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

P. Wang (1) , P. Stammes (1) , R. van Der A (1) , G. Pinardi (2) , M. van Roozendael (2)

1 KNMI - Royal Netherlands Meteorological Institute
2 BIRA-IASB - Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique

The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) algorithm has been used to retrieve cloud information from measurements of the O₂ A-band around 760 nm by GOME, SCIAMACHY and GOME-2. The cloud parameters retrieved by FRESCO are the effective cloud fraction and cloud pressure, which are used for cloud correction in the retrieval of trace gases like O₃ and NO₂. To improve the cloud pressure retrieval for partly cloudy scenes, single Rayleigh scattering has been included in an improved version of the algorithm, called FRESCO+. We compared FRESCO+ and FRESCO effective cloud fractions and cloud pressures using simulated spectra and one month of GOME measured spectra. As expected, FRESCO+ gives more reliable cloud pressures over partly cloudy pixels. Simulations and comparisons with ground-based radar/lidar measurements of clouds shows that the FRESCO+ cloud pressure is about the optical midlevel of the cloud. Globally averaged, the FRESCO+ cloud pressure is about 50 hPa higher than the FRESCO cloud pressure, while the FRESCO+ effective cloud fraction is about 0.01 larger.

The effect of FRESCO+ cloud parameters on O₃ and NO₂ vertical column densities (VCD) is studied using SCIAMACHY data and ground-based DOAS measurements. We find that the FRESCO+ algorithm has a significant effect on tropospheric NO₂ retrievals but a minor effect on total O₃ retrievals. The retrieved SCIAMACHY tropospheric NO₂ VCDs using FRESCO+ cloud parameters (v1.1) are lower than the tropospheric NO₂ VCDs which used FRESCO cloud parameters (v1.04), in particular over heavily polluted areas with low clouds. The difference between SCIAMACHY tropospheric NO₂ VCDs v1.1 and ground-based MAXDOAS measurements performed in Cabauw, The Netherlands, during the DANDELIONS campaign is about ?2.12×10¹⁴ molec cm^?2.

Domaines: Océan, Atmosphère

P. Wang, P. Stammes, R. van Der A, G. Pinardi, M. van Roozendael. FRESCO+: an improved O₂ A-band cloud retrieval algorithm for tropospheric trace gas retrievals. Atmospheric Chemistry and Physics Discussions, 2008, 8 (3), pp.9697-9729. ⟨hal-00304192⟩