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PRELIMINARY RESULTS OF THE MINERAL DUST MODEL
IMPLEMENTED IN BOLCHEM
Mihaela Mircea, Massimo D'Isidoro, Alberto
Maurizi, Francesco Tampieri, Maria Cristina
Facchini, Stefano Decesari, Sandro Fuzzi Istituto
di Scienze dellAtmosfera e del Clima, Consiglio
Nazionale delle Ricerche, 40129 Bologna, Italy
PREAMBLE In the last years, many studies have
shown that the direct forcing of dust aerosol may
be comparable to or even exceed the forcing of
anthropogenic aerosols on both global and
regional scales. The dust aerosols, besides of
changing climate through the scattering and
absorption of solar and thermal radiation, also
affect the environment by fertilizing marine and
terrestrial ecosystems, which in turn influence
the carbon cycle. Moreover, the dust particles
contribute substantially to the total aerosol
mass usually employed in the developing of the
environmental policy regulations, therefore, a
reliable forecast of dust events is mandatory.
Italy is often reached by dust produced in the
Saharan regions. To the scope of predicting the
advection of dust and its physical and chemical
properties over Italy, a dust emission scheme has
been implemented in the air quality model
BOLCHEM, which solves simultaneously the chemical
and meteorological equations. The dust models
rely heavily on the meteorological information,
thus, the online coupling of meteorology to
production, dynamics and chemistry of dust
aerosols is beneficial. This coupling also allows
a better representation of atmospheric processes
(e.g. cloud and rainfall formation, wind speed
and direction), which often have a much smaller
time scale than the meteorological output
frequency involved in dust forecasts. The dust
model simulates the emissions of dust with a
scheme based on preferential dust source areas,
soil types and surface conditions like vegetation
cover and soil moisture. The flux of dust at
surface is a function of friction velocity, which
depends on winds and on threshold friction
velocity, which depends on particle size. Here,
we show a preliminary analysis of model ability
to predict a dust storm over Italy and of model
sensitivity to threshold velocity values.
Dust model sensitivity
BOLCHEM-DUST
BOLCHEM (Mircea et al., 2007) is a modeling
system that comprise the meteorological model
BOLAM (Buzzi et al., 2003), an algorithm for
airborne transport and diffusion of pollutants
and two photochemical mechanisms SAPRC90
(Carter, 1990) and CB-IV (Gery et al., 1989). The
dust model implemented in BOLCHEM was developed
by Tegen et al. (2002) and it is based on the
results from Marticorena and Bergametti (1995).
The horizontal and vertical dust fluxes are
calculated based on the location of the
preferential dust sources, soil texture, surface
roughness, vegetation cover, soil moisture
content and surface wind velocity. The ratio
between the vertical and the horizontal dust
fluxes varies with the type of soil and the size
of the particle mobilized. The size distribution
of the mobilized dust depends on both the surface
properties (soil texture) and the surface wind
speed. The threshold friction velocities used to
initiate the dust emissions are computed as a
function of particle size following Marticorena
and Bergametti (1995), assuming constant
roughness (0.001 cm) within the model grid cells.
The July 2003 dust storm observed over Italy
The graphs above show the ratio of dust
concentrations at surface S1/S2. The different
increase of dust concentration in the coarse mode
with respect to accumulation mode is a
consequence of their different production
mechanisms coarse particle are produced by
deflation and accumulation particles by saltation.
A strong Saharan dust outbreak occurred from 15
to 19 July 2003, transporting the dust particle
almost over the whole Italy. The left graphs
show the SeaWIFS image of the 16/07/2003 at
1235 UTC and the dust loading in mg/m2 for two
simulations performed with BOLCHEM model S1 and
S2. The simulations S1 and S2 were carried out
with threshold friction velocities lowered by a
factor of 0.50 and 0.75 with respect to the
values calculated according with Marticorena and
Bergametti (1995). By comparing BOLCHEM
simulations to the AQUA/MODIS satellite/sensor
image, it can be seen that the model predict
relatively well both the extent and the timing of
the dust event over Italy. In all images, it can
be noted that the plume of dust over the
Mediterranean sea comes from north-west and north
of Africa and goes straightforward to the center
and north of Italy with only a little veil over
Sicily and Messina Strait. However, the dust
loading given by the simulation S1 is much higher
than that given by simulation S2 since more dust
particles are mobilized when the threshold
friction velocity is lowered. The comparison of
model results with the observations (surface
concentrations from EMEP stations and aerosol
optical depth (AOD) from AERONET stations) shows
better agreement in the case S1 than in the case
S2.
The right graph shows the ratio of AOD S1/ S2.
It can be seen that the differences in AOD due to
the threshold velocity are much lower than the
differences in surface concentrations. However,
the lowering of the threshold velocity, produce
important increase of AOD over Italy from 4 to 5
times.
The graphs below show the vertical distribution
of dust concentrations (mg/m3) at Etna, from 14
to 19 July, for the cases S1 and S2. The results
are in agreement with Tafuro et al. (2006), the
dust layers are located below 6 km (ca. level 19).
The two simulations presented here were carried
out on a domain which extends from -15.91 W to
33.58 E and from 11.37 N to 53.37 N with a
resolution of 0.5 degrees. The ECMWF data were
used as initial and boundary conditions for
meteorology. In both simulations, the
sedimentation velocity of the dust particles has
been considered constant, varying only with
particle size.
REFERENCES Buzzi, A., D'Isidoro, M., Diavolio,
S., 2003, Q. J. R. Meteorol. Soc., 129,
1795-1818. Carter, W. P .L., 1990, Atmos.
Environ., 24A, 481-518. Gery, W., Witten, G. Z.,
Killus, J. P., Dodge. M. C., 1989, J. Geophys.
Res., , 94, D10, 12925-12956. Marticorena, B.,
Bergametti, G., 1995, J. Geophys. Res.,
16415-16430. Mircea, M., D'Isidoro, M., Maurizi,
A., Vitali, L., Monforti, F., Zanini, G.,
Tampieri, F., 2007, submitted to
Atmos.Environ. Tafuro,A.M., Barnaba, F., De
Tomasi, F., Perrone, M.R., Gobbi, G.P., 2006,
Atmos. Res., 67-93. Tegen, I., Harrison, S.P.,
Kohfeld, K, Colin Prentice, I, Coe, M.,
Heinmann,M., 2002, J.Geophys.Res., 107, D21, doi
10.1029/2001JD000963.
ACKNOWLEDGEMENTS This work was conducted in the
frame of ACCENT and GEMS EC projects, Italian
MIUR project AEROCLOUDS, and was also supported
by the Italian Ministry of Environment through
the Program Italy-USA Cooperation on Science and
Technology of Climate Change.