Influence of cloudy/clear-sky partitions, aerosols and geometry on the recent variability of surface solar irradiance's components in northern France

Abstract. The surface solar irradiance (SSI) is a fundamental parameter whose components (direct and diffuse) and variabilities are highly influenced by changes in atmospheric content and scene’s parameters. The respective importance of cloudy sky conditions and atmospheric aerosols on SSI evolutions is region dependent and only partially quantified. Here we provide a comprehensive analysis of SSI variabilities recorded in northern France, a region with extensive variabilities of sky conditions and aerosol loads. Through the application of automatic filtering methods on 1 min resolution SSI ground-based measurements over Lille, sky conditions are classified as clear-sky, 11 %, clear-sun-with-cloud, 22 %, and cloudy-sun situations, 67 %, over 2010 to 2022, for which we analyze the statistics and variabilities of the global horizontal (GHI), direct (BHI) and diffuse (DHI) solar irradiances. Coincident photometric measurements of aerosol properties and radiative transfer simulations provide the mean to conduct a multivariate analysis of the SSI observed trends and year-to-year evolutions, and to estimate aerosol and cloud forcings under clear-sun conditions. The analysis of the record value of all sky GHI in spring 2020 attributes 89 % of the changes to the exceptional sunlight conditions (57 % of clear-sun situations). It highlights also for that season the importance of solar zenithal angle’s changes whose positive effect on clear-sun conditions surpasses those due to aerosols. Our results show all-sky GHI and BHI positive trends of around +4 and +4.4 W/m²/year respectively, both in spring and summer, that are explained at more than 60 % by an increase of clear-sun occurrences of +1 % per year. Additional significant BHI’s increase under clear-sun conditions are mainly explained in spring by the negative trend in aerosol optical depth (-0.011 per year), partly by angular effects in summer. Moreover, we find that clear-sun with cloud situations are frequently marked by irradiance enhancement due to clouds, with on monthly-average 13 % more GHI and 10 % additional diffuse proportion than in clear-sky situations. Under such conditions, clouds add on average 25 W/m² of diffuse irradiance that set the amount of solar irradiance at the remarkable level of pristine (aerosol and cloud free) conditions, or even higher by more than +10 W/m² in summer and for low aerosol loads. Overall, our results highlight the dominant and complex influence of cloudy conditions on SSI, which precedes or combines with that of aerosols and geometrical effects, and leads to remarkable global level of SSI in clear-sun with-cloud situations.