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

[摘要] In radiative transfer simulations the simplification of cloud top structureby homogeneous assumptions can cause mistakes in comparison to realisticheterogeneous cloud top structures. This paper examines the influence ofcloud top heterogeneity on the radiation at the top of the atmosphere. Theuse of cloud top measurements with a high temporal resolution allows theanalysis of small spatial cloud top heterogeneities by using the frozenturbulence assumption for the time – space conversion. Radiative observationsare often based on satellite measurements, whereas small spatial structuresare not considered in such treatments. A spectral analysis of the cloud topmeasurements showed slopes of power spectra between –1.8 and –2.0, thesevalues are larger than the spectra of –5/3 which is often applied togenerate cloud field variability. The comparison of 3-D radiative transferresults from cloud fields with homogeneous and heterogeneous tops has beendone for a single wavelength of 0.6 μm. The radiative transfercalculations result in lower albedos for heterogeneous cloud tops. Thedifferences of albedos between heterogeneous and homogeneous cloud topdecrease with increasing solar zenith angle. The influence of cloud topvariability on radiances is shown. The reflectances for heterogeneous topsare 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, whichis 30% of illumination.

[学科分类] 大气科学