Spectral signature of ice clouds in the far-infrared region: Single-scattering calculations and radiative sensitivity study

We investigate the spectral signature of ice clouds in the far‐infrared (far‐IR) spectral region from 100 to 667 cm−1 (15–100 μm). Individual particle scattering properties (extinction efficiency, absorption efficiency, and the asymmetry factor of the scattering phase function) are calculated for small particles using circular cylinders and for large crystals using hexagonal columns. The scattering properties are computed for particle sizes over a size range from 1 to 10,000 μm in maximum dimension from a combination of the T‐matrix method, the Lorenz‐Mie theory, and an improved geometric optics method. Bulk scattering properties are derived subsequently for 30 particle size distributions, with effective particle sizes ranging from 15 to 150 μm, obtained from various field campaigns for midlatitude and tropical cirrus clouds. Furthermore, a parameterization of the bulk scattering properties is developed. The radiative properties of ice clouds and the clear‐sky optical thickness computed from the line‐by‐line method are input to a radiative transfer model to simulate the upwelling spectral radiance in the far‐IR spectral region at the research aircraft height (20 km). On the basis of the simulations, we investigate the sensitivity of far‐IR spectra to ice cloud optical thickness and effective particle size. The brightness temperature difference (BTD) between 250 and 559.5 cm−1 is shown to be sensitive to optical thickness for optically thin clouds (visible optical thickness τ < 2). At the other extreme, for optically thick ice clouds (τ > 8), the BTD between 250 and 410.2 cm−1 is shown to be sensitive to the effective particle size up to a limit of 100 μm.