Fernerkundung inhomogener Bewölkung und deren Einfluss auf die solare Strahlungsbilanz

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.