Three-dimensional distribution of a major Saharan dust outbreak in June 2011 derived from IASI satellite observations
Résumé
The Sahara is the world’s largest source of mineral dust in the atmosphere, which is a key player of
the Earth system. At the regional scale, the environmental impacts of desert dust strongly depend on
the three-dimensional (3D) distribution of dust plumes resulting from long-range transport. Dust
layers can impact chemical balances, atmospheric stability or cloud properties depending on the
altitude at which they are transported. Near the surface, dust can directly affect air quality and settle
down on the surface by dry deposition. The quantification of such impacts are highly uncertain,
particularly due to the sporadic character of dust emissions as well as the large variability of dust
properties and occurrence linked to the meteorological controls. A better understanding of the 3D
distribution of Saharan dust and the associated mechanisms for uplifting and redistributing it in the
vertical is fundamental for quantifying its environmental impacts.
This research work presents the first observation of the 3D distribution of a major Saharan dust
outbreak over the Sahara in June 2011, using new aerosol retrievals derived from IASI (Infrared
Atmospheric Sounding Interferometer). For this, we use a novel auto-adaptive Tikhonov-Philips-type
approach called AEROIASI to retrieve vertical profiles of dust extinction coefficient at 10 μm for
most cloud-free IASI pixels, both over land (including bright desert surfaces) and ocean. The dust
vertical distribution derived from AEROIASI is shown to agree remarkably well with along track
transects of CALIOP space-borne lidar vertical profiles as well as with aerosol optical depth derived
from AERONET sun photometer measurements over West Africa. A comparison is as well made
with airborne lidar and in situ measurements over the Sahara performed in the framework of the
FENNEC field campaign. We also compare AEROIASI with several satellite retrievals available
over the Sahara (e.g. MODIS-DeepBlue, OMI, SEVIRI).
Using AEROIASI observations, we analyse the 3D pathways of the Saharan dust plumes as they are
transported across the Sahara. Particularly, we focus our study on the role of cold pools generated by
mesoscale convective system, convective mixing in the Saharan atmospheric boundary layer and
strong Harmattan winds as mechanisms for vertically redistributing the Saharan dust plumes.