data, surface-based visual observations summarized in a climatic atlas (Warren et
al. 1988), and visual observations on the Russian North Pole (NP) drifting stations.
The figure suggests that, compared to visual observations, the satellite data over-
estimate the mean monthly cloud amount in the winter and underestimate it in
the summer.
In the last few years, several works have attempted to evaluate the influence
of ice crystals (Curry and Ebert 1992, Overland and Guest 1991) and Arctic haze
(Blanchet 1989, Zachek 1996) on the radiation and thermal state of the Arctic
atmosphere and the surface. For example, Curry and Ebert (1992), building on
Huschke's (1969) cloud statistics, estimate the annual variation of total cloud
amount by including the effects of low, medium, and high-level clouds and lower
tropospheric ice crystal precipitation (Fig. 2). They evaluate the role and quantity
of ice crystal precipitation by comparing the results of calculations from numeri-
cal radiation models that do not incorporate ice crystal precipitation with mea-
surements of the longwave radiation balance or the incoming longwave radiation.
Overland and Guest (1991) also note a discrepancy between observations and
model calculations of incoming longwave radiation and likewise suggest that the
missing modeled downward longwave radiation might be explained by ice crys-
tal precipitation, "diamond dust." They did not, however, rule out other explana-
tions such as blowing snow or optically thin clouds. Alternatively, on comparing
incoming longwave radiation measurements with theoretical estimates of the
effective radiant emittance of the atmosphere, Zachek (1996) shows that the tem-
poral variability of this radiation is closely connected with the temporal variabil-
ity of the concentration of atmospheric aerosols, especially in FebruaryMay, when
this concentration has its maximum.
Although it is important to continue research on the above-cited phenomena
and to develop methods to account for these in calculations of incoming longwave
radiation with atmospheric radiation models, here we will consider the simpler
and more conventional characteristics of cloudiness observable visually during
standard meteorological observations. These are total (n) and low (nL) cloud
amount, the cloud parameters most frequently used in climate research for calcu-
lating the radiative fluxes.
The atlases of Gorshkov (1983) and Prik (1965), among many others, give the
spatial and temporal variability of several climatic variables in the Arctic Basin
based on generalized data from polar land stations and Russian drifting stations
10
total
8
low
6
ice crystals
mid
Figure 2. Annual cycle in
4
amounts of low-level, mid-
level, high-level, and total
high
2
clouds and in ice crystal pre-
cipitation in the lower tropo-
0
sphere. (Adapted from Curry
0
50
100
150
200
250
300
350
and Ebert 1992.)
Day
2
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