Seasonal and Diurnal Variation of UVEry and UVyitD

Spectral measurements of solar UV irradiances, which have been undertaken over several years by NIWA (National Institute of Water and Atmospheric Research) at Lauder, New Zealand, have been used to demonstrate the seasonal and diurnal variability of vitamin D weighted UV and erythemally weighted UV irradiances at this mid-latitude site. Results for these weighted irradiances for a wide range of observing conditions at Lauder are shown in Figs. 2.6 and 2.7. The changes over one year of observations (Fig. 2.6) are due to the combined effects of changes in SZA, clouds, ozone, and earth-sun separation. While the mean of daily values, including cloud effects, is about 70% of the clear sky mean, on some days clouds attenuate the irradiances to less than 30% of the clear sky values. The upper envelopes approximately correspond to clear sky conditions (though there are occasional higher values attributable to cloud enhancements), and represent the

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Figure 2.6 Seasonal variation in weighted irradiances at Lauder, New Zealand-45°S. Upper panel: UVEry. Lower panel: UVVitD

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Figure 2.6 Seasonal variation in weighted irradiances at Lauder, New Zealand-45°S. Upper panel: UVEry. Lower panel: UVVitD

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Figure 2.7 Upper panel: Diurnal variations in vitamin D-weighted and erythemally-weighted UV irradiances on clear days near the summer and winter solstice periods at Lauder, New Zealand, 45°S, in 2003. Lower panel: Corresponding ratios of UVVitD/UVEry. On the summer day, the total ozone was 300 DU and the minimum SZA was 21.6°. On the winter day, the total ozone was 310 DU and the minimum SZA was 68.5°

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Figure 2.7 Upper panel: Diurnal variations in vitamin D-weighted and erythemally-weighted UV irradiances on clear days near the summer and winter solstice periods at Lauder, New Zealand, 45°S, in 2003. Lower panel: Corresponding ratios of UVVitD/UVEry. On the summer day, the total ozone was 300 DU and the minimum SZA was 21.6°. On the winter day, the total ozone was 310 DU and the minimum SZA was 68.5°

seasonal variability in noon-time UV irradiances. The seasonal variation in vitamin D weighted UV (Fig. 2.6) is very large at this site, with noon values at mid winter being only 5% of those in summer. This seasonal swing is significantly larger than for UVEry, where the winter values are about 10% of those in summer. The large swings are in contrast to the situation found for noon-time UVvitD at several sites in the USA over several months (Kimlin et al., 2007), where UVVitD remained relatively constant over the summer months. However, those were based on measurements at lower latitudes where seasonal changes are smaller. Furthermore, in the northern hemisphere, the seasonal changes in sun-earth separation tend to cancel some of the effects due to seasonal changes in SZA.

Figure 2.7 shows the diurnal variability of UVEry and UVVitD and their ratios on a summer day and a winter day at Lauder. The total column amount of ozone was stable through both of these days, and was quite similar: 300 DU for the summer day, and 310 DU for the winter day. Most of the disparity is therefore attributable to differences in SZA and earth-sun separation. The peak UVEry is a factor of 10 less than for the summer day, and the peak UVVitD is nearly a factor of 20 less than the summer peak. At larger SZAs, the UVVitD/UVEry ratio is greatly reduced compared with that for high sun, resulting in lower ratios in the winter compared with the summer, and lower ratios at twilight compared with midday. The decrease in this ratio with increasing SZA arises because UVEry includes a contribution from the UVA region, which is less attenuated. At the largest SZA (SZA > 85°), there is a reversal in this trend which arises because at these SZAs, the direct beam contribution is negligible, and most of the radiation is from Rayleigh scattering from the zenith sky for which the scattering efficiency reduces with wavelength.

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