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 st...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/8395/2017/acp-17-8395-2017.pdf |
| Summary: | 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. SeaWiFS (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 single-scattering 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° W, 14.74° 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. |
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| ISSN: | 1680-7316 1680-7324 |