The Clouds-Induced Bimodal Characteristics and Enhancements of Surface Irradiance as Observed over a Brazilian Grassland

If clouds are not distributed continuously over the sky, ground surfaces, which are located at the end of paths of solar beams passing through gaps formed by individual clouds in neighboring skies, may actually receive more irradiance than under a clear sky. This enhancement of surface irradiance is due to light reflections from the sides of clouds and can lead to surface irradiance higher than extraterrestrial solar radiation. This phenomenon has been termed ‘cloud-gap effect’. The impacts of the cloud-gap effect depends on not only magnitude, duration and frequency of irradiance fluctuations but also the paces of biological responses. The former factors are determined by cloud types, wind fields, altitudes, at the location. The latter factor is largely controlled by responses of stomata to changes in light. In this study, we analyzed high resolution surface irradiance measurements obtained from a Brazilian grassland for magnitude, duration and frequency of the cloud-gap effect and their relationships with cloud types and impacts on net exchanges of carbon dioxide and water vapor on this grassland.