Technical Note: The effect of sensor resolution on the number of cloud-free observations from space J. M. Krijger1, M. van Weele2, I. Aben1, and R. Frey3 1SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 2KNMI, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands 3Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, Madison, WI, USA
Abstract. 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).
(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 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 (ITCZ).
Citation: Krijger, J. M., van Weele, M., Aben, I., and Frey, R.: Technical Note: The effect of sensor resolution on the number of cloud-free observations from space, Atmos. Chem. Phys., 7, 2881-2891, doi:10.5194/acp-7-2881-2007, 2007.