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 driving mechanisms vary greatly with cloud type. In this study, we performed numerical experiments to understand which main mechanisms drive surface solar irradiance (SSI) variations across a diverse set of observation-based cloud conditions. The results show that four mechanisms capture the essence. We find that for optically thin (τ<6) clouds, scattering in the forward direction (forward escape) dominates. In cloud fields with enough optically thin area, such as altocumulus, forward escape alone can drive areas of irradiance enhancement of over 50 % of clear-sky irradiance. For flat, optically thick clouds (τ>6), irradiance is instead scattered diffusely downward (downward escape), and (extreme) enhancements are thus found directly below the cloud. For vertically structured clouds, side escape dominates the domain-averaged diffuse irradiance enhancement until the sides become shaded by anvil clouds. Lastly, under optically thick cloud cover, surface albedo enhances radiative fluxes due to multiple scatterings between surface and cloud. This brightens shadows and contributes 10 % to 60 % of the total irradiance enhancement for low (0.2) to high (0.8) albedo. With these four mechanisms, we 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.