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

摘要

This dissertation consists of three parts, each devoted to a particularissue of significant importance for satellite-based remote sensing of cirrus clouds.In the first part, we develop and present a fast infrared radiative transfermodel on the basis of the adding-doubling principle. The model aims to facilitatethe radiative transfer computations involved in hyperspectral remote sensingapplications. The model is applicable to a variety of cloud conditions, includingvertically inhomogeneous or multilayered clouds. It is shown that forhyperspectral applications the model is two order-of-magnitude faster than thewell-known discrete ordinate transfer (DISORT) model, while maintains a similaraccuracy.The second part is devoted to the investigation of uncertainties in theFSSP (Forward Scattering Spectrometer Probe) measurement of cloud extinctionby small ice particles. First, the single-scattering properties of small ice particlesin cirrus clouds are derived and compared to those of equivalent spheres according to various definitions. It is found that, although small ice particles incirrus clouds are often ?quasi-spherical?, their scattering phase functions andasymmetry factors are significant different from those of ice spheres. Suchdifferences may lead to substantial underestimation of cloud extinction in FSSPmeasurement, if small ice particles are assumed to be spheres.In the third part, we present a comparison of cirrus cloud optical thicknessretrievals from two important instruments, MODIS (Moderate Resolution ImagingSpectrometer) and POLDER (Polarization and Directionality of Earth?sReflection), on board NASA?s A-train satellite constellation. The comparisonreveals a large difference. Several possible reasons are discussed. It is foundthat much of the difference is attributable to the difference between the MODISand POLDER retrieval algorithm in the assumption of cirrus cloud bulk scatteringproperties. Potential implications of the difference for climate studies areinvestigated. An important finding is that the use of an unrealistic cirrus bulkscattering model might introduce artificial seasonal variation of cirrus opticalthickness and shortwave radiative forcing into the retrieval.