Geoscience Reference
In-Depth Information
12.3 × 10
6
km
2
with a volume of 26.5 × 10
6
km
3
, contains an adjusted 58 meters
of sea level equivalent (Huybrechts et al.,
2001
; BEDMAP 2). Nevertheless, we
are obviously dealing with a large amount of ice. J. Zwally and M. Giovinetto
(
2001
) estimated that 88 percent of the coterminous Greenland ice sheet lies in
the accumulation zone (where annual mass gains exceed mass losses), with the
other 12 percent lying in the ablation zone (where annual mass losses exceed
mass gains). The changing mass balance of the ice sheet will be addressed
shortly.
Climate data for Greenland are available from several field programs. Beginning
in 1987, an automatic weather station (AWS) network was established in Greenland
by C.R. Stearns. Data from these stations provide a valuable addition to the few
previous expedition measurements discussed by P. Putnins (
1969
) and Barry and G.
Kiladis (
1982
). Since 1995, extensive climatic data have been collected by AWSs
through NASA's Program for Arctic Regional Climate Assessment (PARCA)
(Steffen et al.,
1996
; Thomas,
2001
). The Greenland Climate Network (GC-Net)
established under PARCA provides climate data from eighteen AWSs. The sites are
distributed to sample from different climatic zones (
Figure 8.1
) and range in altitude
from 568 m (JAR-2) to 3,208 m (Summit).
8.1.2
Surface Air Temperature
The high elevation, large extent, and high albedo of the ice sheet are significant fac-
tors for local and regional surface air temperatures although latitude and distance
inland are also involved. K. Steffen and J. Box (
2001
) provide a useful summary.
For both the eastern and western slopes of the ice sheet, surface air temperatures
decrease by about 0.8°C per degree of latitude. As for general elevation effects, from
normalizing all AWS station data to 70°N, annual mean surface air temperatures
decrease at a rate 0.71°C per 100 m. The slope gradient in temperature shows large
seasonality, however. Based on differences between selected stations, the decline
ranges from 0.9-1.0°C per 100 m in November to as low as 0.4°C per 100 m in
June. Regarding free-air lapse rates (as measured by radiosondes), the ice sheet is
characterized by pronounced low-level inversions (see
Chapter 5
), which are most
strongly expressed during the winter season.
Figure 8.2
gives the mean annual cycles of temperature for sixteen of the PARCA
sites. February is the coldest month at every site, while July is the warmest. The
annual mean temperature is only −30°C at NGRIP (75.1°N, 42.3°W, 2950 m), rang-
ing from −14°C in July to −46°C in February. The only station with a mean July
temperature exceeding the freezing point is JAR-1 (+0.2°C), located on the west
slope (69.5°N, 49.7°W, 962 m). At Summit, summer daily high temperatures are
near −8°C and winter minima are around −53°C. There is strong daily variability
in winter, which is associated with synoptic activity and katabatic winds. Steffen
and Box (
2001
) find that the annual mean temperature range is between 23.5°C to
30.3°C for the western slope.
