Abstract
In order to test the validity of ultraviolet index (UVI) satellite products and UVI modelsimulations for general public information, intercomparison involving three satellite in-struments (SCIAMACHY, OMI and GOME-2), the Chemistry and Transport Model,Mod ́elisation de la Chimie Atmosph ́erique Grande Echelle (MOCAGE), and ground-5based instruments was performed in 2008 and 2009. The intercomparison highlighteda systematic high bias of∼1 UVI in the OMI clear-sky products compared to the SCIA-MACHY and TUV model clear-sky products. The OMI and GOME-2 all-sky productsare close to the ground-based observations with a low 6% positive bias, comparable tothe results found during the satellite validation campaigns. This result shows that OMI10and GOME-2 all-sky products are well appropriate to evaluate the UV-risk on health.The study has pointed out the difficulty to take into account either in the retrieval al-gorithms or in the models, the large spatial and temporal cloud modification effect onUV radiation. This factor is crucial to provide good quality UV information. OMI andGOME-2 show a realistic UV variability as a function of the cloud cover. Nevertheless15these satellite products do not sufficiently take into account the radiation reflected byclouds. MOCAGE numerical forecasts show good results during periods with low cloudcovers, but are actually not adequate for overcast conditions; this is why M ́et ́eo-Francecurrently uses human-expertised cloudiness (rather than direct outputs from Numeri-cal Prediction Models) together with MOCAGE clear-sky UV indices for its operational20forecasts. From now on, the UV monitoring could be done using free satellite products(OMI, GOME-2) and operational forecast for general public by using modelling, as longas cloud forecasts and the parametrisation of the impact of cloudiness on UV radiationare adequate.