tuned GOCART aerosol module. Comparisons of simulations with satellite and ground-based observations show that WRF-Chem is able to reproduce the Aerosol Optical Depth (AOD) distribution and associated changes of haboob in the meteorological fields with temperature drops of about 9 °C and wind gust 20 m·s–1. The WRF-Chem Convection-permitting model (CPM) shows strong 10-meter winds induced a large dust emission along the leading edge of a convective cold pool (LECCP). The CPM indicates heavy dust transported over the West African coast (16°W-10°W; 6°N-21°N) which has a potential for long-distance travel on 27th June between 1100 UTC and 1500 UTC. The daily precipitation is improved in the CPM with a spatial distribution similar to the GPM-IMERG precipitation and maximum rainfall located at the right place. As well as raising a large amount of dust, the haboob caused considerable damage along its route

Impacts of COVID-19 Restrictions on Regional and Local Air Quality Across Selected West African Cities

The emergence of COVID-19 brought with it panic and a sense of urgency causing governments to impose strict restrictions on human activities and vehicular movements. With anthropogenic emissions, especially waste management (domestic and municipal), traffic, and industrial activities, said to be a significant contributor to ambient air pollution, this study assessed the impacts of the imposed restrictions on the concentrations and size distribution of atmospheric aerosols and concentration of gaseous pollutants over West African subregion and seven major COVID-19 epicenters in the subregion. Satellite retrievals and reanalysis data sets were used to study the impact of the restrictions on Aerosol Optical Depth (AOD) and atmospheric concentrations NO2, SO2, CO, and O3. The anomalies were computed for 2020 relative to 2017–2019 (the reference years). In 2020 relative to the reference years, for area-averaged AOD levels, there was a consequential mean percentage change between −6.7% ± 21.0% and 19.2% ± 27.9% in the epicenters and −10.1% ± 15.4% over the subregion. The levels of NO2 and SO2 also reduced substantially at the epicenters, especially during the periods when the restrictions were highly enforced. However, the atmospheric levels of CO and ozone increased slightly in 2020 compared to the reference years. This study shows that “a one cap fits all” policy cannot reduced the level of air pollutants and that traffic and industrial processes are not the predominant sources of CO in major cities in the subregion