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

S. E. Nichol National Institute of Water and Atmospheric Research, Lauder, New Zealand

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G. Pfister National Institute of Water and Atmospheric Research, Lauder, New Zealand

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G. E. Bodeker National Institute of Water and Atmospheric Research, Lauder, New Zealand

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R. L. McKenzie National Institute of Water and Atmospheric Research, Lauder, New Zealand

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S. W. Wood National Institute of Water and Atmospheric Research, Lauder, New Zealand

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G. Bernhard Biospherical Instruments, Inc., San Diego, California

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Abstract

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.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: S. E. Nichol, NIWA, Private Bag 14901, Kilbirnie, 6003 Wellington, New Zealand. s.nichol@niwa.co.nz

Abstract

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.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: S. E. Nichol, NIWA, Private Bag 14901, Kilbirnie, 6003 Wellington, New Zealand. s.nichol@niwa.co.nz

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