Skin temperature SAT and vertical structure 591 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 SAT, the vertical temperature structure

Tundra -o— Forest

A

H

h\

f— ^

Figure 5.11 Daily average surface energy balance components for forest and tundra during (a) summer and

(c) spring and the average hourly sensible heat flux for forest and tundra during (b) summer and

(d) spring. Standard errors are indicated by the bars. Time is Alaska Daylight Time (ADT) (adapted from Beringer etal, 2001,byper-mission of AGU).

Time (ADT)

^NET

S

(c)

■ÉB

— r

Tundra

1_1 Forest

Surface Sensible Heat Flux Arctic

Time (ADT)

Time (ADT)

of the lower troposphere, and the surface energy budget over the central Arctic sea ice cover. In summary, the winter skin and air temperatures, along with the vertical temperature structure, 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 temperature, thus the upward longwave radiation, changes 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 adjustments 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). This means that 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. Clearly, toward the Atlantic side of the Arctic, the effects of temperature advection become pronounced. As we have also seen, very large turbulent fluxes can be found over leads. However, as a general statement, the winter SAT regime is strongly coupled to the skin temperature and the longwave fluxes.

The situation is quite different in summer and the concept of the RBL falls apart. The surface energy budget is fundamentally driven by the shortwave, rather than the longwave fluxes (although the latter is certainly important, as with regard to the net cloud radiative forcing). As we have seen, over a snow-free tundra surface in summer, vertical sensible and latent heat fluxes can be large, strongly driving the surface and air temperature. Over the sea ice, the surface energy budget is strongly impacted by the melting surface, fixing surface temperature to the freezing point (hence fixing the upwelling longwave flux) and greatly limiting variations in near-surface temperature.

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