A radiative transfer case study for 3-d cloud effects in the UV

Satellite UV mapping is usually based on the independent pixel approximatio n (IPA) which neglects horizontal photon transport between adjacent columns . Horizontal inhomogeneity of cloud fields therefore causes uncertainties i n the derived UV radiation fields. While these effects are small for large pixel satellites, the broken-cloud errors increase as the pixel size decrea ses. By comparing results of 1-d and 3-d UV radiative transfer calculations for three selected cloud scenes that cover a rather broad range of cloud i nhomogeneity the main 3-d cloud effects on the atmospheric UV transmission are identified and quantified in their order of magnitude. With respect to the different spatial resolutions of satellite instruments it is further sh own how 3-d cloud effects average out with increasing spatial scab. It turn s out that locally the IPA cause maximum uncertainties up to +/- 100% for a spatial resolution of about 1 x 1 km(2) (e.g., AVHRR), they are reduced to +/- 10% for a resolution of about 15 x 15 km(2) and below 5% for a resolut ion greater than 30 km (e.g., TOMS).