Geoscience Reference
In-Depth Information
space. Once the cloud becomes optically thick enough so that cloud top radiative
cooling becomes large, turbulent kinetic energy is produced and vertical mixing
occurs, producing more condensation because of adiabatic cooling. The subsequent
study of J. Curry, E. Ebert, and G. Herman (
1988
) supports this basic view. The
springtime increase in cloud cover takes place prior to the onset of widespread sur-
face melting (Barry et al.,
1987
) implying that evaporation from the pack ice surface
is not important. The observed persistence of the stratus is viewed in terms of the
sluggishness of effective dissipating processes (e.g., precipitation, radiative heat-
ing, convective heating, synoptic activity). Several mechanisms have been proposed
to account for the observed multiple layering, which are reviewed by Curry et al.
(
1996
).
As pointed out by J. Beesley and R. Moritz (
1999
), for the central Arctic Ocean,
there is a fairly sharp rise in the vapor flux convergence between May and June
(Walsh et al.,
1994
; Serreze et al.,
1995a
; Cullather, Bromwich, and Serreze,
2000
).
This increase occurs about a month earlier than the sharp spring rise in the amount
of Arctic stratus, hence casting Herman and Goody's (
1976
) interpretation into ques-
tion. Based on results from a radiative-turbulent column model, Beesley and Moritz
(
1999
) suggest that a more dominant role on the seasonality of low-level stratus is
played by the temperature-dependent formation of atmospheric ice. Briefly, at tem-
peratures below freezing, the saturation vapor pressure over ice is lower than over
liquid water, such that ice particles grow at the expense of the liquid condensate.
The concentration of ice crystals is smaller than that of CCN. Hence a given mass of
frozen condensate is distributed among smaller numbers of larger nuclei that grow
rapidly to precipitable sizes when the environment is supersaturated with respect
to ice, disfavoring the development of stratus. As discussed by Beesley and Moritz
(
1999
), this can be viewed as a “preemptive dissipation” of stratus during the cold
season, as the process can prevent the humidity of clear air from reaching saturation
with respect to liquid water. This idea is supported by numerous observations of ice
crystal precipitation during the winter months. During summer, when temperatures
are higher, the ice-crystal scavenging processes are less effective and stratus is more
likely to form and persist.
Figure 2.26
provides the corresponding ICOADS-derived mean annual cycle in
cloud cover for the Atlantic sector of the Arctic Ocean, taken as the region between
60°N to 65°N latitude and 40°W to 40°E longitude. Note in comparison to the cen-
tral Arctic Ocean, the lack of seasonality in the means of both total and low cloud
cover hover at about 85 percent and 75 percent, respectively. This manifests the per-
sistence for all months in this region of frequent synoptic activity (see
Chapter 4
)
and fairly high atmospheric humidity and temperatures.
The advantage of satellite data, of course, is the regular Arctic-wide coverage.
Figure 2.27
provides mean fields of cloud cover (percent) for the four mid-sea-
son months based on the APP-x data set, which is considered to be improved over
ISCCP-D. The fields are averaged for the years 1982 through 1999. To summarize:
(1) cloud cover is rather extensive over most of the Arctic during all seasons; (2)
lesser amounts are found over central Greenland, as the ice sheet extends above
