Characterisation of cloud shadow transition signatures using a dense pyranometer network

Abstract. Small-scale variability of solar radiation including 3D radiative effects is poorly observed and understood. In this study, we characterise the transition of global solar horizontal irradiance from sunshine to cloud shadow and attribute the transition signature to 3D radiative effects. This analysis is based on 5 case days with shallow cumulus clouds at the ARM Southern Great Plains Central Observatory. Observations are conducted by PyrNet, a network of 60 autonomous pyranometer stations deployed during a field campaign in summer 2023. Complementary observations of cloud mask and shadow motion are derived from the Clouds Optically Gridded by Stereo (COGS) product. Concentrating on shallow cumulus clouds, we explore how geometrical effects and the macro- and microphysical properties of clouds affect the pattern of solar irradiance variation close to the cloud shadow edge. Individual cloud entities and cloud motion vectors are identified using COGS.  We discovered that the amplitude of radiation enhancement can reach 20 % above the clear sky values. Significant influence factors are the size of the cloud gaps, the height of the cloud base and the geometry between the sun and the clouds. The distance from the cloud at which radiation enhancement remains significant depends on the effective radius of the cloud droplets, cloud optical depth, and solar zenith angle. Our findings underscore the necessity of accounting for these 3D effects in atmospheric modelling to enhance the representation of solar radiation processes and are a step towards the development of transition signature parametrisations for photovoltaic energy applications.