S. E. Nichol

Moderation of Cloud Reduction of UV in the Antarctic Due to High Surface Albedo

To gauge the impact of clouds on erythemal (sunburn causing) UV irradiances under different surface albedo conditions, UV measurements from two Antarctic sites (McMurdo and South Pole Stations) and a midlatitude site (Lauder, New Zealand) are examined. The surface albedo at South Pole remains high throughout the year, at McMurdo it has a strong annual cycle, and at Lauder it is low throughout the year. The measurements at each site are divided into clear and cloudy subsets and are compared with modeled clear-sky irradiances to assess the attenuation of UV by clouds. A radiative transfer model is also used to interpret the observations. Results show increasing attenuation of UV with increasing cloud optical depth, but a high surface albedo can moderate this attenuation as a result of multiple scattering between the surface and cloud base. This effect is of particular importance at high latitudes where snow may be present during the summer months. There is also a tendency toward greater cloud attenuation with increasing solar zenith angle.

Ozone-related trends in solar UV-B series

It is now well established that stratospheric ozone has become depleted on a global scale, with downward trends of several percent per decade in populated mid-latitude regions, but evidence of the expected parallel increases in solar UV-B radiation has remained elusive to date; if anything, past studies have actually reported decreases. The broken, ten-year record of Robertson-Berger meter UV-B measurements for a clean-atmosphere, mid-latitude site in the Southern Hemisphere that we analyse here is incapable of independently confirming a long-term upward trend, but it does provide specific evidence in support of the trend hypothesis. In particular, there is a very strong relationship between daily clear-sky UV-B and parallel ozone measurements, with correlation coefficients in the range of 0.84 to 0.95, and the trends and variations in ozone are very well matched by corresponding opposite trends and variations in UV-B.

Intercomparison of total ozone measured at low sun angles by the Brewer and Dobson spectrophotometers at Scott Base, Antarctica

The total ozone measurements made at Scott Base (78°S, 167°E) with Dobson spectrophotometer #17 and Brewer spectrophotometer #50 during the 1991 Antarctic ozone hole season are compared. Due to the high latitude of Scott Base, these measurements have been made using low-sun measurement techniques, which are considered to be less reliable and less accurate. The differences between the quasi-simultaneous total ozone measurements, and the mean daily total ozone values, from Dobson instrument 17 and Brewer instrument 50 are generally between ±5%. The Dobson-Brewer differences can, on occasions, be as great as 10% for measurements made at airmass values between 5 and 6. The Dobson and Brewer ozone data are generally within ±5% of the Total Ozone Mapping Spectrometer (TOMS) value.

Intercomparison of total ozone data from a Dobson spectrophotometer, TOMS, visible wavelength spectrometer, and ozonesondes

Comparisons of total column ozone measurements from three ground-based instruments (Dobson spectrophotometer, ozonesonde, and visible wavelength grating spectrometer) and the TOMS instrument are presented for the period 1991 to 1994 at Arrival Heights/McMurdo, Antarctica. The primary purpose of these comparisons is to investigate the accuracy of the visible spectrometer, while a second aim is to compare version 6 and version 7 TOMS satellite retrievals with the ground-based measurements. While the Dobson, TOMS and ozonesonde measurements show good agreement, there is a seasonal dependence between data from the visible wavelength spectrometer and each of the other three instruments. However, the visible spectrometer produces valuable data for the late autumn and early spring, when data are not available from the Dobson and TOMS.