Calculation of the single-scattering properties of randomly oriented hexagonal ice columns: a comparison of the T-matrix and the finite-difference time-domain methods

We calculated the scattering and absorption properties of randomly oriented hexagonal ice columns using T-matrix theory, employing analytic orientatio n averaging, and the finite-difference time-domain method, which uses a num erical procedure to simulate random orientation. The total optical properti es calculated are the extinction efficiency, absorption efficiency, single- scattering albedo, and the asymmetry parameter. The optical properties are calculated at the wavelengths of 0.66, 8.5, and 12 mum, up to a size parame ter of 20 at 0.66 mum and 15 at the two other wavelengths. The phase-matrix elements P11, P12, and P22 are also calculated and compared, up to a size parameter of 20 at 0.66 mum and 15 at 12.0 mum. The scattering and absorpti on solutions obtained from the two independent electromagnetic methods are compared and contrasted, as well as the central processing unit time and me mory load for each size parameter. It is found that the total optical prope rties calculated by the two methods are well within 3% of each other for al l three wavelengths and size parameters. In terms of the phase-matrix eleme nts it is found that there are some differences between the T-matrix and th e finite-difference time-domain methods appearing in all three elements. Di fferences between the two methods for the P11 element are seen particularly at scattering angles from approximately 120'degrees to 180 degrees; and at the scattering angle of 180 degrees. relative differences are less than 16 %. At scattering angles less than 100 degrees, agreement is generally withi n a few percent. Similar results are also found for the P12 and P22 element s of the phase matrix. The validity of approximating randomly oriented hexa gonal ice columns by randomly oriented equal surface area circular cylinder s is also investigated in terms of the linear depolarization ratio. (C) 200 1 Optical Society of America.