Impact of Saharan Dust on North Atlantic Marine Stratocumulus Clouds: Importance of the Semi-direct Effect

One component of aerosol-cloud interactions (ACI) involves dust and marine stratocumulus clouds (MSc). Few observational studies have focused on dust-MSc interactions, thus this effect remains poorly quantified. We use observations from multiple sensors in the NASA A-Train satellite constellation from 2004 to 2012 to obtain estimates of the aerosol-cloud radiative effect, including its uncertainty, for dust and Atlantic MSc off the coast of North Africa. To calculate the radiative effect, we use two methods following Quaas et al. (2008) (Method 1) and Chen et al. (2014) (Method 2). These two methods yield similar results at -3.99±0.78 and -3.21±3.61 W/m^2, respectively, for the annual mean aerosol-cloud radiative effect. Thus, Saharan dust modifies MSc in a way that acts to cool the planet. There is a strong seasonal variation, with the aerosol-cloud radiative effect switching from significantly negative during the boreal summer to weakly positive during borealwinter. Method 1 (Method 2) yields -3.81±2.51 (-4.27±4.01) during summer, and 0.97±2.91 (0.63±0.48) W/m^2 during winter. In Method 1, the aerosol-cloud radiative effect can be decomposed into two terms, one representing the first aerosol indirect effect and the second representing the combination of the second aerosol indirect effect and the semi-direct effect (i.e., changes in liquid water path and cloud fraction in response to changes in aerosol concentration). The first aerosol indirect effect is relatively small, varying from -0.65±0.61 in summer to 0.05±0.5 W/m^2 in winter. The second term, however, dominates the overall radiative effect, varying from -3.16±2.45 in summer to 0.92±2.86 W/m^-2 during winter. Studies show the semi-direct effect can result in a positive (e.g., absorbing aerosol lies above low clouds like MSc) or negative (e.g., absorbing aerosol lies within low clouds) aerosol-cloud radiative effect. CALIPSO shows that 50\% to 90% of Saharan dust resides above North Atlantic MSc during summer for most of our study area. This is consistent with a relatively weak first aerosol indirect effect, and also suggests the second aerosol indirect effect plus semi-direct effect (the second term in Method 1) is dominated by the semi-direct effect. In contrast, the percentage of Saharan dust above North Atlantic MSc is much lower during the winter, ranging from 10% to 40%. Because the aerosol-cloud radiative effect is positive during winter, and is also dominated by the second term, this again supports the importance of the semi-direct effect. We conclude that Saharn dust-MSc interactions off the coast of north Africa are likely dominated by the semi-direct effect.