Aspects of time variability of the surface solar irradiance as measured and analysed from ground-based measurements with a distinction of cloudyness

We provide here insight into the time variability of the surface solar irradiance (SSI) and of its characteristics obtained from an analysis of ground-based measurements from a western Europe location with both a high cloudiness and a highly variable load in aerosols. We detail here the application of automatic filtering methods on 1 min resolution SSI measurements that lead to the distinction of clear-sky, clear-sun-with-cloud, and cloudy-sun situations. Coincident photometric measurements of aerosol properties and radiative transfer simulations provide the mean to conduct a multivariate analysis of some SSI observed trends and year-to-year evolutions, and to estimate aerosol and cloud forcings under clear-sun conditions. The analysis focuses in particular on clear-sun-with-cloud situations that are often associated with positive cloud enhancement effects. On monthly-average 13 % more global horizontal irradiance and 10 % additional diffuse proportion are encountered in clear-sun-with-cloud situations compared with clear-sky situations, setting the amount of solar irradiance at the remarkable level of pristine (aerosol and cloud free) conditions but with a proportion of diffuse component multiplied by 2.5. With a synergy of observations (pyrheliometer and lidar) that identify cloud covers, we analyze the statistical characteristics of the surface solar irradiance per cloud cover category and how they vary with the width of a temporal averaging window. Results show that positive cloud radiative forcing are more frequent and higher for Cumulus situations compared with Cirrus one, but attenuates faster with time. Our results address the capacity of atmospheric modelling and of satellite-based surface radiation data set to generally not yet represent these important details in SSI variability’s features.