Abstract
In this study, the long term trend of the observed visibility data used directly (without conversion into dust concentrations) over Sahel was investigated between 1957 and 2013. Then, to review the influence of atmospheric factors and land surface conditions on this trend, the coevolution between the visibility and the dust surface mass concentration from MERRA-2 (Modern-Era Retrospective analysis for Research and Applications) reanalysis, the in-situ surface meteorological data (rainfall, relative humidity, wind speed, and air temperature), as well as the Normalized Difference Vegetation Index (NDVI) were analyzed from 2000 to 2013. We showed that the horizontal visibility has significantly decreased since the 1970s. The coevolution between the visibility and the dust surface mass concentration revealed that visibility decreased significantly with increments in dust concentrations. Visibility increases with rainfall and relative humidity. It is greater in areas of high vegetation cover than in deforested areas. Visibility is weakly correlated with wind speed and air temperature but generally, wind leads to a decrease in visibility, while warm air temperature is associated with a clearer sky and hence, high visibility. The worst visibility in the dry season results from high dust concentrations due to warm and dry wind conditions and less vegetation cover. Rainfall, relative humidity and vegetation cover are the dominant factors contributing to the decrease of dust loading in the Sahel.
Highlights
The increase of Aeolian processes observed in most arid and semi-arid areas of the world over the last decades has been observed to be responsible for the environmental stresses and global climate change [1] [2] [3]
To review factors of the atmosphere and of the land surface conditions that influence the visibility in the Sahel, the coevolution between the visibility and the dust surface mass concentration, the in-situ surface meteorological data, as well as the Normalized Difference Vegetation Index (NDVI) were analyzed from 2000 to 2013
The highest visibility, relative humidity, temperature and NDVI were recorded in the rainy season, while the highest dust surface mass concentration and wind speed were measured in the dry season
Summary
The increase of Aeolian processes observed in most arid and semi-arid areas of the world over the last decades has been observed to be responsible for the environmental stresses and global climate change [1] [2] [3]. The vast majority of North African dust emissions occur within largely uninhabited regions and there is a paucity of both meteorological and surface observations in these locations, homogeneous measurements of each that span time scales of years to decades [9]. In this regard, numerical models play a crucial role in understanding the processes. Models are often validated using surface visibility observations from a limited number of sampling stations and retrievals of aerosol optical depth from satellites and ground-based instrumentations, none of them is based on direct measurements of dust concentrations [10]. Several studies have used visibility observational data to investigate the spatial and temporal variation of the optical properties of the atmosphere, mainly with respect to pollutants emissions and aerosol loads [13] [14]
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