Geoscience Reference
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the sea ice and overlying snow cover. No lateral heat advection by the atmospheric
circulation was permitted. The model was calibrated against observed clear-sky
temperature profiles. The modeled downward longwave radiation to the surface
was less than observed. It was concluded that the “missing” downward radiation
could be accounted for by including the emissivity effects of “diamond dust” in the
lower troposphere (Curry,
1983
; Curry et al.,
1990
). A correction for an ice crystal
layer with an emissivity of 0.21 provided the necessary downward longwave flux to
match observations.
After calibration, the model was initialized with a linear temperature profile and
allowed to evolve over ninety days. Initially, a realistic temperature inversion was
observed to develop. However, the temperature maximum layer radiated strongly
into space, and cooled rapidly. With less downwelling longwave radiation directed
downward to the surface, the surface temperature declined, less slowly than the tem-
perature maximum layer because of compensation by the conductive flux through
the sea ice and snow cover. In accord with the dependency of longwave emission
on the fourth power of temperature, as the temperature of the profile declined over
time, the cooling rate declined. The end result was an essentially isothermal profile.
Adding an extra sensible heat flux from leads could not make up for the radiative
loss to space.
Overland and Guest (
1991
) then repeated the experiment by including a steady
lateral heat advection from lower latitudes using values estimated by Nakamura
and Oort (
1988
). Eureka! With the inclusion of advection to offset radiative loss
to space, the model temperature profile evolved to develop a steady inversion. In
summary, these results point out that while the winter inversion structure can be
considered to a crude first order in the context of a radiative equilibrium (coupling
the skin temperature to the atmospheric temperature maximum) an inversion cannot
be maintained in the absence of lateral advection. These results also point to the
significance of “diamond dust” in maintaining the downward longwave flux. Note
that other factors, such as subsidence under anticyclonic condition, could affect the
inversion structure.
Cloud cover has strong influences on inversion structure. As discussed, clouds
increase the downward flux. Serreze et al. (
1992b
) compared winter mean tem-
perature profiles over the Arctic Ocean (using rawinsonde data from the NP pro-
gram) for relatively clear (less than 50 percent cloud cover) and relatively cloudy
conditions (greater than 50 percent cloud cover). For relatively cloudy conditions,
the mean temperature difference between the inversion base and top was 9.2 K,
compared with 13.2 K for clear skies. Temperatures were higher in the cloudy cases
throughout the atmospheric profile from the inversion base to inversion top, but with
the temperature difference largest near the surface. The SAT (based on the lowest
temperature level in the soundings) under cloudy skies was 245K, compared to
238K for clear skies.
In explanation, some of the upward longwave radiation from the surface and from
the atmospheric layers below the cloud base that would escape to space is absorbed
by the cloud and reradiated downward. If we assume that there is no difference
