An intense dust event over Mediterranean basin during June 2006: model description and evaluation with in situ data

The application of Numerical Prediction Models to mineral dust cycle is considered of prime importance in climate change due to aerosol and non-CO2 greenhouse gases. In fact, the mineral dust aerosol injected into the atmosphere and transported away, may modify the direct radiative forcing, the marine biochemistry, cloud microphysics dynamic, the air quality with a large number of consequences even though on human health. Furthermore it can play a potential role in the neutralization of acid rain. In this framework, a comprehensive atmospheric, emission and dispersion modelling system was developed in order to provide a regional characterization of Saharan dust intrusions over Mediterranean basin, focussing on the Italian Peninsula. The aim of the study is the identification specific atmospheric circulation responsible for the long-range dust transport, the evaluation of the entire modeling system and its potential implementation within an operational framework. This analysis is carried on using a comprehensive modeling system in combination with remote sensing data analysis in order to simulate the entire mineral dust cycle in the atmosphere. In order to characterize the processes influencing dust emission we first discriminate sparse vegetation cover, soil roughness and moisture. Soil type was most efficiently mapped using optical reflectance data (using MODIS and the spectral mixing analysis) whereas aerodynamic roughness was obtained from synthetic aperture radar (SAR) backscatter measurements. The modelling system then combine two major physical mechanisms: firstly, a wind stress lifting mechanism which is able to raise up dust particles from some type of bare soil surfaces; then, a long range transport mechanism with a high degree of spatial coherence. The modeling system was based on three different modules: the atmospheric model, the dust emission model and transport/deposition model. The Regional Atmospheric Modelling System (RAMS), forced by the Reanalysis2 dataset, provides input data for the other modules. The dust emission model, called DUSTEM, simulates the emission of four particle categories, based on the content of clay, small-sit, large-silt and sand from the Saharan desert. The dispersion model - Comprehensive Air quality Model with extensions (CAMx), takes the meteorological inputs from RAMS and the emission amount from DUSTEM. The modeling system domain is 16E-60W, 4.7S-77N, with 60 km of horizontal resolution for a relevant episode of Saharan dust intrusions in June 2006. The higher horizontal and vertical resolution of the three-dimensional modeling system improves, with respect to global datasets, the description of dynamical features linking tropics and sub tropics within the Afro - Mediterranean region. The evaluation of the performed analysis on the selected case study, shows a good agreement with the in-situ measurements of some specific crustal markers in the PM10 fraction.