Atmospheric Chemistry and Physics Solar irradiance at the earth’s surface: long-term behavior observed

• Main Authors: At The South Pole, J. E. Frederick, A. L. Hodge
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2011
• Subjects: Austral; South Pole; South pole
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.685.9561; http://www.atmos-chem-phys.net/11/1177/2011/acp-11-1177-2011.pdf

Abstract. This research examines a 17-year database of UV-A (320–400 nm) and visible (400–600 nm) solar irradi-ance obtained by a scanning spectroradiometer located at the South Pole. The goal is to define the variability in solar ir-radiance reaching the polar surface, with emphasis on the influence of cloudiness and on identifying systematic trends and possible links to the solar cycle. To eliminate changes associated with the varying solar elevation, the analysis fo-cuses on data averaged over 30–35 day periods centered on each year’s austral summer solstice. The long-term aver-age effect of South Polar clouds is a small attenuation, with the mean measured irradiances being about 5–6 % less than the clear-sky values, although at any specific time clouds may reduce or enhance the signal that reaches the sensor. The instantaneous fractional attenuation or enhancement is wavelength dependent, where the percent deviation from the clear-sky irradiance at 400–600 nm is typically 2.5 times that at 320–340 nm. When averaged over the period near each year’s summer solstice, significant correlations appear be-tween irradiances at all wavelengths and the solar cycle as measured by the 10.7 cm solar radio flux. An approximate 1.8 ± 1.0 % decrease in ground-level irradiance occurs from solar maximum to solar minimum for the wavelength band 320–400 nm. The corresponding decrease for 400–600 nm is 2.4 ± 1.9%. The best-estimate declines appear too large to originate in the sun. If the correlations have a geophysical origin, they suggest a small variation in atmospheric attenua-tion with the solar cycle over the period of observation, with the greatest attenuation occurring at solar minimum.