Aerosol properties near clouds: Insights from MODIS and CALIPSO observations

Several recent studies have found that, due to processes such as aerosol swelling in humid air, clouds are surrounded by a transition zone of rapidly changing aerosol optical properties. Characterizing this transition zone is important for better understanding aerosol-cloud interactions and aerosol radiative effects, and also for improving satellite retrievals of aerosol properties. This study examines the transition zone by analyzing CALIPSO and MODIS data, by taking advantage of the two instruments' capabilities and limitations complementing each other (e.g., vertical information but larger noise for CALIPSO, and spectral and cross-track information but 3-D radiative effects for MODIS). The analysis of CALIPSO data shows that optical thickness and particle size increase in a nearly 15 km wide zone around clouds over most oceanic regions, and that this increase is most pronounced near cloud top altitude. MODIS data shows that clear-sky reflectance enhancements near clouds are stronger at shorter wavelengths, near thicker clouds, and near sunlit cloud sides. These behaviors indicate that three-dimensional radiative processes play a significant role in shaping radiation fields, and can affect aerosol retrievals in the transition zone. The findings highlight a dilemma near-cloud aerosol changes and remote sensing uncertainties pose to researchers: Excluding the transition zone in order to avoid its remote sensing uncertainties can bias a study toward low aerosol optical depths and radiative effects, but including the transition zone despite the remote sensing uncertainties can bias the study toward too high aerosol optical depths and radiative effects. The presentation concludes by outlining some possible strategies for addressing this dilemma.