Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds

摘要

Aircraft borne optical in situ size distribution measurements were performedwithin Arctic boundary layer clouds with a special emphasis on the cloud toplayer during the VERtical Distribution of Ice in Arctic clouds (VERDI)campaign in April and May 2012. An instrumented Basler BT-67 researchaircraft operated out of Inuvik over the Mackenzie River delta and theBeaufort Sea in the Northwest Territories of Canada. Besides the cloudparticle and hydrometeor size spectrometers the aircraft was equipped withinstrumentation for aerosol, radiation and other parameters. Inside thecloud, droplet size distributions with monomodal shapes were observed forpredominantly liquid-phase Arctic stratocumulus. With increasing altitudeinside the cloud the droplet mean diameters grew from 10 to 20 μm.In the upper transition zone (i.e., adjacent to the cloud-free air aloft)changes from monomodal to bimodal droplet size distributions (Mode 1 with20 μm and Mode 2 with 10 μm diameter) were observed. Itis shown that droplets of both modes co-exist in the same (small) air volumeand the bimodal shape of the measured size distributions cannot be explainedas an observational artifact caused by accumulating data point populationsfrom different air volumes. The formation of the second size mode can beexplained by (a) entrainment and activation/condensation of fresh aerosolparticles, or (b) by differential evaporation processes occurring with clouddroplets engulfed in different eddies. Activation of entrained particlesseemed a viable possibility as a layer of dry Arctic enhanced backgroundaerosol (which was detected directly above the stratus cloud) might form asecond mode of small cloud droplets. However, theoretical considerations andmodel calculations (adopting direct numerical simulation, DNS) revealed that,instead, turbulent mixing and evaporation of larger droplets are the mostlikely reasons for the formation of the second droplet size mode in theuppermost region of the clouds.