1. Introduction
High and cold cirrus clouds are possibly responsible for an enhancement of the Ž
. atmospheric Greenhouse effect as it had be shown already by Hansen et al. 1981 . They
further dry the upper troposphere, but they can also heat it by absorption of solar radiation and form an effective shield against cooling to space. Sedimenting ice crystals
can also trigger precipitation in lower-layer clouds. Cirrus can also be enhanced by aircraft contrails, although the net effect on the radiation budget components is even
Ž over specific regions with enhanced air traffic still relatively small e.g., Gierens et al.,
. 1999 . Relatively little is known on cirrus cloud fields over both polar regions and over
the tropics due to their difficult accessibility. Therefore, an accurate monitoring and modeling of cirrus clouds and its properties in time and space is essential for climate and
weather inventory research. A comprehensive review of the influence of cirrus on
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weather and climate processes has been given, for instance, by Liou 1986 . Since 1987, investigations of mid-latitude cirrus clouds in Europe have been orga-
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nized within the frames of the International Cirrus Experiment ICE and its successor,
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the EUropean Cloud and Radiation EXperiment EUCREX Raschke et al., 1990,
. 1998 . The opportunity of the experiment ARKTIS 93, which was organized by the
Institute for Meteorology in Hamburg and the Alfred-Wegener-Institute for Polar- and Marine Research in Bremerhaven and took place in March 1993 near Island Svalbard
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Spitsbergen Archipel , to perform measurements inside of cirrus and also low level clouds at high latitudes and over the marginal ice zone.
This campaign, named with the German word ARKTIS 93, has been designed to study energy and momentum exchanges over the Arctic sea ice. It included also a
component on the differential heating effects on clouds from below by low amounts of long-wave radiation emerging from cold sea ice and higher long-wave radiation emerg-
ing from the warmer open sea. During this experiment, the solar and thermal radiative fluxes have been measured with Eppley Pyranometers and the cloud microphysics with
the known PMS FSSP 100X and OAP-2D2-C particle probes. Some preliminary results
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and the instrumental performance have been discussed by Albers et al. 1993 , Koch Ž
. Ž
. 1996 and Raschke et al. 1998 .
The objective of this paper is to compare in more detail the airborne measurements of solar radiation flux densities with results from radiative transfer calculations, which are
based on the simultaneous microphysical measurements, and to identify possible error sources to be considered in future experiments.
In Section 2, the microphysical measurements of the campaign ARKTIS 93 are described. The radiation model is outlined in Section 3. The comparisons of calculated
and measured values of the solar radiative flux density are provided in Section 4. Some effects of horizontal inhomogeneities in the cloud field on the transfer of solar radiative
flux density are described in Section 5.
2. Measurements
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