Short Description

Aerosols affect the radiative balance of the Earth-Atmosphere system not only by scattering or absorbing solar radiation directly (aerosol direct effect) but also through their effect on cloud microphysical and macrophysical properties (aerosol indirect effects). Increased aerosol concentrations are expected to increase cloud droplet concentration, total droplet cross-sectional area and by extension cloud albedo for clouds with stable liquid water path (LWP). This is the first indirect effect. Moreover, smaller droplets are expected to delay precipitation formation and increase cloud lifetime and cloud cover (second aerosol indirect effect or cloud lifetime effect). Another effect is the so-called "semi-direct effect" which is actually the "burning off" of clouds in the presence of absorbing aerosols in the atmosphere. Aerosol direct and indirect effects are still considered a highly uncertain forcing to the climate on a global scale. The mean annual aerosol direct radiative effect has been estimated at -0.4±0.2Wm-2 while the first indirect effect has been estimated at -0.7±0.4Wm-2. Models show that the second indirect effect is about the same order of magnitude as the first but considerable uncertainties remain. The quantification of the aerosol indirect effect is crucial since it represents a significant fraction of the anthropogenic impact on climate.

The proposed research aims at the quantification of the aerosol direct and indirect radiative effects and the investigation of the aerosol, cloud droplet and cloud cover relations from satellite observations over the region of Eastern Mediterranean [30oN-45No, 17.5oE-37.5oE]
. Aerosol and cloud optical properties data from various satellite instruments (MODIS TERRA, MODIS AQUA, Earth Probe TOMS, OMI and CALIOP/CALIPSO), ECMWF ERA-interim reanalysis wind speed data and data from a global chemical-aerosol-transport model will be jointly analyzed for the compilation of a new high and moderate resolution (0.1x0.1 degrees and 1x1 degrees) spatially and temporally homogenized gridded dataset. The relative contribution of maritime, anthropogenic and dust aerosols will be determined. These parameters will be used as an input to recently proposed satellite-based parameterizations in order to quantify the direct and indirect radiative effects of different aerosol types. The first and second indirect effects of aerosols, through their impact on the droplet size and lifetime of clouds, will also be investigated analyzing 2D (latitude/longitude) and 3D (latitude/longitude/height) satellite data. The position of the aerosol layer relative to clouds seems to be critical for the effect of aerosols on clouds. In addition, the direct and indirect radiative effect of aerosols also depends on the relative position of aerosol and cloud layers. The high resolution of the analysis applied here is expected to reveal local features that cannot be discriminated with the usually used (100 times lower) 1x1 degree resolution. The procedure will be repeated using a 1x1 degree resolution, in order to examine the footprint of the aerosol direct and indirect effects. The investigation of the local features will help us to better understand the aerosol-cloud-radiation interactions. Within this research simulations will be implemented using REGCM4 regional climate model. Results from these model runs will be compared with the satellite-based observations in order to evaluate REGCM4's ability to simulate aerosols and their direct radiative effect.

The proposed research focuses on the region of Eastern Mediterranean [30oN-45No, 17.5oE-37.5oE], since, apart from the obvious local interest arising from the fact that Greece is situated in this area, this is at the crossroad of different aerosols types. Anthropogenic aerosols from continental Europe, Saharan dust aerosols, maritime aerosols, biomass burning aerosols from agricultural fires in Southeastern and Eastern Europe) are accumulated over this region. So, Eastern Mediterranean consists a "natural laboratory" ideal for the investigation of the direct and indirect effects of the various aerosol types. Previous aerosol and radiative forcing (direct radiative effect only) studies in the area have only focused on specific spots or have been done at a much coarser resolution than the 0.1x0.1 degree resolution proposed here.

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