Uncertainties in satellite-derived estimates of surface UV doses

Satellite-derived maps of surface UV irradiance are currently limited by th eir poorly assessed accuracy. Here we use an extensive data set of ground-b ased spectral measurements from two Alpine sites to examine the level of un certainty expected in model results due to the time-limited nature of some satellite data. When the ozone column, cloud optical depth, and aerosol opt ical depth supplied to a radiative transfer model are fixed to a single noo ntime value, the root-mean-square difference between calculations and measu rements of the erythemal daily dose is about 20%. The corresponding uncerta inty in the monthly dose is less than 5%. The modeled results also show a s ystematic error that depends on cloud optical depth. The results suggest th at satellite-derived maps of UV irradiance cannot be expected to produce ac curate values of the daily dose if they rely on a single estimate of the cl oud conditions but may be able to provide reasonable estimates of the month ly dose.