Information Content for Cloud Ice Microphysics in the FIR Radiance Spectrum

Mid-infrared (MIR) radiance measurements in the atmospheric window (8 - 12 mu m) have demonstrated sensitivity to ice particle microphysics, due to the spectral variation of the index of refraction of ice within the spectral range. The far infrared radiance (FIR) spectrum (17 - 50 mu m) also contains a region of spectrally varying index of refraction. The longer wavelengths imply a smaller scattering size parameter, which in turn gives the ice particles different scattering properties in the FIR relative to the MIR. Generally, the FIR shows larger single scatter albedos and weaker forward scattering. Although the atmosphere has high opacity in the FIR due to the rotational absorption band of water vapor, high altitude ice clouds would be observable in the upwelling radiance spectrum even in moist tropical atmospheres. Thus, the FIR spectrum could be used for cloud property retrieval from satellite sensors. Using a modeling study, we investigate the information content for ice microphysical retrievals to characterize how extending a high spectral resolution measurement from the MIR into the FIR could improve ice microphysics retrievals. The results show which regions in state space are better constrained with the far infrared information, and shows the advantage of a combined far-infrared and mid-infrared measurement for ice microphysical retrievals from satellites.