The influence of cloud top variability from radar measurements on 3-D radiative transfer

In radiative transfer simulations the simplification of cloud top structure by homogeneous assumptions can cause mistakes in comparison to realistic heterogeneous cloud top structures. This paper examines the influence of cloud top heterogeneity on the radiation at the top of the atmosphere. The use of cloud top measurements with a high temporal resolution allows the analysis of small spatial cloud top heterogeneities by using the frozen turbulence assumption for the time – space conversion. Radiative observations are often based on satellite measurements, whereas small spatial structures are not considered in such treatments. A spectral analysis of the cloud top measurements showed slopes of power spectra between –1.8 and –2.0, these values are larger than the spectra of –5/3 which is often applied to generate cloud field variability. The comparison of 3-D radiative transfer results from cloud fields with homogeneous and heterogeneous tops has been done for a single wavelength of 0.6μm. The radiative transfer calculations result in lower albedos for heterogeneous cloud tops. The differences of albedos between heterogeneous and homogeneous cloud top decrease with increasing solar zenith angle. The influence of cloud top variability on radiances is shown. The reflectances for heterogeneous tops are explicitly larger in forward direction, in backward direction lower. The largest difference of the mean reflectances (mean over cloud field) between homogeneous and heterogeneous cloud top is approximately 0.3, which is 30% of illumination.