Development and Testing of A Radiation Model for Interpreting ARM Data Page: 3 of 9
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Light scattering by spherical particles in an absorbing medium occurs in the earth-
atmosphere system, such as cloud particles surrounded by water vapor, and air bubbles in
the ocean and sea ice. We have developed analytic solutions for the single-scattering
properties of a spherical particle embedded in an absorbing medium (Fu and Sun 2001a).
We have derived absorption and scattering efficiencies by using the near filed at the
particle surface, which avoids difficulty in obtaining the extinction based on the optical
theorem. We found that an absorbing medium can significantly affect the light scattering
by a sphere, but have little effect on the particle absorption.
*To improve our understanding of radiative properties of nonspherical ice particles, we
have developed a new three-dimensional finite-difference time-domain (FDTD) scheme
for light scattering by nonspherical dielectric particles (Sun and Fu 2000). The perfectly
matched layer (PML) absorbing boundary condition (ABC) is used to truncate the
computational domain. As a result, the present FDTD program requires much less
memory and CPU time than those using traditional truncation techniques. Appropriate
treatments of the particle surface and related electric fields are also introduced to improve
the FDTD accuracy. For spheres with size parameters as large as 40, we show that the
errors in extinction and absorption efficiencies due to the FDTD are less than ~1% and
the errors in asymmetry factor are less than 0.1%.
Using the FDTD for nonspherical particles with small size parameters and geometric
optics method (GON4) for large size parameters, we have examined a number of
commonly used methods for the calculation of the scattering and absorption properties of
nonspherical ice crystals at thermal infrared wavelengths. It is found that the anomalous
diffraction theory and GOM can underestimate the cirrus emissivity by as large as 30%
and 15%, respectively. The Mie theory can either underestimate or overestimate the
cirrus emissivity, depending on the definition of equivalent spheres.
*We have examined the outgoing longwave radiation (OLR) bias due to the neglect of
cloud horizontal inhomogeneities (Fu et al. 2000b). We found that the OLR bias is most
significant for cirrus clouds which are semi-transparent and located in the cold upper
troposphere. For two cirrus cases observed at the ARM SGP CART site by cloud radar
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FU, Qiang. Development and Testing of A Radiation Model for Interpreting ARM Data, report, November 1, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc780478/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.