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Influence of clouds on OMI satellite total daily UVA exposure over a 12-year period at a southern hemisphere site

ABSTRACT This research investigated and evaluated the influence of clouds on the total daily UVA (320–400 nm) exposures calculated from the three Ozone Monitoring Instrument (OMI) UV spectral irradiances at solar noon. These evaluated satellite total daily UVA exposure data were compared to the total daily UVA exposures of a ground-based instrument over the period of October 2004 to December 2014 at a sub-tropical Australian site (27.5°S, 151.9°E) under all cloud cover conditions including sun obscured and not obscured conditions. The aim was to evaluate the influence of clouds on the total daily UVA. When the sun was not obscured by clouds, there was good agreement between satellite and ground-based daily UVA exposure measurements with coefficient of determination (R2) between 0.80 and 0.84, for the cloud conditions 0 to 2, >2 to 4, >4 to 6 and >6 to 8 oktas. For sun obscured by clouds, the R2 was 0.71, 0.64 and 0.75, respectively, for >2 to 4, >4 to 6 and >6 to 8 oktas. The method was validated using total daily UVA exposures from ground measurements taken in 2015 and 2016 giving a mean absolute error of 84.2 kJ m−2 (10%) and 138.8 kJ m−2 (30%) respectively, for the cases of sun not obscured cloudy days and sun obscured by cloud cover. Total daily UVA exposures were able to be calculated from the OMI satellite spectral irradiance for all cloud conditions, including cases where the sun was obscured, demonstrating the potential application of the technique to be applied in locations that do not record surface UVA measurements directly.