Aerosol Effect on the Phase of Low-Level Clouds in the Arctic

Cloud phase (liquid or ice) affects the cloud's persistence and optical properties, and thus the atmospheric radiation balance. In the warming Arctic, the transition from ice to liquid clouds has a potential to enhance the warming rate, however, changes in aerosol properties can affect the cloud phase even more substantially. Here, a characterization of low level clouds and their dependence on dust aerosols, which are assumed to be the most important ice nuclei (IN) over the Arctic area, is presented based on measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The level 2 aerosol and cloud layer products with 5 km horizontal resolution from the CALIOP lidar (Cloud-Aerosol Lidar with Orthogonal Polarization) were employed for the period June 2006 to December 2015. The three available cloud phases (ice, water, unknown) were linked with the coinciding aerosol type characterization (dust, polluted dust, smoke and background aerosols). Based on the ice cloud fraction calculations done in this study, clouds freeze in warmer temperatures in the presence of dust particles than with other aerosol types. A difference in the cloud freezing temperature as high as 7o C was observed. Thus, dust aerosols have a high potential to affect the properties of Arctic clouds. Furthermore, possible increments in efficient IN due to long-range transported aerosols or new local aerosol sources due to anthropogenic activities may lead to optically thinner clouds modifying the radiation balance over Arctic.