Geoscience Reference
In-Depth Information
160°W
120°W
80°W
40°W
0°
40°E
80°E
120°E
160°E
80°N
(a)
(b)
Lack of
rainfall
60°N
40°N
Less
ascent
Lack of
vegetation
20°N
0°
(c)
Higher
albedo
20°S
Reduced
radiation
6
40°S
4
ALBEDOES PRESCRIBED
Idealised margional
area (0.14 or 0.35)
Idealised permanent
desert (0.35)
2
SAHEL
60°S
Humid area (0.14)
Ice cover (0.7)
0
1
2
3
4
80°S
Time (weeks)
Figure 24.2
The first GCM modeling study demonstrating the effect of land surfaces on climate: (a) the
'desertification' hypothesis that motivated the study; (b) areas of the globe where different albedo values were used in
the model, including the Sahelian region; and (c) the modeled difference in precipitation in the Sahel. (Data from
Charney
et al.
, 1975.)
driven positive feedback process. The hypothesis was that overgrazing reduced
vegetation cover and increased the surface albedo, and this in turn reduced the
energy entering the atmosphere and consequently atmospheric ascent, causing
precipitation and vegetation cover to be further reduced. In this model
experiment, Charney increased the albedo in selected regions of the globe
including the Sahel, (Fig. 24.2b) and modeled a reduction in precipitation of
about a factor two (Fig. 24.2c).
A second important early modeling study also used such a simple land surface sub-
model to represent the land surface. By making GCM runs with land surface evapo-
ration across the globe fixed first to zero and then to the potential evaporation rate,
Shukla and Minz (1982) demonstrated that water evaporated from continental
surfaces recycles in the atmosphere and can contribute significantly to modeled
precipitation (Fig. 24.3).
Developing realism in SVATS
There have been rapid developments in the realism of land-surface sub-models
in meteorological and hydrological models over the past decades, motivated
partly by the sensitivity of climate to the land surface as demonstrated by early
experiments such as those just described, and stimulated by the need for better
predictions of human influence on the atmosphere resulting from land use
