Cloud cover fraction
As additional service to interpret the SO2 vertical column densities,
the web pages that show images of the SO2 data also show an image
of the cloud cover fraction for the same region, taken from the same
instrument. Note that currently there is no cloud screening performed
on the data: all SO2 slant column retrieval results are shown as-is.
The same cloud fraction is also used to determine the air-mass factor (AMF)
when taking the AMF from look-up tables made with a
radiative transfer model in order to compute the SO2 vertical column
density. Note that the AMF and thus the vertical column cannot be calculated
of the cloud cover fraction is missing.
For the daily data at orbit (like the image shown here) and at grid
coordinates the cloud cover fraction is presented in the same way and available in the
accompanying data files. Monthly averages if the cloud fraction are not
made: there is no cloud screening method applied to the SO2 data, so that a
monthly average cloud fraction does not provide any useful additional
The cloud fraction is a number between zero (clear-sky: no clouds) and one
(fully clouded, also known as overcast). For the Volcanic & Air Quality
SO2 Services the cloud data is taken from existing data products:
For SCIAMACHY data, the cloud
fraction is taken from the data files generated by the FRESCO algorithm,
which derives the cloud data from the Oxygen A-band (between 758-775 nm).
See this page at the TEMIS website
for more information and data.
The FRESCO cloud data is either in "normal mode" or in "snow/ice mode". In
the latter case, a cloud fraction of one is assumed with the cloud at ground
level when determining the AMF for the SO2 vertical column. In the
datafiles, however, the cloud fraction in the "snow/ice mode" is set to
'-1', and it is shown in a separate colour in the cloud fraction images
(indicated simply as "snow" in the colour bar).
Note that the FRESCO cloud cover data retrieval stricktly speaking is based
on sunlight reflected by a combination of clouds and (reflecting) aerosols:
such aerosols are effectively treated as clouds. This means that the FRESCO
data product contains a (small) aerosol contribution.
For OMI data, the cloud fraction is taken from the level-2 data product
provided by NASA/NOAA.
The "quality flags" of the ground pixel in that data product gives the
presence of snow or ice. The following situations are taken to represent the
"snow/ice mode": permanent ice, dry snow, and a sea ice concentration of
more than 50% -- all other cases are considered to be "normal mode"
The GOME-2 SO2 data are generated in near-real time at DLR and that data
product contains cloud information based on the OCRA/ROCINN method,
developed at DLR. The maps on the web pages show the cloud cover fraction as
determined by OCRA.