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.
