On the Sensitivity of Satellite-Derived Cloud Properties To Sensor Resolution and Broken Clouds

The sensitivity of cloud properties derived from meteorological imagers to sensor resolution is investigated by a comparison of results obtained from 1x1km2 versus down-sampled 3x3km2 resolution MODIS reflectances. Focus is put on deviations caused by the plane-parallel albedo bias for completely overcast, single-phase pixels (67% of cloudy pixels), and by broken clouds (20% of cloudy pixels). For pixels corresponding to overcast water clouds, a low bias of -5.6% in cloud optical thickness and -3.8% in liqud water path is observed. A method to significantly reduce these biases is described, if information on unresolved variability in reflectance is available. For broken clouds, retrieval results are found to be rather unreliable. Again, a significant improvement in accuracy is achieved, if information on the sub-pixel

cloud fraction is used in the retrieval.