Assessment of the accuracy in UV index modelling using the UVIOS2 system during the UVC-III campaign

Abstract. The third campaign for the calibration and intercomparison of solar UV radiometers (UVC III) took place at Davos, Switzerland in June–August 2022. More than 70 radiometers participated in the campaign and measured side-by-side with the portable reference spectroradiometer QASUME. By using inputs from various sources, the UVIOS2 system was used to estimate the UV index (UVI) for the site of the campaign. The UVIOS2 system is a flexible UVI modelling tool that can be exploited for different applications depending on the inputs. Thus, different combinations of satellite, reanalysis, and/or ground-based inputs were used to test the UVIOS2 performance when it is used as a tool for UVI nowcasting or for climatological studies. While UVIOS2 provided quite accurate estimates of the average (for the period of the campaign) UVI levels, larger deviations were found for individual estimates. The average agreement between the UVI from the UVIOS2 and QASUME was better than 1 % for all the different sets of inputs that were used for the study. The range of the variability was of the order of 40 % for instantaneous measurements (15 min), mainly due to the model’s inability to capture the instantaneous effects of cloudiness, especially under broken cloud conditions. Under clear-sky conditions the model was found to perform much better, with the differences between the model estimates and the QASUME measurements being smaller than 12 % for 95 % of the studied cases. Even at the pristine environment of Davos, single scattering albedo (SSA) was found to contribute significantly to the modelling uncertainties under cloudless conditions. For relatively small Aerosol Optical Depth (AOD), of the order of 0.2 – 0.4, the role of the SSA was found to be comparable to the role of AOD in the modelling of the UVI. Radiometers that were not properly maintained and/or calibrated were found to provide UVI measurements with uncertainty that was comparable to the uncertainty of the UVIOS2 estimates, which highlights the significance of systematic maintenance and calibration of the UV radiometers.