Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-4465-2006 http://www.atmos-chem-phys-discuss.net/6/4465/2006/ http://www.atmos-chem-phys-discuss.net/6/4465/2006/acpd-6-4465-2006.html http://www.atmos-chem-phys-discuss.net/6/4465/2006/acpd-6-4465-2006.pdf 4465 4494 2006-06-07 The effect of sensor resolution on the number of cloud-free observations from space J. M. Krijger 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 two days of MODIS data are analyzed. Globally the fraction of cloud-free observations drops from 16% at 100 km² resolution to only 3% at 10 000 km² 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 km² 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.