Microstructures of low and middle‐level clouds over the Beaufort Sea
Authors: Hobbs Peter V. , Rangno Arthur L. 
Published: February 2007

Airborne measurements in low and middle‐level clouds over the Beaufort Sea in April 1992 and June 1995 show that these clouds often have low droplet concentrations (<100 cm3) and relatively large effective droplet radii. The highest average droplet concentrations overall were measured in altocumulus clouds that formed in airflows from the south that passed either over the North American continent or were from Asia. Droplet concentrations in low clouds tended to be higher in April than in June. The low clouds in June occasionally contained drops as large as 35 μm diameter; in these clouds the collision‐coalescence process was active and produced regions of extensive drizzle. Cloud‐top droplet concentrations were significantly correlated with aerosols beneath their bases, but appeared to be relatively unaffected by aerosols above their tops. Anthropogenic sources around Deadhorse, Alaska, increased local cloud droplet concentrations.

Ice particle concentrations were generally low in April, but high ice particle concentrations were encountered in June when cloud‐top temperatures were considerably higher. On two days in June, tens per litre of columnar and needle ice crystals were measured in stratocumulus with top temperatures between −4 and −9 °C. Ice particle concentrations were poorly correlated with temperature (r = 0.39) but, for the two data sets as a whole, the concentrations of ice particles tended to increase with increasing temperature from −30 to −4.5 °C. Ice particle concentrations correlated better with the size of the largest droplets (r = 0.61).

The most common mixed‐phased cloud structure encountered was a cloud topped by liquid water that precipitated ice. Liquid‐water topped clouds were observed down to temperatures of −31 °C. They are likely common in the Arctic, and may play an important role in the radiation balance of the region.

Temperature lapse rates in the clouds were generally complex, reflecting either layering, due to differential advection, or radiational effects. In these cases, vertical profiles of liquid‐water content and effective cloud droplet radius did not vary systematically with height above cloud base, as they do in well‐mixed clouds. However, when the temperature in a cloud decreased with height at or very near the pseudo‐adiabatic lapse rate, the cloud liquid‐water content (and various measures of the broadness of the cloud droplet size distribution) generally increased monotonically with height until very close to cloud top.

For clouds consisting entirely of droplets, or droplets and ice crystals, cloud coverage increased by about 10% when the definition of a cloud was changed from 10 to 5 droplets per cubic centimetre. For clouds containing ice particles, cloud coverage and/or cloud depth increased by about 40% when the definition of a cloud was changed from 1 to 0.1 ice particles per litre.

The significance of these observations with respect to the effects of clouds on the radiation budget of the Arctic, and the potential for modification of arctic clouds by pollution, are discussed.