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3 Climatology of Ultraviolet Radiation at High Latitudes Derived from Measurements of the National Science Foundation's Ultraviolet Spectral Irradiance Monitoring Network

Germar Bernhard, Charles R. Booth, and James C. Ehramjian

5340 Riley Street Biospherical Instruments Inc., San Diego, CA, USA E-mail: [email protected] E-mail:[email protected] E-mail:[email protected]

Abstract Solar ultraviolet (UV) radiation has been measured at seven sites of the National Science Foundation's UV Spectral Irradiance Monitoring Network (UVSIMN) for up to 20 years. Data are used to establish a UV climatology for each site and to quantify differences between sites. Most locations are at high latitudes and include the South Pole; two research stations at the Antarctic coast (McMurdo and Palmer); the city of Ushuaia at the tip of South America; the Arctic village of Barrow; and Summit, a research camp established at the top of Greenland's ice sheet. UV levels at San Diego, California were also analyzed as an example of a lower-latitude location. The climatologies focus on the UV Index, which was derived from measured solar spectra of global irradiance. For each site and day of year, the average, median, and maximum UV Index at solar noon, as well as 10th and 90th percentile values, were calculated. Measurements were also compared with pre-ozone-hole UV levels estimated from historical measurements of total ozone. The analysis indicates a large effect of the ozone hole on the UV Index at the three Antarctic sites, and to a lesser extent at Ushuaia. UV Indices measured at South Pole during the ozone hole period (October and November) are 20% - 80% larger than measurements at comparable solar elevations during summer months. During October and November, the average UV Index between 1991 and 2006 was 55% - 85% larger than the estimate for the years 1963 - 1980. The UV Index at McMurdo shows a similar asymmetry about the solstice. In October and November, the average UV Index is about 30% - 60% higher now than it was historically. The largest UV Index ever measured at Palmer was 14.8. This value exceeds the maximum UV Index of 12.0 observed at San Diego. While the average

UV Index at Ushuaia is fairly symmetrical about the solstice, maximum UV Indices as high as 11.5 have occurred in October at times when the ozone hole passed over the city. The annual cycle of UV radiation at Barrow is governed by large seasonal changes of total ozone, albedo, and cloud cover. The UV Index does not exceed 5 due to less severe ozone depletion over the Arctic: changes in UV over the last 30 years are on average less than ±8%. A comparison of UV levels at network locations reveals that differences between sites greatly depend upon the selection of the quantity used for the comparison. Average noontime UV Indices at San Diego during summer are considerably larger than noontime UV levels under the ozone hole at all Antarctic sites. The difference diminishes, however, when daily doses are compared because of the effect of 24 hours of sunlight during Antarctic summers. Data analysis further revealed that broken clouds at the South Pole can enhance spectral UV irradiance at 400 nm by up to 30% above the clear-sky value due to multiple reflections between the snow-covered surface and the cloud ceiling.

Keywords solar ultraviolet radiation, Antarctica, Arctic

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