Remote sensing of optical and microphysical properties of cirrus clouds using Moderate-Resolution Imaging Spectroradiometer channels: Methodology andsensitivity to physical assumptions

A methodology for the retrieval of cirrus cloud microphysical and optical p roperties based on observations of reflected sunlight is introduced. The re trieval method is based on correlation of the bidirectional reflectance of three channels, 0.65, 1.6, and 2.2 mu m, that are available onboard Earth O bserving System (EOS) Moderate-Resolution Imaging Spectroradiometer (MODIS) . Validation studies using microphysical measurements and MODIS airborne si mulator (MAS) observations illustrate the nature of the potential errors as sociated with the retrieved optical depth and mean effective ice crystal si ze. The effects of the physical assumptions involving ice crystal size dist ribution and shape employed in the algorithm are subsequently assessed. In terms of the microphysical models used for radiation calculations the ice c rystal shape assumption is found to have the most significant impact on the retrieved parameters. The effect of the background surface reflectance on the retrieval results is further examined, and we show that in order to rel iably infer nonblack cirrus parameters from solar reflectance measurements it is essential to properly account for the background radiation over both land and ocean surfaces. Finally, we present the measured ice microphysical data for tropical cirrus as a function of cloud development and ambient te mperature to illustrate the importance of vertical inhomogeneity for valida tion studies.