Cloud modulation of surface solar irradiance at a pasture site in Southern Brazil

Broken cloud fields create mosaic radiative landscapes with interchanging cloud-shaded and sunlit areas. While clouds attenuate solar radiation incident on cloud-shaded areas, sunlit ground surfaces may actually receive more irradiance than under a clear sky due to light scattering and reflection from neighboring clouds. In this paper, we studied these two opposite but closely related aspects of cloud modulation of surface solar irradiance at a pasture site in southern Brazil. We analyzed a high-resolution time series of surface measurements obtained during the 1999 wet season. Surface solar irradiance frequently (more than 20% of the time) exceeded clear-sky levels and occasionally surpassed the extraterrestrial radiation. Clouds created a bimodal frequency distribution of surface solar irradiance, producing an average of approximately 50 and 14% for attenuation and enhancement, respectively, as compared to corresponding clear-sky level irradiance. The average duration of enhancement periods was about 1/3 of the average duration of attenuation periods. On the daily basis, cloud-induced enhancement contributed an average of 4% of the daily solar input to the surface and compensated for more than 10% of the attenuation due to the presence of clouds. Through spectral analysis, two temporal regimes were shown to modulate the surface irradiance by clouds. One was a convective/mesoscale of tens of minutes to hours and the other was a turbulent scale of several minutes corresponding to the classical Kolmogorov^f-5/3power law.