In recent years, there has been increasing interest in the far-infrared radiation spectrum (from 100-667 cm^-1, or 15 to 100 µm) in atmospheric research. For example, it has been confirmed that the radiation balance in the troposphere is influenced by radiative cooling caused by water vapor at far-infrared wavelengths. The present study explores the potential advantages of using the far-infrared spectral signature for investigating the microphysical and radiative properties of ice clouds (cirrus clouds, in particular). These clouds have a significant impact on the terrestrial climate system through their radiative effect. It is quite challenging to reliably derive the microphysical and radiative properties of these clouds.

The spectral signature of ice clouds in the far-infrared (far-IR) spectral region is investigated in detail. Several state-of-the-art scattering computational methods are used in this study. Furthermore, the bulk scattering properties of ice clouds at far-IR wavenumbers are developed from scattering computations and in-situ measured microphysical properties, as well as a parameterization of the bulk scattering properties.

Extensive sensitivity studies are carried out to understand the effect of ice cloud effective particle size and optical thickness on far-infrared radiance. It is found that the brightness temperature difference (BTD) between 250 and 559.5 cm^-1 is quite sensitive to optical thickness for optically thin clouds (visible optical thickness t<2). At the other extreme, for optically thick ice clouds (t>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. The conclusion of this study is that the use of the far-IR spectral signature may provide complementary information to what may be inferred by current methods using satellite imagery such as MODIS, and will be useful in gaining a better understanding of the role of ice clouds in the Earth’s radiation budget.