Computation of the scattering properties of nonspherical ice crystals

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Title: Computation of the scattering properties of nonspherical ice crystals
Author: Zhang, Zhibo
Abstract: 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 .
URI: http : / /hdl .handle .net /1969 .1 /1267
Date: 2004-11-15


Computation of the scattering properties of nonspherical ice crystals. Available electronically from http : / /hdl .handle .net /1969 .1 /1267 .

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