Validation of GOME cloud cover fraction relevant for accurate ozone retrieval

The Global Ozone Monitoring Experiment (GOME), launched on board of ERS-2 in April 1995, is a spectrometer measuring the Earth's reflectivity between 240 and 790 nm with a spectral resolution of 0.2-0.4 nn. The spatial resolution of GOME can be varied between 40 x 40 km(2) and 40 x 320 km(2). The main geophysical product from COME is the total ozone column, which is derived from the COME reflectivity measurements using the Differential Optical Absorption Spectroscopy method. For accurate retrievals of the total ozone column, the presence of clouds has to be taken into account. Therefore, cloud cover fraction is derived from the COME reflectivity measurements around the O-2 A-band using the the Initial Cloud Fitting Algorithm (ICFA). The ICFA results are input to the COME ozone column retrieval algorithm. In this study, the ICFA cloud fraction results are validated in two ways. Firstly, a statistical approach is followed by comparing monthly averaged ICFA results with monthly averaged cloud fractions from the International Satellite Cloud Climatology Project (ISCCP). Secondly, a detailed comparison is performed for a limited number of data using cloud fractions derived from Along Track Scanning Radiometer-2 (ATSR-2) data and synoptical observations (SYNOP). The main conclusion is that the ICFA cloud fractions generally show a reasonable correlation with ISCCP, ATSR-2 and SYNOP data. However, the difference between cloud fractions over land and sea which shows up in the ISCCP results is not present in the ICFA results. Furthermore, the ICFA cloud fractions are too high over highly reflecting surfaces (e.g. desert and snow). The magnitude of errors in the COME total ozone column due to errors in cloud fraction and cloud top height has been investigated using a detailed radiative transfer model. From this analysis, it is concluded that errors in these cloud parameters may introduce errors in the ozone vertical column of typically 2-3%.