Geoscience Reference
In-Depth Information
8.1.3
Radiation Regime
Some aspects of the Greenland's radiation were already examined in
Chapter 5
. To
summarize and expand, during winter, net allwave radiation is strongly negative.
This is a function of the high elevation of the ice sheet and the relative lack of cloud
cover compared to other areas of the Arctic, which helps to keep the downward
longwave radiation flux low. Solar radiation is of course essentially absent in win-
ter. It follows that compared to other Arctic regions, the summer downward solar
radiation flux over the ice sheet is quite high (
Figure 5.1
), again owing to more lim-
ited cloud cover and (from the high elevation) less attenuation of the incident flux
by aerosols and water vapor. But even in July, net allwave radiation over the higher
elevation parts of the ice sheet is less than about 40 W m
−2
(
Figure 5.6
). In part, this
manifests to the fairly small flux of downward longwave radiation. There is hence
strong compensation between the two incoming radiation terms. However, the much
larger contributor to the low net allwave radiation in summer is the high albedo -
most of the incident solar flux is lost to space. From
Figure 5.4
it is seen that even
in July, the mean surface albedo over most of the ice sheet is around 0.80 although
much lower values can be found in the warmer parts subject to summer melt.
The role of albedo is highlighted in radiation measurements collected by
Konzelmann and Ohmura (
1995
) in 1990 and 1991 at the Swiss Camp (69.6°N,
49.3°W, 1149 m). Swiss Camp is located in West Greenland, near the mean equilib-
rium line. These data show that the decrease in incoming solar radiation through the
summer melt season associated with increased cloud cover (principally observed
in August as a result of stratus) is essentially offset by the tendency for clouds to
increase the downwelling longwave radiation. Because of the melting surface, the
outgoing longwave radiation flux is steady. Hence, variations in the net surface (all-
wave) radiation are determined largely by variations in albedo. Albedo decreased
from 0.86 in May to 0.72 in July. Overall, a positive net radiation balance can be
expected from late May to mid-August. But even in July, net radiation averaged
for the two years was only 37 W m
−2
. This is consistent with the satellite-derived
estimates given in
Figure 5.8
.
8.1.4
Wind Regime and Tip Jets
A prominent feature of the Greenland climate is its katabatic wind regime. In the
right conditions, such drainage winds can reach gale force. In its most generic
sense, a katabatic wind (from the Greek word
katabikos
- to go down) refers to any
downslope wind flowing from high elevation mountains, plateaus, or hills down to
valleys or plains. But more commonly, the term is reserved for winds that, despite
the effects of adiabatic compression during descent, are colder than the air displaced
at the bottom of the incline. A wind that is warmer than the air being displaced at the
bottom of an incline is commonly referred to as a foehn, although there are many
local names (e.g., Chinooks in the lee of the Rocky Mountains and the Santa Ana in
the lee western side of the San Bernadino Mountains, California).
