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Abstract
Abstract. Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside 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 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.
Highlights
The Arctic sea ice plays an important role in the global climate system as it partly insulates the Arctic Ocean especially in the winter season and diminishes the exchange of heat, momentum and gases between ocean and atmosphere (Lemke et al, 2007)
It consists of a Cloud Droplet Probe (CDP) and a Cloud Imaging Probe (CIP grey scale – denoted as CIPgs in the following) which in combination cover a size diameter range from 2 to 960 μm, including large aerosol particles, liquid cloud droplets and small frozen hydrometeors
If the aircraft had moved through one patch of air containing only Mode 1 particles and afterwards through a different air parcel with mostly droplets from Mode 2 – for example because the two parcels were from different entrainment filaments – a bimodal size distribution (SD) might result from data averaging over a sufficiently long flight path stretch
Summary
The Arctic sea ice plays an important role in the global climate system as it partly insulates the Arctic Ocean especially in the winter season and diminishes the exchange of heat, momentum and gases between ocean and atmosphere (Lemke et al, 2007). Arctic boundary layer clouds lead to a net warming effect of the below-cloud atmosphere depending on the season 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 (Wendisch et al, 2013; Shupe and Intieri, 2004). In April–July 1998, the FIRE ACE campaign was performed to improve (a) the satellite retrieval of cloud and surface characteristics in the Arctic, and (b) the representation of Arctic clouds and radiation in general circulation models For this purpose, four aircraft were equipped with instrumentation as described in Curry et al (1999). The purpose of this study is to analyze in further detail the microphysical properties inside and at the top of the boundary layer stratus clouds in the Beaufort Sea area with emphasis on the occurrence of monomodal and bimodal distributions and the connection to turbulent mixing processes at the cloud top levels
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