CCN as a Modulator for Ice Processes in Arctic Mixed-Phase Clouds

The effect of aerosol on mixed-phase cloud forcing is critical for understanding climate change in the Arctic, one of the most climatically sensitive regions of the planet. Mixed-phase clouds are thermodynamically unstable, and yet often persist in the Arctic springtime. At temperatures above ~ -38oC, aerosol particles are needed to overcome the energy barrier for freezing. Thus it is typically assumed that glaciation of super-cooled clouds relies on the concentration of this special subset of the aerosol population known as ice nuclei (IN). However, it has been historically difficult to show a clear relationship between measured IN concentrations and ice crystal concentrations. Similarly, cloud top temperatures have been only weakly linked to the number of ice crystals observed. In contrast, ice crystal concentrations have been shown to be strongly correlated to the presence of large droplets (~>20 um) in a variety of mixed-phase cloud types [Hobbs and Rangno, 1985]. We posit that cloud condensation nuclei (CCN) concentrations can be as important as IN concentrations for the rate of ice formation in Arctic mixed-phase clouds, through modification of the droplet size distribution. Aircraft observations from the 'Aerosol, Radiation, and Cloud Processes affecting Arctic Climate' (ARCPAC) study in northern Alaska April 2008 are presented. Measurements on board the NOAA-P3 allow for identification and characterization of both aerosol and trace gas phase pollutants, which are then compared with the cloud microphysical properties. We obtain results consistent with the results from Hobbs and Rangno [1985], by positively correlating ice crystal concentrations with the concentration of large droplets. We are further able to link these microphysical conditions to the haze pollution advected from lower latitudes, which is identified as originating mainly from biomass burning emissions. The case studies presented show that polluted mixed-phase clouds have a much narrower droplet size distribution and contain less ice precipitation than clean clouds, suggesting an Indirect Effect leading to greater cloud lifetime and greater cloud emissivity, both of which are expected to cause warming of the springtime Arctic surface. This is opposite to the “Glaciation Indirect Effect” proposed previously, whereby polluted clouds precipitate more readily due to an increase in black carbon particles acting as IN. The observations are discussed in the context of different droplet freezing modes.