2008-12-18: FRESCO+: an improved O2-A band cloud retrieval algorithm for trace gas retrievals

More than 85% of the ground pixels of GOME(-2) and SCIAMACHY are influenced by clouds. Therefore, detection of clouds is essential in obtaining precise tropospheric pollution information from these satellite sensors.
Wang et al. [2008] have improved the FRESCO cloud detection algorithm by including Rayleigh scattering. Validation of the improved algorithm, FRESCO+, with groundbased radar observations shows that the FRESCO+ cloud pressure is the mid-level of clouds. It appears that FRESCO+ gives more reliable cloud pressures over partly cloudy pixels than FRESCO. Globally averaged, the FRESCO+ cloud pressure is about 50 hPa higher than FRESCO. Application of FRESCO+ to cloud correction of tropospheric NO2 measurements from SCIAMACHY shows that the algorithm works better than the previous version. FRESCO+ is now the operational cloud algorithm for the TEMIS data products from GOME and SCIAMACHY (see: http://www.temis.nl).
(a) Frequency distributions of FRESCO+ and FRESCO effective cloud fractions for one month of global GOME data in January 2000. (b) Same as (a) but for cloud pressures. The distributions are normalized to 1. The average values are given in the legends. The snow/ice pixels are not included in the distributions.