Impact of Sahara dust on solar radiation at Cape Verde Islands derived from MODIS and surface measurements

Based on radiometer measurements of solar irradiance (direct and diffuse light) and Aeronet-based aerosol optical depth (AOD) obtained at the Cape Verde atmospheric observatory during a major cloud-free dust outbreak event on February 7, 2012, the relationship between Saharan mineral dust outbreaks and a reduction of solar irradiance is quantified. The investigation is representative of the eastern subtropical North Atlantic region where the wind mobilization of mineral desert dust from the Sahara results in aerosol signals that are large enough to outweigh those from other aerosol types such as anthropogenic and marine aerosols. Ground-based estimates of AOD show frequency dependence as is expected from Mie theory. Our AOD signals agree well with satellite-based MODIS products and reveal AOD values exceeding 2.5 during the investigated dust storm event. We also demonstrate the use of satellite imagery with an atmospheric trajectory model to simulate time series of measurements at a given location. Using this approach, variations in AOD observed during February 7, 2012 can be rationalized as spatial inhomogeneities in the atmospheric dust load being advected laterally over the observing site. Our measurements suggest a dust forcing efficiency of around − 90 W/m2/AOD at a wavelength of 380 nm, which is about 10-15% greater than reported in the literature indicative of a possible non-linear behavior at high AODs.