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as instrument problems, it primarily reflects local variability in site properties and
cloud conditions. Nevertheless, at both sites there is a general decline in net radia-
tion from June through August. Net radiation in June at the coastal and upland sites
is fairly similar. Like net radiation, the sensible heat flux generally declines through
the growing season, with no consistent difference between the upland and lowland
sites in any month. By contrast, the latent heat flux increases from June to July, then
decreases in August. This is associated with the seasonal pattern of leaf area devel-
opment. In turn, the latent heat flux is higher in the upland than coastal sites during
July and August, manifesting site differences in leaf area and air temperature.
Building on Ohmura's (
1984
) observations, a conclusion that can be drawn from
a comparison between
Table 5.4
and the SHEBA results is the much larger frac-
tion of the net radiation over tundra represented by the sensible and latent heat
fluxes. Furthermore, over the tundra, there is a summer conductive heat flux into the
ground, warming the soil; whereas over the sea ice, nearly all of the net radiation
goes into melt. The higher and upward-directed sensible heat fluxes over the tundra
are in turn consistent with higher surface air temperatures.
5.9.3
Contrasts across the Arctic Treeline
As another component of the LAII program, surface energy fluxes for spring
and summer were collected at two sites near Council on the Seward Peninsula of
Alaska. The sites represent tundra and adjacent white spruce forest. Measurements
were made from tower-mounted instruments. At the boreal forest site, they were
measured at the top of the canopy. The contrasts in surface energy budgets are con-
sidered to be representative of those expected along the boreal forest - tundra tran-
sition. Part of the interest in collecting these data was to reexamine the idea that
heating contrasts along the transition zone may help to “anchor” the location of the
summer Arctic frontal zone (see
Chapter 4
).
J. Beringer et al. (
2001
) examined daily averages and the mean diurnal cycle of
energy balance components during the spring transition (May) and summer (June-
August) based on data for the years 1999 and 2000. During summer, the mean
albedo of the forest canopy of 0.095 was lower than that of the tundra because of its
darker color and more complex canopy structure. The forest site absorbed 11 per-
cent more net radiation on a daily basis (11 Wm
−2
). In turn, the greater leaf area in
the forest yielded less radiation at the ground surface and hence a smaller ground
heat flux than at the tundra. Bowen ratios (Q
H
/Q
E
) between the two sites showed no
consistent differences.
As a result, on average, the summer sensible flux over the forest averaged
60 Wm
−2
higher than over the tundra two hours either side of solar noon (second
panel). This is a large difference. Pielke and Vidale (
1996
) argue that a maintained
heating contrast of 50 W m
−2
would be sufficient to manifest itself as a synoptic
front. However, the observed contrast only occurs for a few hours per day. When
average daily results are considered, the contrast in the sensible heat flux is only
about 5 W m
−2
, similar to daily mean contrasts of about 12 W m
−2
observed over
