A study of the absorption and extinction properties of hexagonal ice columns and plates in random and preferred orientation, using exact T-matrix theory and aircraft observations of cirrus

Absorption and extinction properties of the finite hexagonal ice column and hexagonal ice plate in random and preferred orientation are studied at the wavelength of 80 mum using a new implementation of exact T-matrix theory. For the case of random orientation at size parameters around two, it is sho wn that the hexagonal ice column and hexagonal ice plate absorption resonan ces are diminished relative to Mie theory, and the same behaviour is also n oted for an aggregate particle consisting of eight hexagonal elements. The absorption properties of the aggregate particle have been calculated using the finite-difference time-domain method. It is also shown that extinction and absorption solutions for the hexagonal ice column and hexagonal ice pla te can differ significantly if incidence occurs perpendicular or parallel t o the cylindrical axis of the hexagon. For the case of perpendicular incide nce on the edge of the hexagon, absorption solutions can exceed those of Mi e theory, and for the case of parallel incidence, behaviour of the extincti on solutions for hexagonal ice columns and hexagonal ice plates is shown to be similar to previously published work based on the prolate and oblate sp heroid. Interference structure, associated with surface waves, is resolved on the hexagonal column extinction solution and the hexagonal plate absorpt ion solution, thereby demonstrating that surface waves can exist on a non-a xisymmetric geometry. The usefulness of assuming the hexagonal ice column i n retrieval of ice crystal effective size is also investigated using aircra ft based radiometric observations of semi-transparent cirrus at the wavelen gths of 8.5 and 11 mum (C) 2001 Elsevier Science Ltd. All rights reserved.