Technical Note: The effect of sensor resolution on the number of cloud-free observations from space

Abstract

Air quality and surface emission inversions are likely to be focal points for future satellite missions on atmo-spheric 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 quan-tified the increase in number of useful footprints, i.e. foot-prints which are sufficient cloud-free, as a function of sen-sor resolution (footprint area). High resolution (1 km×1 km) MOD1S 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 100km2 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 observa-tions for different sensor resolutions is also quantified as a function of geolocation and season, showing examples over Europe and northern South America (ITCZ).