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
Only during 2 days in March 1993, fields of well-developed but still not ideally uniform frontal cirrus clouds were located in the reachable neighborhood of the airport
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in Longyearbyen Svalbard , about 120 km away. They originated from two decaying frontal systems arriving in the area from southwest. In both cases, the cirrus cloud field
had a geometrical thickness of more than 5 km between about 8.5 and 3 km altitude. It overlaid lower broken stratocumulus. Up to 10 level flights, each about 60 to 100 km
long, could be made in a vertically staggered array.
The aircraft, a FALCON 20 jetliner of the DLR in Oberpfaffenhofen, was equipped to measure particles with the probes PMS-FSSP-100X and PMS-OAP-2D2-C, and also
with its standard equipment the ambient temperature and humidity and wind fluctua- tions, and also upward and downward, broadband solar and infrared radiation flux
densities. The PMS OAP-2D2-C probe is designed to measure particles in the size range from 25 to 800 mm. To evaluate its data, we used in this study the methods discussed in
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Heymsfield and Baumgardner 1985 . In particular, for the calculation of the sampling Ž
. area we used the ‘‘depth of field’’ formula derived by Knollenberg 1970, 1981 and the
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‘‘entire-in-method’’ Heymsfield and Baumgardner, 1985 for the determination of the effective array width.
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In addition, we made use of airspeed corrections Albers, 1989 , to take into account the effect of the electronic response time of the probe, which is important at the high
airspeeds of a jetliner. While the nominal size resolution of the 2D-C probe is 25 mm at 100 mrs, the resolution in flight direction is decreased to approximately 50 to 70 mm
because of the high true air speeds in the range of 150 to 180 mrs, which were used during our field campaign. The relative accuracy of 2D-C probes and different process-
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ing methods were discussed previously e.g., Gayet et al., 1993 . The PMS FSSP probe measures the forward scattering instrument of light and can
detect cloud particles with sizes ranging from 2 to 47 mm with a resolution of 3 mm Ž
. range 0 . For the probe data analysis, we considered each particle as a ice sphere. This
is in fact questionable, because we do not know a priori the scattering function of the small cirrus ice particles. On the other hand, there is a lack of data in the size range
below that of the 2D-C probe. Ž
. Ž
. Gardiner and Hallet 1985 and Heymsfield et al. 1990 described some of the
problems using the FSSP for measurements in ice clouds. They described an artificial enhancement of the particle spectra measured by the FSSP caused by large ice crystals.
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However, Gayet et al. 1996 indicated that the data of the FSSP may be used if the agreement of the 2D-C and FSSP size distributions is good, which is the case in our
study. Anyhow, the FSSP data have to be treated with great care and a large uncertainty is inevitable, but it might be better to use these data than to accept a total lack of
information about particles in the small size range. For the radiative transport studies, which are described in the following sections, it was shown that the small ice particles
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contribute only little to the cloud optical thickness Koch, 1996 . The synoptic situation on 12 March 1993 was characterized by a lower cyclone west
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of Iceland with a central pressure of 975 h Pa and a high pressure system 1020 h Pa over North Greenland with a ridge extending from the Spitsbergen archipel towards
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North Norway. A new low cyclone 995 h Pa at 1200 UT had rapidly developed at an air mass boundary around 728N, 08E with a wide cirrus field towards Svalbard. In this
cirrus cloud field, a strong wind shear occurred both in direction and speed.
Fig. 1. Particle size spectra measured in a decaying frontal cirrus near Svalbard on 12, March 1993, where for clarity the spectra of levels 9 to 2 are shifted by one order of magnitude, respectively.
With respect to the extreme climatic situation, it was of interest to examine the suitability of our instrumentation and method under these conditions. This concerned the
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functioning of the PMS particle probes OAP-2D2-C and FSSP-100X as well as possible difficulties in solar radiation measurements at low sun elevations. In contrast to
former flights in mid-latitude cirrus clouds, the particle probes were exposed from the beginning to low temperatures of about y208C at ground. All particle probes required
careful shielding against electromagnetic noise in the aircraft.
The domes of the radiation instruments were possibly subject to icing. This icing was observed at ground, however, only after the return from the missions. This ice might
have formed during the last portion of the descending phase from high altitudes. However, no indication in the data was visible that icing happened also during the flight
in cirrus clouds. Otherwise, such an experiment would entirely be in doubt.
Simultaneously measured leg-averaged particle size spectra from both particle instru- ments could be combined, as shown in Fig. 1, where a bimodal structure can be
observed with a systematic shift of the second maximum towards larger particles with decreasing altitude. This shift is smaller in the lowest layers, possibly due to increasing
sublimation of the particles. In general, this shift indicates the growth of crystals while
Ž Fig. 2. Horizontal inhomogeneities of cirrus properties, ice water content, effective radius, and number
. Ž .
Ž . density and of solar radiation at 8520 m a and 4860 m b . The effective radius corresponds to the radius of
a sphere with same area.
sedimenting downward, but possibly also a reduction of their number density occurs due Ž
. to instabilities splintering and aggregation . Such time histories of size spectra have
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been simulated by Zhang et al. 1992 with a rather simple model considering terminal velocities, heating and cooling by radiation and sublimationrevaporation, respectively,
and also the effects of large scale lifting of cirrus layers. The upper parts of the cirrus cloud layer was horizontally quite inhomogeneous as
shown in the example in Fig. 2, where the effective radii, as derived from the measurements of the OAP-2D2-C probe alone, are plotted. In the lower layers with
relatively higher amounts of diffuse radiation, much smoother radiation fields could be observed.
Below the cirrus cloud layer, some cumuliform clouds were observed, which may also have contained ice particles but will be assumed in this study as water clouds.
3. Radiative transfer model
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