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

Abstract

Abstract. A methodology for the retrieval of cirrus cloud microphysical and optical properties based on observations of reflected sunlight is introduced. The retrieval method is based on correlation of the bidirectional reflectance of three channels, 0.65, 1.6, and 2.2 /xm, that are available onboard Earth Observing System (EOS) Moderate-Resolution Imaging Spectroradiometer (MODIS). Validation studies using microphysical measurements and MODIS airborne simulator (MAS) observations illustrate the nature of the potential errors associated with the retrieved optical depth and mean effective ice crystal size. The effects of the physical assumptions involving ice crystal size distribution and shape employed in the algorithm are subsequently assessed. In terms of the microphysical models used for radiation calculations the ice crystal 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 reliably 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 temperature to illustrate the importance of vertical inhomogeneity for validation studies. IntroductionIn view of their physical location and the complex nature of their optical and physical properties, cirrus clouds have been identified as one of the major unsolved components in weather and climate research [Liou, 1986[Liou, , 1992. The influence of cirrus clouds on the radiation field of the Earth-atmosphere system depends on the solar and thermal IR radiative properties, referred to as the greenhouse versus albedo effect, which in turn are modulated by the cloud composition and physical location in the atmosphere. Thus, in order to quantify the radiative effects of cirrus, microphysical (ice crystal size distribution and shape), structural (height and spatial extent), and optical (optical depth) properties are required. A realistic treatment of cirrus cloud physical and radiative properties is essential to achieve physically based climate modeling and predictions. Satellite remote-sensing methods must be developed and validated to provide the required cirrus cloud parameters on a global scale in conjunction with general circulation models (GCMs) and climate modeling development. A number of approaches based on the principles of radiativeCopyright 2000 by the American Geophysical Union. Paper number 2000JD900028.0148-0227/00/2000JD900028509.00 transfer have been developed to infer cloud optical depth and mean effective ice crystal size from satellites. They can be divided into two groups, with some overlap: reflection and emission techniques. The former relies on the reflectance characteristics of solar wavelengths, implying daytime use, while the latter uses radiation emitted in...

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