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Description:
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This thesis is made up of three parts on the computation of scattering properties of nonspherical
particles in the atmosphere . In the first part , a new crystal type -droxtal -is introduced to make a better
representation of the shape of small ice crystals in the uppermost portions of midlatitude and tropical cirrus
clouds . Scattering properties of droxtal ice crystals are investigated by using the Improved -Geometric Optic
(IGO ) method . At the visible wavelength , due to the presence of the hexagonal structure , all elements of the
phase matrix of droxtal ice crystals share some common features with hexagonal ice crystals , such as 220
and 460 halos . In the second part of this thesis , the possibility of enhancing the performance of current
Anomalous Diffraction Theory (ADT ) is investigated . In conventional ADT models , integrations are
usually carried out in the domain of the particle projection . By transforming the integration domain to the
domain of scaled projectile length , the algorithm of conventional ADT models is enhanced . Because the
distribution of scaled projectile length is independent of the particle's physical size as long as the shape
remains the same , the new algorithm is especially efficient for the calculation of a large number of particles
with the same shape but different sizes . Finally , in the third part , the backscattering properties of
nonspherical ice crystals at the 94GHz frequency are studied by employing the Finite -Difference Time -
Domain (FDTD ) method . The most important factor that controls the backscattering cross section is found
to be the ratio of the volume -equal radius to the maximum dimension of the ice crystal . Substantial
differences in backscattering cross sections are found between horizontal orientated and randomly oriented
ice crystals . An analytical formula is derived for the relationship between the ice water (IWC ) content and
the radar reflectivity ( e Z ) . It is shown that a change to the concentration of ice crystals without any
changes on the size distribution or particle habits leads only to a linear e Z IWC - relationship . The famous
power law e Z IWC - relationship is the result of the shift of the peak of particle size distribution . |