SAHARAN DUST OVER ITALY: SIMULATIONS WITH THE REGIONAL
AIR QUALITY MODEL BOLCHEM
Mihaela Mircea, Massimo D'Isidoro, Alberto Maurizi, Francesco Tampieri, Maria Cristina Facchini, Stefano Decesari,
Sandro Fuzzi
Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, Bologna, Italy
PREAMBLE
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 (Mircea et
al., 2007; P 4.5). The flux of dust at surface is parameterized as a function of friction velocity, which depends on winds and on threshold friction velocity, and on particle size. Here,
we show the sensitivity of the dust model to threshold velocity values and to number of dust size bins. And a preliminary analysis of model ability to predict a dust storm over Italy.
BOLCHEM-DUST
The July 2003 dust storm over Italy
BOLCHEM (Mircea et al., 2007) couples on-line, one-way, meteorology with
chemistry and dust production. The dust model implemented in BOLCHEM was
developed by Tegen et al. (2002), 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 soil texture and surface wind speed. The threshold friction
velocities used to initiate dust emissions are computed as a function of particle
size (Marticorena and Bergametti, 1995), assuming constant roughness (0.001
cm) within model grid cells.
A strong Saharan dust outbreak occurred from 15 to 19 July 2003 over Italy. The
figures below show the SeaWIFS image of the 16 July at 12:35 UTC and the dust
loading in g/m2 for two simulations performed with BOLCHEM model: V1 and V2. By
comparing BOLCHEM simulations V1 and V2 to the AQUA/MODIS satellite/sensor
image, the model seems to 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 V1 is much higher
than that given by simulation V2 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 V1 than in the
case V2.
Dust model sensitivity
16 July 2003, 13:00 UTC
Accumulation mode
Coarse mode
BOLCHEM-DUST-V1
AQUA/MODIS, 16 July 2003,12:35 UTC
Sensitivity to threshold friction velocity: The graphs above show the ratio of dust
concentrations at surface for the simulations V1 (threshold friction velocities lowered
by a factor of 0.50) and V2 (threshold velocities lowered by a factor of 0.75) with
respect to the values calculated according with Marticorena and Bergametti (1995).
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.
B1
Accumulation mode
B1
Coarse mode
BOLCHEM-DUST-V2 (same as V1, with
lowered friction velocity)
B2
Accumulation mode
B1
Coarse mode
BOLCHEM-DUST-V2.1 (same as V2,
with changed sedimentation)
The figures below show the vertical distribution in function of forecast time of dust
concentrations (g/m3) at Etna, from 14 to 19 July, for the cases V1 and V2. The
results are in agreement with Tafuro et al. (2006), the dust layers are located below
6 km (ca. level 19).
Sensitivity to number of dust size bins: The graphs above show the ratio of
dust loading for two cases: 12 bins (B1) and 48 bins (B2) with respect to the
reference case with 24 bins. The results show that the dust loading increase with
increasing the number of bins and decrease substantially by reducing the number
of bins in both accumulation and coarse mode.
REFERENCES
Buzzi, A., D'Isidoro, M., Davolio, 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:
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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.
BOLCHEM-DUST-V1
BOLCHEM-DUST-V2
The 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. The
sedimentation velocity of the dust particles has been considered constant in the
simulations V1,V2 and variable in the simulations V2.1, B1 and B2.