Synergetic use of NOAA/AVHRR and Meteosat cloud information for space-based UV measurements

An efficient method for calculating and mapping surface UV radiation by use of satellite data and based on the independent pixel approximation has been developed. The method principally allows to take into account the total ozone column amount, the cloud optical depth, the surface elevation and albedo (snow, no snow) and the solar zenith angle. It is currently used for creating a UV climatology to study the effects of UV daily doses on marine ecosystems around the Norwegian Lofoten Islands. This study focuses on the temporal sampling of the cloud field and its effects on the calculation of UV daily doses. Three-dimensional radiative transfer calculations have been performed to estimate how many satellite overpasses or slots are needed to derive daily UV doses within certain limits of error. Shown are results for a synthetic cloud field having a pronounced asymmetry in cloud cover around noon time. For this case the daily dose can be derived with an error of 10% if the cloud field is at minimum probed every 60 minutes within the time interval from 10:00 to 14:00 assuming a constant cloud cover from sunrise to 10:00 and from 14:00 to sunset. Such a time sampling may be obtained from Meteosat data or from a combination of Meteosat and NOAA/AVHRR data. By using only NOAA/AVHRR data with a temporal resolution of typically 100 minutes at least 3 overpasses are needed to obtain the 10% relative difference between the derived daily dose and the exact reference value.