Evidence for a Glaciation Aerosol Indirect Effect from Ship Tracks

Ship tracks are a prominent manifestation of the aerosol indirect effect that provides a unique opportunity to study aerosol interactions in both warm and mixed-phase clouds. While ample evidence supports that an increase in aerosol concentration generally suppresses warm phase precipitation leading to longer cloud lifetime and more reflected sunlight (Albrecht, 1989) there is less understood about these effects in mixed-phase clouds. Lohmann, (2002) propose that an increase in IN (Ice Nuclei) may cause a glaciation indirect effect which results in more frequent glaciation of super-cooled droplets via the Bergeron process thereby increasing the amount of precipitation, which could decrease cloud cover, cloud longevity, and reflected sunlight. In this study, over 200 ship tracks are identified in mixed phase clouds using MODIS (MODerate resolution Imaging Spectroradiometer) imagery. Retrievals of the ice phase are obtained using CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations). These measurements provide evidence that glaciation is more frequent in polluted clouds compared to the unpolluted clouds that lie adjacent to ship tracks. Larger ice fractions may result from the increased IN emitted from the ship or by other processes (e.g., immersion/contact freezing) that lead to faster ice multiplication in polluted clouds with smaller and more numerous supercooled droplets. Observations from the profiling radar on CloudSat show that aerosol suppresses warm phase precipitation but enhances the cold phase precipitation. For mixed-phase clouds, these differences roughly cancel resulting in small changes in precipitation between polluted and unpolluted clouds. When cloud tops are warm, aerosol decreases precipitation rates and cloud water paths due to the entrainment effect but the differences in cloud water amount are considerably smaller than those found in cold phase clouds. These results provide the first glance of ship tracks in mixed-phase clouds and provide evidence for a glaciation aerosol indirect effect, which may decrease the overall longevity of boundary layer clouds in a cold environment.