70
60
50
40
30
20
10
Figure 13. Temporal variability of
0
the longwave radiation balance
10
and the sensible heat flux for the
20
middle 10 days in May 1992 on
30
Ice Station Weddell. The radiation
data are hourly averages; the sen-
50
sible heat flux values derive from
40
profile measurements of wind
speed and air and surface tem-
30
perature (Andreas et al. 1992).
20
10
0
10
20
30
10
11
12
13
14
15 16 17
18
19
20
21
May 1992
perature in winter over Arctic sea ice depends largely on longwave radiation pro-
cesses, that temperature, in effect, contains information on cloud amount.
Another process that dictates the close agreement among the equilibrium ice
thicknesses in experiments 5, 6, and 9 is the feedback between the longwave
radiation balance and the vertical turbulent surface flux of sensible heat, the main
components of the surface heat budget in winter. That feedback is clear in the data
from Ice Station Weddell: Figure 13 shows that an increase in the radiative cooling
of the snow surface causes an increase in the temperature gradient in the atmo-
spheric surface layer (and a corresponding increase in the downward sensible heat
flux). Consequently, the traces of longwave radiation balance and sensible heat flux
are almost mirror images. The one-dimensional sea ice model of Makshtas and
Timachev (1992) also reproduces this feedback between sensible heat flux and net
longwave radiation.
The large differences between the computed ice thicknesses in experiments 5,
6, and 9 and the thickness in experiments 7 and 8 result because of the absence of
feedback between the sensible heat flux and the net longwave radiation in sum-
mer. During the summer, the snow and ice surface is near the melting point for
long periods and, thus, accommodates changes in the net longwave radiation by
changing phase rather than by exchanging sensible heat. Also, the strong depen-
dence of Fdn on cloud amount (see eq 12 or 16), which in summer is usually 810
tenths (Table 2), sharpens the difference between the results for the two groups of
23
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