The influence of cloud top variability from radar measurements on 3-D radiative transfer F. Richter1, K. Barfus1, F. H. Berger1,2, and U. Görsdorf2 1TU Dresden, Faculty of Forest, Geo and Hydro Sciences, Institute of Hydrology and Meteorology, Dresden, Germany 2German Meteorological Service, Lindenberg, Germany
Abstract. 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 0.6 μm. 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.
Citation: Richter, F., Barfus, K., Berger, F. H., and Görsdorf, U.: The influence of cloud top variability from radar measurements on 3-D radiative transfer, Atmos. Chem. Phys., 7, 4699-4708, doi:10.5194/acp-7-4699-2007, 2007.