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the northern Canadian treeline (Lafleur and Rouse, 1995 ). Gradual forest-tundra
transitions, which are common, are likely to show weaker heating gradients than
between the distinct stands of tundra and boreal forest examined in the Beringer
et al. ( 2001 ) study.
The heating contrast is much sharper in spring because of masking of snow by
the forest canopy. This resulted in a much higher albedo over the tundra (0.68) com-
pared to the forest (0.16) and 142 W m −2 more sensible heating on a daily basis.
However, this transition period lasts only two to three weeks when solar radiation
is strong and the surface is still snow covered. But it is only in summer that the
Arctic frontal zone can be clearly distinguished from mid-latitude frontal activity.
Atmospheric soundings made over the two sites show that in both spring and sum-
mer, heating differences are also confined to a shallow layer. These findings lend
further credence to the idea that the summer Arctic frontal zone is primarily driven
by large-scale land-ocean heating contrasts and topography (see Chapter 4 ).
5.10
Skin Temperature, Surface Air Temperature,
and Vertical Structure
5.10.1 The Radiative Boundary Layer
A number of winter studies (e.g., Overland and Guest, 1991 ; Overland et al., 1997 ;
Overland et al., 2000 ) have examined linkages between the skin temperature (the
temperature at the very surface), the overlying surface air temperature (SAT, the
temperature at about 2 m height), the vertical temperature structure of the lower
troposphere and the surface energy budget over the central Arctic sea ice cover. In
summary, the skin and air temperature, along with the vertical temperature struc-
ture, can be considered as primarily driven by the longwave radiation exchanges. As
we saw earlier, the mean surface net radiation deficit of typically 20-40 W m −2 in
winter is maintained by small energy transfers to the surface of sensible heat and a
small upward conduction of heat through the sea ice and snow cover. The skin tem-
perature, and thus the upward longwave radiation flux, change primarily in response
to changes in downward longwave radiation and attendant adjustments in the small
non-radiative terms. As the skin temperature adjusts to the downward longwave
flux, the overlying SAT comes toward equilibrium with the surface through adjust-
ments in the sensible heat flux. In general, the snow surface temperatures do not
depart from the surface air temperature by more than a few degrees, except in very
calm conditions. The same general statements can be made with respect to much of
the Arctic land surface in winter.
The Arctic in winter is hence considered to have a radiative boundary layer
(RBL) - that is, the surface energy balance changes when the downward longwave
radiation flux changes, such as would attend alterations in cloud cover. This is
not always the case as during storms with strong temperature advection. Clearly,
toward the Atlantic side of the Arctic, the effects of temperature advection become
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