A comparison of MODIS-derived cloud amount with visual surface observations

Two main sources for global cloud climatologies are visual surface observations and observations made by spaceborne sensors. Satellite observations compared with surface data show in most cases differences ranging from - 15% upto - 1%, depending on sensor and observation conditions. These differences are partially controlled by sensors' cloud detection capabilities - a higher number of spectral bands and higher spatial resolution are believed to allow discrimination of clouds from land/ocean/snow background. A Moderate-Resolution Imaging Spectroradiometer (MODIS) produces images of the atmosphere in 36 spectral bands with a spatial resolution of 2501000 m, thus having a capacity for cloud detection far more advanced than other operating sensors. In this study, instantaneous MODIS cloud observations were compared with surface data for Poland for January (winter) and July (summer) 2004. It was found that MODIS observed 4.38% greater cloud amount in summer conditions and 7.28% in winter conditions. Differences were greater at night (7-8%) than in daytime (0.5-7%) and correlations ranged between 0.577 (winter night) and 0.843 (winter day, summer day and night). (C) 2009 Published by Elsevier B.V.