Sensitivity of passive measurements in VIS, SWIR, and TIR to cirrus microphysical vertical profile: application to cloud remote sensing from MODIS

For inferring cirrus optical and microphysical properties from satellite imagery, a common assumption is that the radiative properties of a cirrus cloud may be represented by those associated with a specific ice crystal habit, a single particle size distribution and Ice Water Content (IWC). Various algorithms have been developed to retrieve cirrus optical and microphysical properties in the past 20 years. They can be categorized into the techniques based on either thermal infrared or solar reflection measurements. However, in-situ measurements have shown that shapes, sizes and IWC of ice crystals may vary substantially with height within the clouds. Given the different sensitivity of thermal infrared and solar wavelength to cloud microphysics, it is unlikely that a single cloud layer with homogeneous cloud properties can be used to reproduce both type of measurements. Thus, it is necessary to assess the effect of vertical inhomogeneity within cirrus on the radiative transfer calculations and on the retrieval techniques. The purpose of this study is to investigate a microphysical cirrus model composed of different layers in terms of ice crystal habit, size and IWC. The vertical structure will be given by simple analytic formula derived from various prescribed physical constraints. The primary goal of this study is to determine a simple cloud model that can be used to retrieve consistent information from both solar and thermal measurements. For this purpose, we examine the sensitivity of cirrus reflectances and brightness temperature to its vertical description for a suite of MODIS (MODerate-resolution Imaging Spectroradiometer) bands spanning visible, near infrared and thermal infrared wavelengths. Results of this study are presented and potential application to remote sensing of cirrus clouds with MODIS are discussed.