Cloud occurrence and phase at Arctic atmospheric observatories

Clouds occur frequently throughout the Arctic and play a unique role in Arctic climate that is distinct from the role of clouds in lower latitudes. Arctic clouds warm the surface through most of the year apart from a period of cooling in the mid-summer. Thus, Arctic clouds tend to modulate and counteract the background annual cycle of surface cooling and warming. With recent declines in Arctic sea-ice concentration there is a heightened focus on Arctic climate and its variability. The role of clouds in that variability is sensitive to a number of cloud properties including presence, vertical distribution, phase partitioning, and microphysical composition. Detailed information on all of these key cloud properties is being produced from measurements at a number of Arctic atmospheric observatories as part of the Arctic Observing Network. This observing network includes surface-based stations at Barrow and Atqasuk (Alaska), Eureka (Canada), Ny'Alesund (Svalbard, Norway), Summit (Greenland), and over the Arctic Ocean that have each been in operation for some portion of the last ten years. Suites of complementary instruments at each site produce synergistic measurements that are optimally combined to provide a comprehensive characterization of cloud macrophysical properties. Specifically, cloud occurrence and phase results are presented here.

Among the observing stations, clouds occur most frequently at Barrow and over the Arctic Ocean and least frequently, although still nearly 60% of the time, at Summit Greenland. Many stations show an annual cycle in cloudiness with maximum occurrence fraction in the late summer and fall. Ny'Alesund differs markedly from this trend, having a near-steady cloud fraction of approximately 60-65% for all months of the year. Distinguished by cloud phase, ice phase clouds tend to occur 40-50% of the time at most stations, with a maximum occurrence fraction at about 6 km altitude. Liquid-only clouds are at a maximum of 40% occurrence fraction in the middle of the summer but occur infrequently in the winter. At some observatories, mixed-phase clouds are the most abundant cloud type, although apart from a general trend towards larger mixed-phase occurrence fractions in the fall there are few similarities among sites in terms of the annual cycle of mixed-phase cloud occurrence. At both Barrow and over the western Arctic Ocean, liquid-water in clouds (often in mixed-phase clouds) occurs about 80% of the time in summer, decreasing to 15-40% in the winter. In general, there are fewer liquid water-containing clouds at Eureka, particularly in the summer. Arctic clouds are persistent at most locations, with the average persistence increasing as the clouds are lower in the atmosphere. Low-level clouds over Barrow last for nearly 7 hours, on average, while over the western Arctic Ocean these low clouds persist twice as long on average.

These results on cloud occurrence and phase are derived from the first data set of its kind at this scale, and represent a baseline to which future observations of Arctic cloudiness can be compared. Moreover, the results can be used to evaluate satellite-based Arctic cloud observations and to validate Arctic model simulations.