Mechanisms of surface solar irradiance variability under broken clouds

Abstract. Surface solar irradiance variability is present under all broken clouds, but the patterns, magnitude of variability, and mechanisms behind it vary greatly with cloud type. In this study, we performed numerical experiments to understand which main mechanisms drive surface solar irradiance extremes across a diverse set of cloud conditions based on observations. The results show that we can capture the essence in four mechanisms. We find that for optically thin (τ < 6) and clouds, scattering in the forward direction (forward escape) is the dominant mechanism. In cloud types such as altocumulus, it is able to produce irradiance enhancements of up to 50 % of clear-sky values due to small gaps in the cloud field. For flat, optically thick clouds (τ > 6) like stratus, downward escape becomes the dominant mechanism, and the irradiance extremes are found underneath the cloud edge or gaps. Albedo has a significant effect under optically thick cloud cover, contributing 10 to 60 % of the total irradiance enhancement for low (0.2) to high (0.8) albedo. For deep convective clouds, side escape is the dominant mechanism enhancing domain-averaged diffuse irradiance. This effect has a large area of influence, extending over 20 km from the sunlit side of the cloud. Extreme irradiance enhancement, however, comes from downward escape and forward escape just underneath the cloud edge on the sunlit side, not from side escape. These results provide a framework for understanding the vast diversity and complexity found in surface solar irradiance and cloudiness. A next step is to apply this analysis to multi-layered cloud fields and non-isolated deep convective clouds.