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

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.

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

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-14-14599-2014 ◽

2014 ◽

Vol 14 (10) ◽

pp. 14599-14635 ◽

Cited By ~ 1

Author(s):

M. Klingebiel ◽

A. de Lozar ◽

S. Molleker ◽

R. Weigel ◽

A. Roth ◽

...

Keyword(s):

Boundary Layer ◽

Droplet Size ◽

Model Simulation ◽

Size Distributions ◽

Arctic Boundary Layer ◽

Cloud Droplets ◽

Boundary Layer Clouds ◽

Droplet Size Distributions ◽

Size Mode

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. The observations were carried out within a joint research activity of seven German institutes to investigate Arctic boundary layer-, mixed-phase clouds 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 μm 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 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 two droplet populations from different air volumes. The formation of a 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 was detected directly above the stratus cloud might form a second mode of small cloud droplets. However, theoretical considerations and a model simulation revealed that, instead, turbulent mixing and evaporation of larger droplets most likely are the main reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

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Investigation of Droplet Size Distributions and Drizzle Formation Using A New Trajectory Ensemble Model. Part II: Lucky Parcels

Journal of the Atmospheric Sciences ◽

10.1175/2008jas2789.1 ◽

2009 ◽

Vol 66 (4) ◽

pp. 781-805 ◽

Cited By ~ 31

Author(s):

L. Magaritz ◽

M. Pinsky ◽

O. Krasnov ◽

A. Khain

Keyword(s):

Boundary Layer ◽

Droplet Size ◽

Threshold Value ◽

Ensemble Model ◽

Size Distributions ◽

Cloud Droplets ◽

Stratocumulus Clouds ◽

Water Vapor Mixing Ratio ◽

Large Eddies ◽

Droplet Size Distributions

Abstract A novel trajectory ensemble model of the cloud-topped boundary layer containing 1340 Lagrangian parcels moving with a turbulent-like flow with the observed statistical properties was applied to investigate the formation of the microphysical structure of stratocumulus clouds (Sc) in a nonmixing limit (when turbulent mixing between the parcels is not taken into account). The Sc observed in two research flights during the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II)—RF01 (no drizzle) and RF07 (weak drizzle)—are simulated. The mechanisms leading to a high variability of droplet size distributions (DSDs) with different spectrum width and dispersion are discussed. Drizzle formation was investigated using the radar reflectivity–LWC and LWC–effective drop radius diagrams simulated by the model in the nondrizzle and drizzle cases. It is shown that in the RF07 case large cloud droplets that trigger drop collisions and drizzle formation form only in a small fraction (about 1%) of the parcels (which will be referred to as lucky parcels) in which LWC exceeds ∼1.5 g m−3. This value exceeds the horizontally averaged LWC maximum value of 0.9 g m−3 by two to three standard deviations, indicating a small amount of lucky parcels. In a nondrizzling cloud simulation this threshold is exceeded extremely rarely. The dependence of the threshold value of LWC on aerosol concentration is discussed. The lucky parcels (at least in the nonmixing limit) start their updraft in the vicinity of the surface, where the water vapor mixing ratio is maximum, and ascend to the highest levels close to the cloud top. It is shown that the lucky parcel tracks are related to the large eddies in the boundary layer, which indicates the substantial role of large eddies in drizzle formation.

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