page last modified:
27 September 2004
Clouds influence the depth of absorption lines of a trace gas in the
spectrum, in particular if the trace gas concentration is high in the
troposphere. For the retrieval of tropospheric trace gas columns it is
therefore essential to have information on the cloud cover conditions. Both
the cloud fraction and the cloud top pressure are needed as input for the
trace gas retrieval algorithms. For the TEMIS project, the
(effective) cloud fraction and cloud top pressure
are derived with the algorithm called Fast Retrieval
Scheme for Clouds from the Oxygen A band (FRESCO).
The FRESCO algorithm
The cloud top pressures and effective cloud fractions shown here are derived
from calibrated level-1 reflectivity data of the GOME or SCIAMACHY spectrum
of the oxygen A-band (between 758-775 nm). Three one-nanometer wide parts of
the oxygen A-band spectrum are used in the FRESCO near-real time retrieval,
both inside and outside the oxygen A-band, namely at 758 nm (no absorption),
761 nm (strong absorption), and 765 nm (moderate absorption).
The reflectivity outside the oxygen A-band is almost independent of cloud
top pressure, but depends mainly on cloud fraction, cloud optical thickness,
and surface albedo. The reflectivities inside the band depend on cloud top
pressure as well, and are used to derive cloud top pressure. An effective
cloud fraction and cloud top pressure are derived for each GOME or SCIAMACHY
pixel using non-linear least-squares fitting of a measured spectrum to a
The effective cloud fractions are derived by assuming that the clouds have
an albedo of 0.8, and must therefore be interpreted as effective cloud
fractions. Note that the derived cloud top pressures are rather insensitive
to the assumed cloud albedo.
For areas with effective cloud fractions smaller than 0.05, cloud top
pressures cannot be derived reliably. These areas show up in blue in the
effective cloud fraction and cloud top pressure maps.
No attempt is made to account for the presence of snow, ice, or sun-glint.
Thus if cloud-free land or ocean is covered by snow or ice shelves, or if a
pixel is affected by sun-glint, these areas will show up as having
low-altitude clouds with high cloud coverage.
- Koelemeijer, R. B. A.,
P. Stammes, J. W. Hovenier, and J. F. de Haan, "A fast method for retrieval of cloud
parameters using oxygen A band measurements from the Global Ozone Monitoring Experiment",
J. Geophys. Res. 106, 3475-3490, 2001.
- Koelemeijer, R. B. A.,
P. Stammes, J. W. Hovenier, and J. F. de Haan, "Global distributions of effective cloud
fraction and cloud top pressure derived from oxygen A band spectra measured by the Global
Ozone Monitoring Experiment: comparison to ISCCP data," J. Geophys. Res. 107,
doi 10.1029/2001JD000840, 2002.