Geoscience Reference
In-Depth Information
analyses, they suggest that the penetration distance for precipitation from cyclones
located off the coasts is about 200 km. The low precipitation over the high central
parts of the ice sheet is consistent with this basic view.
The analysis of Ohmura et al. (
1999
) is in general accord with the model gen-
erated precipitation map given in
Figure 6.6
. Although maps of accumulation and
precipitation are not directly comparable, Ohmura's (
1999
) map is broadly similar
to the accumulation map given in
Figure 6.5
that shows local peaks greater than
2,000 mm. But
Figure 6.5
shows differences with the estimate of accumulation
discussed by R. Bales et al. (
2001
). Details of precipitation and accumulation over
Greenland hence still seem to be somewhat uncertain.
Sublimation was briefly introduced in
Chapter 5
. It refers to the exchange of
water vapor between ice and the overlying atmosphere during sub-freezing condi-
tions (typical of Greenland) in which water molecules are transferred directly from
the solid to gas phase, or vise-versa. This contrasts to evaporation, which deals with
transfers between the liquid and gas phases. The combination of the two is termed
evapo-sublimation.
In the ablation area of the ice sheet, Ohmura et al. (
1999
) estimate annual evapo-
sublimation at between 60-70 mm. Over the higher parts of the ice sheet it is prob-
ably 20-30 mm during the three summer months. Box and Steffen (
2001
) esti-
mated the sublimation term separately over the ice sheet based on application of
two methods to GC-Net data. One is from single-level “bulk” estimates of the type
commonly used in global climate models, whereas the second uses measurements
of wind and humidity at two levels. Details are provided in that paper.
Sublimation over the ice sheet is highly variable in both space and time. Maximum
sublimation rates from the surface to the atmosphere tend to occur when temper-
atures are relatively high and winds are strong. Vertical temperature differences
allow for gradients in specific humidity, which in turn drive sublimation. Large
vertical temperature differences do not occur under strong winds without a heat
source to the surface. Hence, large sublimation rates can occur when melt episodes
reduce the surface albedo, promoting increased absorbtion of solar radiation. Under
clear skies, sublimation rates are largest toward the middle of the day when solar
heating is strongest. Deposition can occur under favorable synoptic conditions with
a reversed humidity gradient. Deposition can also occur at nighttime owing to radi-
ative cooling. Sublimation between even nearby sites may differ greatly.
The annual map from Steffen and Box (
2001
) from the two-level method shown
in
Figure 8.4
represents a trend-surface fit to calculated sublimation in terms of ele-
vation and longitude. Even though results from the single and two-level approaches
differ in the magnitude of sublimation rates, the spatial patterns are very similar.
They both show sublimation as upward (surface to atmosphere, positive values in
the convention of the figure) over most of the ice sheet, and greatest in the warmer
lower elevations during the summer season. The highest elevations show a small
vapor transfer from the atmosphere to the surface (deposition, negative values in
the convention of the figure). Overall, they estimate that mass losses by sublimation
account for at least 12 percent and possibly up to 23 percent of annual precipitation,
