From hygroscopic aerosols to cloud droplets: The HygrA-CD campaign in the Athens basin - An overview.

The international experimental campaign Hygroscopic Aerosols to Cloud Droplets (HygrA-CD), organized in the Greater Athens Area (GAA), Greece from 15 May to 22 June 2014, aimed to study the physico-chemical properties of aerosols and their impact on the formation of clouds in the convective Planetary Boundary Layer (PBL). We found that under continental (W-NW-N) and Etesian (NE) synoptic wind flow and with a deep moist PBL (~2-2.5km height), mixed hygroscopic (anthropogenic, biomass burning and marine) particles arrive over the GAA, and contribute to the formation of convective non-precipitating PBL clouds (of ~16-20µm mean diameter) with vertical extent up to 500m. Under these conditions, high updraft velocities (1-2ms-1) and cloud condensation nuclei (CCN) concentrations (~2000cm-3 at 1% supersaturation), generated clouds with an estimated cloud droplet number of ~600cm-3. Under Saharan wind flow conditions (S-SW) a shallow PBL (<1-1.2km height) develops, leading to much higher CCN concentrations (~3500-5000cm-3 at 1% supersaturation) near the ground; updraft velocities, however, were significantly lower, with an estimated maximum cloud droplet number of ~200cm-3 and without observed significant PBL cloud formation. The largest contribution to cloud droplet number variance is attributed to the updraft velocity variability, followed by variances in aerosol number concentration.

Natural and anthropogenic aerosols in the Eastern Mediterranean and Middle East: possible impacts.

The physical and chemical properties of airborne particles have significant implications on the microphysical cloud processes. Maritime clouds have different properties than polluted ones and the final amounts and types of precipitation are different. Mixed phase aerosols that contain soluble matter are efficient cloud condensation nuclei (CCN) and enhance the liquid condensate spectrum in warm and mixed phase clouds. Insoluble particles such as mineral dust and black carbon are also important because of their ability to act as efficient ice nuclei (IN) through heterogeneous ice nucleation mechanisms. The relative contribution of aerosol concentrations, size distributions and chemical compositions on cloud structure and precipitation is discussed in the framework of RAMS/ICLAMS model. Analysis of model results and comparison with measurements reveals the complexity of the above links. Taking into account anthropogenic emissions and all available aerosol-cloud interactions the model precipitation bias was reduced by 50% for a storm simulation over eastern Mediterranean.