Diapositive 1

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Preliminary validation of the mineral dust
transport model
C. Schmechtig, L. Menut, B. Marticorena, B.
Chatenet LISA, UMR 7583, 61 avenue du Général de
Gaulle, 94010 Créteil Cedex France
schmechtig_at_lisa.univ-paris12.fr http//www.lisa.u
niv-paris12.fr/mod/chimeredust/chimdust.html
Abstract This poster presents the preliminary
validation of ChimereDust, a model which takes
into account emissions and transport of Desert
dust. For the validation, we use SeaWiFS data,
AERONET Data and MERIS data. Even if there is
still ongoing work for the intensity (Factor of
5), results are very encouraging regarding the
transport and the emissions.
The March 2004 dust storm observed over Western
Africa A dust storm occured on early March over
Western Africa. In order to have a view of the
whole dust event, we used SeaWIFS composites
provided by ORBIMAGE and the NASA/GSFC center.
From these images, we defined the simulation
period and selected the MERIS data. A simulation
of the atmospheric dust content was performed
with ChimereDUST using the ECMWF meteorological
fields from 1rst February (accounting for the
spin-up) to 16th March 2004 on the continental
domain for a 1x1 resolution. We also get
AERONET data (Holben et al., 2001) for the same
period.
7)
Figure 3) illustrates time series of AERONET
aerosol optical thickness (AOT) from the 3rd to
the 10th March. The comparison between the
measured and simulated AOT for the Cape Verde
islands and Dakar reveals that (1) in terms of
intensity, the simulated AOT are lower by a
factor of 5. than the AERONET AOT (2) the
temporal variations of the simulated AOT agrees
with observation (4a and 4b) the dust storm
reached Dakar, the 4th March, and Cape Verde
Islands, the 5th of March, while the maximum AOT
is observed and simulated the 5th March in Dakar
and in Cape Verde islands.
All SeaWiFS composites are Provided by the
SeaWiFS Project, NASA/Goddard Space Flight Center
and ORBIMAGE
Conclusion Our first objective was to validate
the simulations with MERIS LEVEL 2 aerosol
products. Figures 9 (a, b, c) present the MERIS
products for the 6th of March. We noticed that
the flag  dust-like absorbing aerosols 
(Figure 9a) only partially detects the desert
dust plume seen by SeaWiFS ( figure 7). In fact,
no aerosol inversion is performed in region of
the dust plume maximum (figure 9b). While the AOT
should reach their maximum, the retrieval is set
to 0.0. The maxium retrieved AOT is 1.6, while
the AERONET AOT measured in Dakar and Cap Verde
Islands can be higher than 2 ( figures 4a) and
4b)). On Figure 9c, we present the radiance in
the band 13 of MERIS at the LEVEL1. This figure
gives a view of the whole dust plume. At this
wavelength (870nm), since the ocean is dark, the
atmospheric contribution to the signal is
dominant. The inclusion of a radiative transfer
module in ChimereDust would allow a direct
simulation of the measured radiance.This approach
would avoid any artefact due to the inversion.
8)
5a)
5b)
The simulated aerosol plume altitude ranges
between the surface and 2000m (5a and 5b). On the
Figure 6 (SeaWiFS image of the 05/03/2004) over
the Cape Verde Republic, the summits of the
Santiago and Sao Nicolau (1300m) Islands emerge
from the dust clouds. This means that this dust
event takes place below these altitudes.
3)
References -B. Marticorena and G. Bergametti,
Modeling the atmospheric dust cycle 1-Design of
a soil derived dust production scheme, J.
Geophys. Res., 100, 16415-16430, 1995 -B.
Marticorena, G. Bergametti, B. Aumont, Y. Callot,
C. N'Doumé and M. Legrand, Modeling the
atmospheric dust cycle 2-Simulations of Saharan
dust sources, J. Geophys. Res., 102, 4387-4404,
1997. -Alfaro S.C. and L. Gomes, Modeling
mineral aerosol production by wind erosion
Emission intensities and aerosol distributions in
source areas, J. Geophys. Res, 106, n D16,
18,075-18,084, 2001. -Holben, B.N., D. Tanré, A.
Smirnov, T.F. Eck, I. Slutsker, N. Abuhassan,
W.W. Newcomb, J. Schafer, B. Chatenet, F. Lavenu,
Y.J. Kaufman, J. Vande Castle, A. Setzer, B.
Markham, D. Clark, R. Frouin, R. Halthore, A.
Karnieli, N.T. O'Neill, C. Pietras, R.T. Pinker,
K. Voss, G. Zibordi ,An emerging ground-based
aerosol climatology Aerosol Optical Depth from
AERONET, J. Geophys. Res., vol. 106, No. D11, pp
12067-12097, 2001.
Perspectives The validation of the simulations
performed with Chimeredust is based both on
available ground-based data and satellite data,
over land and over ocean. The synoptic and
continental domains will be devoted to long-term
simulations. Their spatial resolution is
comparable to the resolution of historical
sensors Meteosat (visible and infrared) and TOMS,
which can be used for the long-term simulations
and validations. The regional domain will be used
for simulating periods for which ground-based
observations are available. The spatial
resolution of this domain is similar to that of
the new generation sensors (MERIS, POLDER,
SEAWIFS et MSG), allowing a fine resolution
validation. It is also foreseen to use AATSR data
or SCIAMACHY data, as validation data. A
climatologic studies of the mineral dust cycle
simulated over North Africa over the past 20
years, when the Sahel experienced the largest
variations in the mineral dust concentrations
will be performed to address the following
questions Are Sahelian dust emissions
responsible for the observed increase of mineral
dust content during drought period ? To what
extend the decrease of the wet deposition flux
due to precipitation deficit participates to this
increase ? Is the synoptic circulation modified
during drought period, and does this possible
change affect the efficiency and main pathways of
the dust atmospheric transport ?