Global distribution of effective cloud fraction and cloud top pressure derived from oxygen A band spectra measured by the Global Ozone Monitoring Experment: Comparison to ISCCP data

Satellite measurements of the Earth's reflected sunlight in the oxygen A band around 761 nm, as measured by the Global Ozone Monitoring Experiment (GOME) on board ERS-2, have been analyzed to obtain global distributions of effective cloud fraction and cloud top pressure for July 1995 and January 1997. These quantities have been compared to effective cloud fractions and cloud top pressures derived by the International Satellite Cloud Climatology Project (ISCCP D2), averaged over the July and January months from 1989 to 1993. We have found a reasonable agreement between the GOME O2 A band cloud results and the climatological ISCCP D2 data. The global monthly average difference in effective cloud fraction is 0.01 for July and vanishes for January; the standard deviation of the difference is 0.10 for both months (absolute values). These effective cloud fractions hold under the assumption of optically thick clouds and are typically 2 times smaller than the real cloud fractions. The global monthly average difference in cloud top pressure is 27 hPa (July) and 36 hPa (January); the standard deviation of the difference is 110 hPa (July) and 104 hPa (January). Largest differences are found over land surfaces.