The July 2003 dust storm over Italy

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.

PREAMBLEThe 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.

ACKNOWLEDGEMENTSThis 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.

REFERENCESBuzzi, 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:

10.1029/2001JD000963.

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

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.

BOLCHEM-DUST

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.

AQUA/MODIS, 16 July 2003,12:35 UTCBOLCHEM-DUST-V1

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.

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).

Accumulation mode Coarse mode

16 July 2003, 13:00 UTC

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.

BOLCHEM-DUST-V2.1 (same as V2, with changed sedimentation)

BOLCHEM-DUST-V2 (same as V1, with lowered friction velocity)

BOLCHEM-DUST-V1 BOLCHEM-DUST-V2

Dust model sensitivity

B1Accumulation mode

B1Coarse mode

B2Accumulation mode

B1Coarse mode