Geoscience Reference
In-Depth Information
Figure 2.25.
Mean annual cycle of cloud cover (total cloud and low cloud, in %) for the
central Arctic Ocean based on ICOADS data through 1995 (by the authors).
based on surface observations and APP-x in generally show poor agreement. A large
negative trend in cloud cover in winter found in the APP-x data is not seen in the
surface observations.
Figure 2.25
shows the mean annual cycle of cloudiness (percent cover) over
the central Arctic Ocean based on surface observations from the International
Comprehensive Ocean Atmosphere Data Set (ICOADS) through 1995. The ICOADS
records for this region are from a combination of observations from the NP program
and ship reports. No adjustments for moonlit versus moonless conditions or for ice
crystal clouds are incorporated. Results are given for the percent of the celestial
dome covered by clouds of all types (total cloud cover) and by only low cloud cover.
During the winter months, there is typically about 60 percent total cloud cover, and
about 45-50 percent low cloud cover. During summer, corresponding values are
about 80 percent and 70 percent. Hence the annual cycle is largely driven by low-
level (stratus) clouds. Note the abrupt increase in central Arctic Ocean cloud cover
between May and June and the abrupt decline between October and November.
These results are broadly consistent with those from other studies.
There has been considerable work addressing summer Arctic stratus. G. Herman
and R. Goody (
1976
) envisioned the formation of summer stratus as an airmass
modification process, whereby relatively warm and moist air of continental origin
in an initially unsaturated airmass moves over the ice cover and condenses because
of radiative and diffusive cooling to the colder surface and longwave emission to
