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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 are 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.
5.10.2 Low Level Temperature Inversions
A prominent feature of the Arctic environment is the frequent occurrence of low-
level temperature inversions (i.e., temperature increases with height). This was first
demonstrated by C. Brooks ( 1931 ) from kite ascents over Siberia. More detailed
studies from kite and captive balloon ascents made by Sverdrup ( 1933 ) during the
Maud expedition provided some of the first detailed information on inversion struc-
ture. H. Wexler ( 1936 ) was the first to address physical controls behind the forma-
tion of Arctic inversions. Other early studies of inversion characteristics include E.
Vowinkel and S. Orvig ( 1967 , 1970 ), based on soundings from the Russian NP data
stations over the central Arctic Ocean (NP-4, NP-6, and NP-7) and Arctic coastal
sites. More recent efforts include J, Kahl ( 1990 ), Overland and Guest ( 1991 ), and
Serreze et al. ( 1992b ).
The usual situation in the Arctic during winter is a surface-based inversion (typi-
cal for land) or an inversion above a shallow (30-80 m) mixed layer (typical for
the ice covered ocean), a broad region of warm air with a temperature maximum
of around 1,000-1,200 m, and a negative lapse rate aloft. During winter, inversions
are evident in nearly all sounding profiles. The temperature difference between the
inversion base and top averages 10-12 K. The frequency of inversions over land
areas decreases through spring and summer, but they are still found in about 50 per-
cent of soundings for June and July. Over the central Arctic Ocean, inversions are
still present in the great majority of summer soundings. However, summer inver-
sions are weaker than their winter counterparts, are thinner and tend to be elevated
above the surface. Over the ocean, the inversion base is about 200-400 m above
the surface during May-August, with the top located between 750-1,000 m. The
SHEBA observations showed an inversion persisting in summer at about 400 m
altitude with an intensity of about 5 K (Uttal et al., 2002 ). Over land, the depth of
the summer mixed layer is greater.
Typical vertical temperature structures for winter are provided in Figure 5.13
based on averaged rawinsonde data from six island stations around the perimeter
of the Arctic Ocean (Overland et al, 1997 ). Also illustrated ( Figure 5.14 ) is the
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