Modeling of Scattering and Absorption by Nonspherical Cirrus Ice Particles at Thermal Infrared Wavelengths

Abstract

This paper examines 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, for randomly oriented nonspherical particles, Mie theory using equivalent ice spheres tends to overestimate the absorption efficiency while the anomalous diffraction theory (ADT) and the geometric optics method (GOM) tend to underestimate it. The absorption efficiency is not sensitive to the particle shape when the size parameter is large. Herein a composite scheme is used that is valid for nonspherical particles with a wide range of size parameters. This scheme is a composite of Mie theory, GOM, and ADT to fit the single-scattering properties of hexagonal particles derived from the GOM for large size parameters and the finite-difference time domain technique for small size parameters. Applying this composite technique, errors in the broadband emissivity of cirrus clouds associated with conventional approaches are examined. It is shown that, when the projected area is preserved, Mie results overestimate the emissivity of cirrus clouds while, when the volume is preserved, Mie results underestimate the emissivity. Mie theory yields the best results when both projected area and volume are preserved (the relative errors are less than 10%). It is also shown that the ADT underestimates cirrus cloud emissivity. In some cases, the relative errors can be as large as 30%. The errors in the GOM are also significant and are largely a result of nonspherical particles with size parameters smaller than 40.