Cloud flagging of UV spectral irradiance measurements

Cloud flags can be used as identifiers to classify measured spectra of UV irradiance as to the conditions of cloudiness during the time of the spectral measurement. That classification is helpful for quality checks of spectra as well as for their further analysis and comparisons with model calculations. A method has been developed to derive cloud flags for UV spectra. It uses readily available ancillary measurements such as direct (or global) and diffuse radiation, relative sunshine duration and hourly observations of cloud cover and visibility as input data. Application of the cloud flagging algorithm to 16,217 UV spectra measured with Brewer spectroradiometers MW and MKIII at the station Potsdam in the years 1995 and 1996 classified 807 UV spectra (5%) as 'clear sky', with almost all of the remaining data, i.e., 15,384 (95%) being classified as 'cloudy', and-due to missing part of corresponding ancillary data-26 spectra classified as 'unknown' with referring to cloud conditions. Among the combinations of ancillary data tested, both direct/diffuse ratios and sky brightness turned out to be the most useful parameters for cloud detection including Cirrus clouds. They can also be used to select those spectra distorted by moving clouds. In addition to the simple 'clear sky vs, cloudy' classification, direct/diffuse ratios together with sky brigthness have been tentatively used as potential classifiers to separate five different conditions of cloudiness as to their optical effect. The cloud flagging algorithm is neither confined to UV spectra nor to meteorological data of a specific site, but it can be applied to any meteorological data at any other site. (C) 1998 Elsevier Science B.V. All rights reserved.