1. 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 dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, 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.
Accumulation mode
Coarse mode
16 July 2003, 13:00 UTC
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 12:35 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.
16 July 2003, 13:00 UTC
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.
AQUA/MODIS, 16 July 2003,12:35 UTC
BOLCHEM-DUST-S1
BOLCHEM-DUST-S2
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). S1 S2
The two simulations presented here were carried out on a domain which extends from W to E and from N to 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,
Carter, W. P .L., 1990, Atmos. Environ., 24A,
Gery, W., Witten, G. Z., Killus, J. P., Dodge. M. C., 1989, J. Geophys. Res., , 94, D10,
Marticorena, B., Bergametti, G., 1995, J. Geophys. Res.,
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.,
Tegen, I., Harrison, S.P., Kohfeld, K, Colin Prentice, I, Coe, M., Heinmann,M., 2002, J.Geophys.Res., 107, D21, doi: /2001JD
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.