Microphysical and short‐wave radiative structure of stratocumulus clouds over the Southern Ocean: Summer results and seasonal differences

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Authors: Boers, R., Jensen, J. B., Krummel, P. B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1998
• Subjects: Atmospheric Science; Southern Ocean
• Online Access: http://dx.doi.org/10.1002/qj.49712454507; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fqj.49712454507; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.49712454507

Abstract Six case‐studies are presented of stratocumulus clouds observed during the summer phase of the Southern Ocean Cloud EXperiment (SOCEX). The experiment was conceived to investigate the microphysical properties of clouds in the unpolluted background region over the Southern Ocean. The case‐studies show evidence of decoupled stratocumulus layers, i.e. situations where cumulus clouds were covered by a uniform sheet of stratocumulus cloud. Optical depths of the clouds ranged from 2 to 23. Cloud droplet concentrations varied from 50 to 180cm −3 , and mostly decreased with altitude, indicating the importance of droplet coalescence in reducing the number of large cloud droplets. Drizzle was observed on four of the six flights. A comparison of the summer results with those published from the winter phase of SOCEX indicates that the droplet concentration in summer was three times that of the concentration in winter. The differences between summer and winter are so large that errors in sampling and measurement uncertainties hardly influence the results. Because these results were obtained under conditions where the air parcels had traversed long distances over the unpolluted Southern Ocean, it suggests a natural cause, the most likely being oxidation products of oceanic dimethylsulphide acting as cloud condensation nuclei. Model results show that the natural variation in droplet number concentration is responsible for a natural variation in cloud albedo of around 30% at a constant solar zenith angle. This may indicate that there is a natural negative albedo feedback counteracting the enhanced greenhouse effect. The feedback links increased emission of dimethylsulphide at higher oceanic temperatures to an increase in cloud condensation nuclei, and thus to an increase in cloud albedo.