Table 1 Coefficients of cloud type k
i
Cloud type k
i
Cirrus 0.04
Cirrostratus 0.08
Altocumulus 0.16
Altostratus 0.20
Cumulus 0.20
Stratocumulus 0.22
Fog 0.25
data recorded in Hamburg over a 10-year period, they obtained the following mean values:
b s y0.75
1
b s 3.4
2
Ž .
Concerning long-wave radiation, Arnfield 1979 proposed a relationship to correlate the sky radiation flux with the amount and type of cloud cover:
´ s ´ 1 q Ýk c c
Ž .
a i
i
In this formula ´ represents the atmospheric emissivity in clear sky condition, k is
i
the ith coefficient related to the individual cloud type i, c the total cloud cover fraction and c the fractional cloud amount of any cloud type. Table 1 shows the k values used
i i
Ž .
by Arnfield 1979 . Ž
. Paltridge 1970 tried to correlate the increase in infrared radiation coming from the
sky with a cloud cover index by analysing a set of data recorded in Aspendale, Australia. He found that any 1r10 of increase in the cloud cover index corresponds to a
mean increase of 0.6 mW cm
y2
in the long-wave radiation flux. This work aims at verifying the admissibility of the daily mean values of infrared
radiation from the sky at the Antarctic measuring site with respect to the values of the Ž
. Ž
. Ž
. Swinbank 1963 , Deacon 1970 and Idso and Jackson 1969 relationships. It shows
how to correlate the incoming solar radiation with the deviation of L from the expected
i
Ž .
Swinbank’s 1963 values for a clear day. It was possible to relate this deviation to the cloud cover amount according to the cloud height using the formula of Kasten and
Ž .
Czeplak 1980 . Finally, a particular geometric surface is modelled to explain the Ž
. deviation from Iqbal’s 1983 formulation of the daily pattern of the albedo values, with
regard to the solar azimuth and elevation angle.
2. Materials and methods
During the 9th and 10th Italian expedition a pyrradiometer and albedometer were set up at the Antarctic site. The biennial experiment, proposed by the Italian Programme of
Ž .
Antarctic Research PNRA , set out to correlate the radiative balance terms with the rise
and development of katabatic flows and, more generally, to collect a data-set on the climatic patterns in this remote area. In the first year, the measurements were performed
Ž
X
at two different sites located near the Italian base of Terra Nova Bay lat 74841 S, long
X
. 164807 E , close to the intersect point of the Priestley Glacier and the Nansen ice sheet
and, in the following year, over the Reeves Neve Glacier at 1200 m a.s.l.
`
The second set of measurement is used for the present work, which were collected Ž
. throughout December every 10 min with a Schenk mod. 8111 net pyrradiometer and a
Ž .
Schenk mod. 8104 albedometer. Both were equipped with two sensors to collect the incoming radiation from the whole sphere. A direct estimation of both the total net
radiation and the albedo was possible. The net pyrradiometer is able to estimate the total net radiation in the 0.3–30 mm.
spectral range. The black surface of the upward sensor estimates the direct and indirect solar radiation and the thermal radiation emitted by the atmosphere. The visible radiation
reflected by the surface and the long-wave radiation emitted from it are detected by the downward-looking sensor.
Ž .
The albedometer is made up of two pyranometers 0.3–3 mm
for the direct determination of the albedo. The upward sensor receives only the direct and indirect
solar radiation from the sky, while the other receives the radiation reflected by the surface.
By means of a resistance thermometer inside the pyrradiometer, it is possible to determine the total radiation coming from the sky and from the surface, according to the
following formulas S q L s V f q ´ s T
4 i
i u
u s
s
and S q L s V f q ´ s T
4 o
o 1 1
s s
respectively, where T is the internal temperature of the instrument and ´ its emissivity;
s s
signals V and V are generated in the upper and lower receiving plates, respectively,
u l
and f and f are the calibration parameters suitable for the conversion in watts per
u l
square meter. Subsequently, using the albedometer signals, the incoming and outgoing long-wave radiative components are determined.
All the analog signals are converted into digital values by a fully programmable Campbell CR-10 data-logger capable to periodically store the values in an internal
memory.
3. Results and discussion
