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Keywords:

  • solar radiation;
  • global dimming;
  • sunshine

Abstract

Temporal and spatial variability in annually averaged global irradiance, diffuse irradiance, and bright sunshine duration over twelve stations of solar radiation network of India Meteorological Department (IMD) were evaluated for the period 1971–2005. Annually averaged all-sky global irradiance decreased between 0.1% (0.3 Wm−2) and 3.6% (9.0 Wm−2) per decade at the stations. The decrease in sunshine duration was concomitant with the decrease in global solar irradiance. The long-term trends in diffuse irradiance differed markedly between the twelve stations. Significant increasing trend in diffuse irradiance were found at the four stations under all-sky conditions, while seven stations showed increasing trend under clear sky conditions. The two most probable causes of long-term trends in radiation are changes in aerosol loading and cloud cover. Trend analyses of Angstrom's turbidity coefficient measured at three stations of IMD during the last three decades indicate that total column aerosol has increased significantly. Trend analysis of bright sunshine duration suggests long-term decrease at all the stations. Reduction in bright sunshine hours varied from 0.0 to 0.5 h per decade. There is a strong northwest-northeast gradient in sunshine duration which contributes to the spatial trends in global and diffuse solar irradiance. This paper examines the trends of global irradiance, diffuse irradiance and sunshine duration over India in all-sky conditions as well as cloud-free skies so the effect of cloud cover can be examined. Copyright © 2011 Royal Meteorological Society