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2.4. SATELLITE GROUP 61
2.4.3 Retrieval of cloud parameters using SCIAMACHY and GOME data
Michael Grzegorski (Thomas Wagner 1 , Tim Deutschmann, Mark Wenig 2 , Ping Wang 3 , Piet
Stammes 3 )
1 also at Max Planck Institute for chemistry, Mainz
2 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3 Royal Meteorological Institute of the Netherlands, KNMI, Utrecht, The Netherlands
Abstract The retrieval of cloud parameters like cloud coverage, cloud top pressure or cloud optical
thickness from satellite is an important issue both for climatology and the analysis of tropospheric
trace gases. The HICRU algorithm improves retrieval of effective cloud fraction using data from
SCIAMACHY on ENVISAT and GOME on ERS-2. The combination of HICRU data with the DOAS
evaluation of O 2 and O 4 allows the retrieval of further cloud parameters.
Figure 2.30: RGB image of the cloud free earth retrieved by HICRU applied to SCIAMACHY data.
The PMD4 (805-900 nm) is used as green channel, because the PMD instruments only provide blue
and red in the visual wavelength bands.
Background The detection of cloud parameters
like cloud coverage, cloud top pressure or
cloud optical thickness from satellite is an important
issue: 1.) for meteorology and the investigation
of climate change and 2.) for the analysis of
tropospheric trace gases from space relevant to environmental
and climatological issues. Although
the retrieval of different cloud parameters is useful
for trace gas retrievals, especially the accurate
identification of completely cloud free regions is
crucial due to the shielding effect, which causes
an underestimation of the vertical column density
of tropospheric trace gases measured by satellite.
Methods and results The Heidelberg Iterative
Cloud Retrieval Utilities (HICRU) retrieve effective
cloud fraction by applying the widely used
threshold method combined with image sequence
analysis and an iterative approach to the socalled
Polarization Monitoring Devices (PMDs)
with higher spatial resolution compared to the
channels used for trace gas retrievals. The algo-
rithm is published in 2006 and was part of validation
studies for the official SCIAMACHY data
product. These studies show the improvement of
HICRU compared to other cloud algorithms, especially
over deserts. Problems of the next release
of the official data product from ESA are worked
out. A DOAS evaluation of O 2 and O 4 is successfully
implemented and it is proven, that the
combination with HICRU allow the retrieval of
at least one further cloud parameter (cloud top
height).
Outlook/Future work The radiation transfer
model TRACY will be used to retrieve modelled
air mass factors of O 2 and O 4 . These will be used
to retrieve a new database containing cloud top
height information.
Funding EVERGREEN (EU project)
Main publication Grzegorski et al. [2006]