FRESCO+: an improved O2 A-band cloud retrieval algorithm for tropospheric trace gas retrievals P. Wang1, P. Stammes1, R. van der A1, G. Pinardi2, and M. van Roozendael2 1Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands 2BIRA-IASB, Belgian Institute for Space Aeronomy, Brussels, Belgium
Abstract. 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 show
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 density (VCD) retrievals 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
NO2VCDs 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
Citation: Wang, P., Stammes, P., van der A, R., Pinardi, G., and van Roozendael, M.: FRESCO+: an improved O2 A-band cloud retrieval algorithm for tropospheric trace gas retrievals, Atmos. Chem. Phys., 8, 6565-6576, doi:10.5194/acp-8-6565-2008, 2008.