Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
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7
11
2007
10.5194/acp-7-2881-2007
http://www.atmos-chem-phys.net/7/2881/2007/
http://www.atmos-chem-phys.net/7/2881/2007/acp-7-2881-2007.html
http://www.atmos-chem-phys.net/7/2881/2007/acp-7-2881-2007.pdf
2881
2891
2007-06-11
Technical Note: The effect of sensor resolution on the number of cloud-free observations from space
J. M. Krijger
krijger@sron.nl
M. van Weele
I. Aben
R. Frey
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
KNMI, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, Madison, WI, USA
Air quality and surface emission inversions are likely to be focal points for future satellite
missions on atmospheric composition. Most important for these applications is sensitivity to
the atmospheric composition in the lowest few kilometers of the troposphere. Reduced sensitivity
by clouds needs to be minimized. In this study we have quantified the increase in number
of useful footprints, i.e. footprints which are sufficient cloud-free, as a function of sensor resolution (footprint area).
High resolution
(1 km×1 km) MODIS TERRA cloud mask observations are aggregated to lower resolutions. Statistics for
different thresholds on cloudiness are applied. For each month in 2004 four days of MODIS data are analyzed.
Globally the fraction of cloud-free observations drops from 16% at 100 km<sup>2</sup> resolution to only 3% at
10 000 km<sup>2</sup> if not
a single MODIS observation within a footprint is allowed to be cloudy. If up to 5% or 20% of a
footprint is allowed to be cloudy, the fraction of cloud-free observations is 9% or 17%, respectively, at
10 000 km<sup>2</sup> resolution. The probability of finding cloud-free observations for different sensor
resolutions is also quantified as a function of geolocation and season, showing examples over
Europe and northern South America (ITCZ).
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