Satellite-based remote sensing of cirrus clouds: hyperspectral radiative transfer modeling, analysis of uncertainties in in-situ cloud extinction measurements and intercomparison of cirrus retrievals from a-train instruments

This dissertation consists of three parts, each devoted to a particular\nissue of significant importance for satellite-based remote sensing of cirrus clouds.\nIn the first part, we develop and present a fast infrared radiative transfer\nmodel on the basis of the adding-doubling principle. The model aims to facilitate\nthe radiative transfer computations involved in hyperspectral remote sensing\napplications. The model is applicable to a variety of cloud conditions, including\nvertically inhomogeneous or multilayered clouds. It is shown that for\nhyperspectral applications the model is two order-of-magnitude faster than the\nwell-known discrete ordinate transfer (DISORT) model, while maintains a similar\naccuracy.\nThe second part is devoted to the investigation of uncertainties in the\nFSSP (Forward Scattering Spectrometer Probe) measurement of cloud extinction\nby small ice particles. First, the single-scattering properties of small ice particles\nin cirrus clouds are derived and compared to those of equivalent spheres according to various definitions. It is found that, although small ice particles in\ncirrus clouds are often “quasi-spherical”, their scattering phase functions and\nasymmetry factors are significant different from those of ice spheres. Such\ndifferences may lead to substantial underestimation of cloud extinction in FSSP\nmeasurement, if small ice particles are assumed to be spheres.\nIn the third part, we present a comparison of cirrus cloud optical thickness\nretrievals from two important instruments, MODIS (Moderate Resolution Imaging\nSpectrometer) and POLDER (Polarization and Directionality of Earth’s\nReflection), on board NASA’s A-train satellite constellation. The comparison\nreveals a large difference. Several possible reasons are discussed. It is found\nthat much of the difference is attributable to the difference between the MODIS\nand POLDER retrieval algorithm in the assumption of cirrus cloud bulk scattering\nproperties. Potential implications of the difference for climate studies are\ninvestigated. An important finding is that the use of an unrealistic cirrus bulk\nscattering model might introduce artificial seasonal variation of cirrus optical\nthickness and shortwave radiative forcing into the retrieval.