Development of bimodal droplet size distributions at the top layer of Arctic super-cooled liquid water boundary layer clouds

Boundary layer clouds act to cool the atmosphere if their radiative effects are annually and globally averaged. This cooling is due to the dominance of the solar albedo effect as compared with the less important emission of thermal-infrared radiation. In the Arctic, boundary layer clouds mostly warm the below-cloud atmosphere. This is due to the generally low sun elevation, the long-lasting polar night and the high solar surface albedo of land/sea ice and snow.

In a joint research activity of seven German institutes, microphysical measurements within Arctic boundary layer-, mixed-phase clouds were conducted during the VERDI (VERtical Distribution of Ice in Arctic Clouds) campaign. VERDI was performed in April and May 2012 out of the base airport located at Inuvik with the main operation area over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. During VERDI the POLAR 5 research aircraft, operated by the Alfred Wegener Institute (AWI), was equipped with instrumentation for aerosol, radiation and cloud particle measurements including four cloud particle spectrometers.

We focus on measurements of the Cloud Droplet Probe (CDP) inside and at the cloud top level of an Arctic boundary layer cloud. Well inside the cloud droplet size distributions with a monomodal shape were measured. With increasing altitude the cloud droplet mean diameter grows continuously. In the upper transition zone of the penetrated boundary layer cloud, changes from monomodal to bimodal droplet size distributions were observed. The development of the second size mode could be explained by condensation and/or evaporation processes. On the one hand, possible activation of particles from a dry Arctic enhanced background aerosol, which was detected directly above the stratus cloud, might form a second mode of small cloud droplets by condensation. On the other hand, theoretical considerations and a model simulation revealed that, instead, evaporation of larger droplets most likely is the main reason behind the formation of the second droplet size mode. In order to characterize the spatial distribution of the droplets from the bimodal size distributions a new method is presented which makes use of the particle-by-particle data of the CDP.