LONGWAVE RADIATIVE PROPERTIES OF POLYDISPERSED HEXAGONAL ICE CRYSTALS

With the use of the anomalous diffraction approximation, analytical ex pressions for the absorption and scattering coefficients and for the s ingle scattering albedo of a polydispersion of horizontally oriented h exagonal columns and hexagonal plates are derived. By comparison of th e anomalous diffraction results with those obtained using the dipole s uperposition method, it is estimated that the accuracy of the derived expressions are within 5% in the two narrow bands from 2.77 to 3.02 an d from 10.6 to 11.1 mu m and within 15% from 0.8 to 3.05 from 5.4 to 6 .1 and from 8.3 to 11.9 mu m. The equivalent spheres of equal volume o r of equal surface area do not provide a suitable approximation for th e scattering properties of the polydispersion of horizontally oriented hexagonal columns or plates. The errors due to the use of equivalent spheres in calculation of the absorption and scattering coefficients a nd in single scattering albedo are typically of the order of several h undred percent. Equal volume or equal surface area cylinders approxima te well the scattering proprieties of hexagonal columns; however, they are not suitable as equivalent particles for the case of hexagonal pl ates.