Geoscience Reference
In-Depth Information
Although the turbulent fluxes are quite small for the SHEBA region, over areas
of thin ice and open water they can be large. In such areas, strong temperature gradi-
ents are formed in the boundary layer, and winter sensible heat fluxes may reach 600
W m
−2
. Measurements over and near freezing leads and polynyas (Badgley,
1966
;
Smith et al.,
1983
; Makshtas,
1984
;) suggest that heat is transferred only 10-15 m
above the surface (Andreas et al.,
1979
) and is relayed back to the ice downwind
by condensation, ice crystal precipitation, the sensible heat flux and longwave radi-
ation from cloud formation downwind of the lead. However, condensate plumes
emanating from wide (greater than 10 km) open water areas (leads) that extend to 4
km in the atmosphere and persist for up to 200 km downwind have been identified
using backscatter measurements from an airborne downward pointing lidar (Schnell
et al.,
1989
; Andreas et al.,
1990
). Such deep convection events appear to require a
combination of a large air-sea temperature contrast (providing a large sensible heat
flux), low wind speeds, and a weak Arctic temperature inversion. This combination,
however, is rare in Arctic conditions (Serreze et al.,
1992a
). Shallow convection
(less than 1 km) above narrow leads is believed to result in locally high concentra-
tions of ice crystals near the surface, substantially augmenting the downwelling
radiation flux (Curry et al.,
1990
).
5.9.2
Characteristics over Tundra
The fundamental difference between the Arctic Ocean and tundra is in how net radia-
tion is apportioned is in the energy used to melt snow and ice (Ohmura,
1984
). Once
the snow is melted from the tundra, energy can be apportioned in sensible heating
and to evaporate water. The snowmelt process over tundra tends to be completed
over a rather short time interval, typically a few weeks (Weller and Holmgren,
1974
).
The consumption of heat through melt on the ice-covered ocean is much larger than
on the tundra. Consequently, and for summer conditions, sensible heat is typically
(but not always as seen in the SHEBA data) transferred from the atmosphere to the
surface of the ocean, while it is carried from the surface to the atmosphere in the
tundra. Evaporation is, on average, the most significant summer heat sink on tundra
and is considerably larger than for the ice-covered ocean.
As shown by Ohmura (
1984
), a similar contrast exists between tundra and gla-
ciers. It is often noted that during summer melt, glaciers exert a cooling effect on the
surface air temperature in comparison with ice-free tundra surfaces at similar eleva-
tions (e.g., Müller and Roskin-Sharlin,
1967
; Ohmura,
1984
; Bradley and Serreze,
1987
), primarily associated with the consumption of energy through ice melt.
However, the higher parts of the Greenland ice sheet do not usually experience melt
under present climate conditions. Here, sublimation can be an important process
(Box and Steffen,
2001
). These sublimation processes are examined in
Chapter 8
.
We can make some comparisons between the Arctic Ocean and the land surface
by comparing the SHEBA results with Alaskan data collected through the NSF LAII
program. A. Lynch et al., (
1999b
) synthesized results from two sets of measurements
for the snow-free period (June, July, and August) of 1995. The first set represents
