The Semi-Direct Aerosol-Cloud Effects

This thesis is composed of two parts. In the first part we investigate the impact of Saharan dust on North Atlantic marine stratocumulus clouds (MSc) using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), Clouds and the Earth's Radiant Energy System (CERES), and European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim). To calculate the aerosol- cloud radiative effect, we use two different methods These two methods yield similar results that 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 boreal winter. When most dust resides above the clouds during summer, aerosol-cloud microphysical effects that involve the co-location of aerosol and cloud, such as the second aerosol indirect effect, would likely be muted relative to the SDE. Moreover, the positive value of the aerosol-cloud radiative effect during winter, when most dust resides within MSc, indicates that the semi-direct effect is dominant$-$ that is the only mechanism by a negative aerosol-cloud radiative effect can be obtained. We conclude that aerosol-cloud radiative effects associated with Saharan dust and North Atlantic MSc are dominated by the semi-direct effect