Microstructures of low and middle‐level clouds over the Beaufort Sea

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Authors: Hobbs, Peter V., Rangno, Arthur L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1998
• Subjects: Arctic; Beaufort Sea; Alaska
• Online Access: http://dx.doi.org/10.1002/qj.49712455012; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fqj.49712455012; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.49712455012

Abstract 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 cm 3 ) 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 ...