Satellite Observations of Cloud-Related Variations in Aerosol Properties

This presentation provides an overview of our efforts to characterize and better understand cloud-related changes in aerosol properties. These efforts have involved primarily the statistical analysis of MODIS and CALIOP aerosol and cloud observations. The results show that in oceanic regions, over half of all aerosol measurements by passive satellite instruments come from near-cloud areas, where clouds and cloud-related processes may significantly modify aerosol optical depth and particle size. As in preceding studies, aerosol optical depth is found to increase systematically with regional cloud amount throughout the Earth. In contrast, it is shown that effective particle size can either increase or decrease with increasing cloud cover. In bimodal aerosol populations, the sign of changes depends on scene characteristics such as aerosol coarse mode fraction or the hygroscopicity and altitude of small and coarse particles. The results also indicate that over large parts of Earth, undetected cloud particles are not the dominant reason for the satellite-observed changes with cloud amount, and that 3D radiative processes contribute about 30% of the observed near-cloud changes. The findings underline the need for improving our ability to accurately measure aerosols near clouds.