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