Improvement of the FRESCO O2 A-band cloud retrieval algorithm for GOME and SCIAMACHY

GOME on ERS-2 and SCIAMACHY on ENVISAT are two UV/visible spectrometers with spectral resolution about 0.2-0.4 nm. SCIAMACHY also has near Infrared channels. GOME and SCIAMACHY are widely used in atmospheric composition studies. However with GOME and SCIAMACHY we have an unique opportunity for an independent measurement of clouds, namely via the absorption of oxygen. Since oxygen is a well-mixed gas, the reflectance in and around the oxygen A-band is a direct measurement of the amount and altitude of clouds.

FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) cloud algorithm retrieves cloud top pressure and effective cloud fraction by simulating the measured reflectance at TOA from 758-766 nm. Until now in FRESCO only O₂ absorptions are considered in the reflectance simulation. The reflectances are pre-calculated, and stored as a look up table. The FRESCO products are mainly used in the cloud correction for trace gas retrievals from GOME and SCIAMACHY measurements. Recent work also shows the possibility to perform climate studies using FRESCO cloud products.

We present an improvement of the FRESCO cloud algorithm by adding the single Rayleigh scattering in the reflectance database and the retrieval. The reflectances are calculated using the latest HITRAN 2004 database and the line-by-line calculations are also improved.

The new reflectance database has been compared with multiple scattering radiative transfer simulations with the DAK model (Doubling Adding KNMI). We have tested the new FRESCO algorithm with SCIAMACHY measurements. In the new version of FRESCO the change in the effective cloud fraction is smaller than 0.05. The main improvement is in the cloud top pressure for less cloudy pixels.