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1 Abstract

Measurements were made between October 26 and November 6, 1992 of properties of the Antarctic skies at two sites, one near Tent Island, and the other at Lake Vanda, the first site being surrounded by sea-ice, while the second site was surrounded over a substantial distance by bare rock. The measurements at Tent Island confirmed the general trends observed during the same period in 1990, and the differences between the two sites confirmed the interpretation suggested by the earlier work, that the distinctive features of skylight over ice (and snow) are due to the high albedo environment. No enhancement of the UVB was observed during this time, and this was consistent with the generally high ozone levels measured at Arrival Heights.

2 Proposed Programme

Our measurements during 1990 identified low maximum polarisation of sky light and strong low-azimuth back-scattered light as two characteristic features of the Antarctic skies. We suggested in our analysis of these features that they came about by multiple reflections from the snow-ice surfaces surrounding the Tent Island site (and most of Antarctica). Our hypothesis was that the skies over the Dry Valleys would differ, since the amount of back-scattering would bevery much reduced. The two sites have essentially the same latitude, and the same unpolluted air, so a direct comparison should put in evidence the effects of surface albedo.

We planned to look for enhancement of UVB radiation at these sites associated with any ozone hole. We planned to study algae sensitivity to UV radiation.

3 Scientific Endeavours and Achievements

The 3/4 channel sky monitor was essentially the same as used in the earlier 1990 study at Tent Island, but the data-logger now used a PC laptop driven from 12V external batteries and could run for a 12 hour period unattended. The computer control of the spectrometer had been substantially altered, and now was much more convenient to use, and could take sky page break spectra more quickly. A diffuse reflectance attachment had been made to allow measurements of the colour of sea-ice algae.

We joined Haskell and colleagues at the camp site near Tent Island, and with good weather we were able to establish our sky monitor and our polarisation spectrometer and start measurements immediately (October 23, 1992). The polarisation and sky light distribution showed very similar behaviour to that observed during 1990. The maximum polarisation in the sun-zenith plane in a direction 90° away from the sun reached 40%, which was the value observed in 1990 and contrasts with a typical value of 70-80% observed in New Zealand. The UVB levels were not enhanced, and this was consistent with the preliminary ozone levels recorded by Sylvia Nicol.

There was very tittle algae under the sea-ice at the Tent Island site at that time. It was clear that we would not be able to get sufficiently dense samples to reliably measure their colour in situ (the intention had been to study the colours and UV sensitivity without artihcial concentration), and this part of the programme was dropped.

We were able to accumulate sufficient sky data within 5 days, and with the improved data programmes we were able to analyse for preliminary results immediately. Bad weather then held up our move to Lake Vanda for 2 days, but we spent time continuing our data analysis and helping with the Haskell event.

We travelled to Lake Vanda by helicopter on 31 October. We spent a few hours opening the station. We were unable to get power from the local battery system (which hovered around 10V), and found the switches to the solar panels confusing- in no position did they appear to produce a charging current. We used our own generator and batteries.

We started our measurements during the evening of the first day there. The weather was very settled, showing the same pattern each day: fine clear skies during the evening and night, but very thin high cloud from mid morning to mid afternoon. On the 4 November the thin high cloud held off till noon so we were able to measure through the maximum of the sun's altitude. The preliminary analysis showed that the maximum sky polarisation was between 50 and 60%, which was substantially higher than at Tent Island, and this confirmed our hypothesis about the effects of the surface reflection (surface albedo).

The high walls of the Wright Valley restricted the north and south horizons and reduce the total intensity of sky tight falling onto the surface. We measured the horizon and will make corrections for this effect when we determine the total surface intensities. On the last day the helicopter stopped at Bull Pass for an hour around noon, and we took total intensity measurements there to compare with those at the Lake Vanda site. The horizon at the Bull Pass site was substantially lower.

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We returned to New Zealand earlier than we had planned. This shortening of the event was the result of the fine weather encountered at both sites. In all we only lost a couple of days due to cloud coverage.

4 Publications

Two papers describing our earlier study at Tent Island have been published:

Antarctic skies I : Diurnal variations of the sky irradiance, and UV effects of the ozone hole, spring 1990, D.Beaglehole and G.G.Carter, J.G.R.Atmospheres D2, 97, 2589-2596, 1992

Antarctic skies II: Characterisation of the intensity and polarisation of skylight in a high albedo environment. D.Beaglehole and G.G.Carter, J.G.R.Atmospheres D2, 97, 2597-2600, 1992.

We are preparing one further report Antarctic skies III : Comparison of high and low albedosites, which will be submitted to J.G.R.

Some preliminary discussion of our comparison between Vanda and Tent Island has been given in our article

Ultraviolet radiation and Sea Ice Algae in Antarctica, K Ryan and D.Beaglehole, to be published in Ultraviolet radiation and biological research in Antarctica, eds S. Weiler and P. Penhale, Antarctic Research Series 1993.

6 Future Research

We have now characterised the Antarctic skies sufficiently for reasonable radiative-transfer models to be applied to the problem with reliability. No one else has made a polarisation and sky intensity distribution study, so we feel that we have made a significant contribution here.The UV monitoring by the NSF network is better than our Victoria group can do with our the limited resources, so while the long term variations in UVB radiation are important, there seems little point in competing with the US who are measuring at McMurdo.

7 Management of Science

We were very pleased with the way we were able get on with our measurements with the shortest delays at Scott Base and the helicopter support. This was at a particularly difficult page break time following the accident and helicopter shortage, and the assistance we were given was very much appreciated. We also appreciated the help given in opening Lake Vanda.

8 Acknowledgements

We very much appreciate the help and cooperation given by all the Scott Base staff, and especially Paul Chaplin, Dave Bryce and Eric Saxby who helped with the logistics and the equipment. The support of Tim, Bob and Joe at the Tent Island camp was also apppreciated. Paul Woodgate's efforts at Christchurch were also much appreciated.