Impact of circumsolar irradiance and aerosol optical depth on CAMS surface solar irradiance estimates

The global horizontal irradiance is a main variable in the integration of solar energy systems in electricity grids, and in climate and weather studies. The physical model McClear is the clear-sky module for the all-sky irradiance retrievals from the CAMS Radiation Service. Validations of McClear against high-quality in situ measurements reveal very accurate global horizontal irradiance estimates with negligible bias. Albeit still good, the beam and diffuse irradiance components exhibit biases generally opposing each other. These biases are mainly attributed to the aerosol optical depth used as input to McClear, and the forward scattered circumsolar irradiance surrounding the solar disc which is not modelled by McClear but intercepted within the solid angle aperture of radiometers measuring beam irradiance (and blocked by those measuring diffuse irradiance). This study performs a global scale validation of a desert-based circumsolar correction, and evaluates the influence of this circumsolar correction and the aerosol optical depth on the beam and diffuse irradiance estimates from McClear. Circumsolar corrected retrievals are enhanced globally, especially for the diffuse component of McClear, with the standard deviation of deviations improving from 26.3 to 24.4%, the bias from 19.0 to 13.1%, and the correlation remaining constant at 0.837. Comparing ground measured and CAMS modelled aerosol optical depth, improvements are expected with further developments to the CAMS aerosol model.