BROADCASTS.com

The influence of cloud inhomogeneity on the remote sensing of cloud parameters and the consequences for the determination of the radiation budget in the solar spectral range are studied. Standard techniques of remote sensing are based on simplifying assumptions on radiative transport: clouds are assumed to be homogeneous throughout each pixel and the interaction between pixels is neglected. The quantification of the resulting uncertainties is of major concern considering the potential of remote sensing for a global characterisation of clouds and their interaction with the radiation field. For this purpose, the three-dimensional radiative transport (using a Monte Carlo model) and remote sensing are simulated for a number of realistic cloud structures. The latter are based on high resolution measurements (15 m horizontal resolution) of marine stratus and stratocumulus from the airborne spectrometer CASI. The development of a novel method is described that allows for the derivation of a horizontal distribution of liquid water path, of a profile of microphysics, and of a realistic cloud top geometry. Based on these cloud structures a systematic investigation of a standard remote sensing technique for the simultaneous derivation of optical thickness and effective droplet size is conducted for different sensor geometries (resolution, viewing angle). While the systematic deviation for the optical thickness of overcast pixels is always lower than 5%, the bias increases for partially covered pixels (10-30%). In contrast, the uncertainty for individual pixels can reach more than 50%. The effective radius is systematically underestimated by 3 to 5%. If the solar part of the radiation budget, e.g. the scene reflection, is determined based on these data, deviations from the actual situation between 3 and 10% do occur.