Local enhanced solar irradiance on the ground generated by cirrus: measurements and interpretation

This study concerns the influence of thin cirrus on the solar irradiance budget on the ground, using data obtained by remote sensing from a set of instruments operated at the Observatoire de Haute Provence (France). Comparisons between solar irradiance ground-based measurements obtained during clear and cloudy days usually show a lower irradiance in the latter case as expected. However, there are cases lasting a maximum of 30 min for which the solar irradiance is greater in the presence of cumulus than in clear-sky conditions, and this phenomenon was named enhanced solar irradiance. Our pyranometer measurements reveal the same phenomenon associated with the presence of thin clouds alone, as revealed by a daytime wide-field camera and occurring both in total and UVB* (280 to 320 nm) solar irradiance. In the case of thin cirrus, this phenomenon can last up to several hours. To understand this phenomenon, we have developed a model of solar light scattering within thin cirrus clouds that takes into account the presence of the atmosphere. The model reproduces the enhanced solar irradiance phenomenon in total and UBV* spectral range. Monoscattering and multiscattering processes inside the cirrus clouds also will be discussed.