Geoscience Reference
In-Depth Information
for different climatic zones (tundra, taiga, prairie, alpine, and maritime regions),
based on the classification of Sturm et al. (
1995
) (see
Chapter 2
). SSM/I cannot
detect areas of thin snow. Areas of thin snow were identified through comparisons
between the SSM/I fields and NOAA weekly show charts and given an assumed
thickness of 3.0 cm.
Figure 9.3
summarizes results for two grid cells over the period of August 31,
1998 through December 31, 2000, which includes two complete freezing and thaw-
ing seasons. The first grid is at 5 1°N at 1664 m in southern Siberia, in a region of
continuous permafrost with taiga snow. The middle panel depicts the time evolution
of the vertical cross section through the soil. Shaded regions in the profiles show
where the soil is thawed. Spring and summer thawing occurs from the top down.
Autumn/winter freezing is also primarily from the top down. Consequently, from
autumn into early winter, there exists a frozen surface layer with a thawed layer
below. It takes until the middle of winter for the thawed layer at depth to completely
refreeze. This late freeze-up reflects the insulating effects of overlying snow cover.
The bottom panel shows the time evolution of the depth of thawed soil. The active
layer represents the maximum depth that experiences seasonal thaw (i.e., the max-
imum value shown in the bottom panel). For the Siberian grid, a fairly deep active
layer develops in each year, largest in 2,000 with a value of 177 cm. The second
grid is located in Canada on Prince Patrick Island at 76°N. This is in a much colder
climatic zone than for the Siberian example. While the ground is snow covered for a
longer period, the snow cover (tundra snow) is thinner because of the scant precip-
itation. In turn, the active layer depth is shallower.
Some idea of the spatial characteristics of the simulated active layer over the
Arctic terrestrial drainage is provided in
Figure 9.4
. The maximum thaw depth
(active layer thickness) is given for 1999, along with the day of year at which the
maximum thaw depth occurred. There is a general decrease with latitude in thaw
depth but with minimum values around coastal Greenland and the Canadian Arctic
Archipelago. Topographic effects are apparent. The area northwest of Hudson Bay
is characterized by high bulk density bedrock areas with high thermal conductivity
and hence deeper maximum thaw depths. In general, as latitude increases, the date
at which the maximum depth occurs shifts to earlier in the year. This relates to the
shorter period of warmth at higher latitudes.
9.3
Land Surface Models
Numerous LSMs of differing complexity and architecture are in existence. It gen-
eral, they are being continually refined to enable the simulation of increasingly
complex climate interactions and feedbacks. A given LSM may exist in a number
of different versions designed for particular applications. A few examples of LSMs
in wide use include:
VIC (Variable Infiltration Capacity Model):
VIC was developed at the University
of Washington Seattle and with the purpose of producing realistic runoff and
