Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-10-8811-2010
http://www.atmos-chem-phys-discuss.net/10/8811/2010/
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http://www.atmos-chem-phys-discuss.net/10/8811/2010/acpd-10-8811-2010.pdf
8811
8858
2010-04-07
Radiative forcing associated with a springtime case of Bodélé and Sudan dust transport over West Africa
C. Lemaître
C. Flamant
J. Cuesta
J.-C. Raut
P. Chazette
P. Formenti
J. Pelon
Laboratoire Atmosphères, Milieux, Observation Spatiales, UMR 8190, CNRS and Université Pierre et Marie Curie and UVSQ, Paris, France
Laboratoire de Météorologie Dynamique, CNRS, Ecole Polytechnique and ENS, Palaiseau, France
Laboratoire des Sciences du Climat et l'Environnement, CEA, CNRS and UVSQ, Saclay, France
Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS and Université Paris Est Créteil Val de Marne/ Université Denis Diderot, Créteil, France
The radiative forcing due to mineral dust over West
Africa is investigated using the radiative code STREAMER, as well as remote
sensing and in situ observations gathered during the African Monsoon
Multidisciplinary Analysis Special Observing Period (AMMA SOP). We focus on
two days (13 and 14 June 2006) of an intense and long-lasting episode of
dust being lifted in remote sources in Chad and Sudan and transported across
West Africa in the African easterly jet region, during which airborne
operations were conducted at the regional scale, from the southern fringes
of the Sahara to the Gulf of Guinea. Profiles of heating rates are computed
from airborne LEANDRE 2 and space-borne CALIOP lidar observations using two
mineral dust model constrained by airborne in situ data and ground-based
sunphotometer obtained during the campaign. Complementary space-borne
observations (from MODIS) and in-situ observations such as dropsondes are
also used to take into account a realistic infrared contribution of the
water vapour. We investigate the variability of the heating rate on the
vertical within a dust plume, as well as the contribution of longwave
radiation to the heating rate and the radiative forcing of dust during the
nighttime. The sensitivity of the so-derived heating rate is also analyzed
for some key variables for which the associated uncertainties are quite
large. During daytime, the warming associated with the presence of dust was
found to be between 1.5 K day<sup>−1</sup> and 4 K day<sup>−1</sup>, on average,
depending on altitude and latitude. Strong warming (i.e. heating rates as
high as 8 K day<sup>−1</sup>) was also observed locally in some limited part of
the dust plumes. Obviously, during nighttime much smaller values of
heating/cooling are retrieved (less than ±1 K day<sup>−1</sup>) but large
enough to modify the low tropospheric equilibrium. Furthermore, cooling is
observed as the result of the longwave forcing in the dust layer, while
warming is observed below the dust layer, in the monsoon layer.
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