Comparing OMI UV index to ground-based measurements at two Finnish sites with focus on cloud-free and overcast conditions

Abstract. Satellite based surface UV product of the Ozone Monitoring Instrument OMI was validated using ground based UV measurements from the two Finnish sites Jokioinen and Sodankylä. The goal was to further investigate the observed positive UV bias of the OMI UV product focusing on how it may be connected to cloudiness during the overpass of the Aura satellite. A total of seven years of summer time data was used to compare OMI UV index to a reference UVI observed on the ground with Solar Light 501 broadband radiometers. Cloudiness during satellite overpass was determined with auxiliary ground based observations on sunshine duration, cloud cover and global radiation as well as the satellite based MODIS cloud cover estimates. The analysis aimed to minimize the error sources from temporal discrepancies and from the differences in the field of view of OMI and its ground based reference data. As a result, OMI UV product was seen to overestimate surface UV index by 21% in average and overcast UV index up to 56%. The study confirms that OMI UV index is overestimated compared to ground based reference, and shows, that the bias is related to cloudiness and is higher during well defined overcast conditions.