Cloud phase discrimination by near-infrared remote sensing.

Abstract

A ground-based near-infrared spectroradiometer was built and used to measure relative spectral reflectance from cumulus congestus and cumulonimbus clouds during the 1985 and 1986 Arizona summer monsoon seasons. Thermodynamic phase was inferred from spectral features in the regions between 1.55-1.75μm and 2.1-2.3μm where there are distinct differences between absorption in liquid water and ice and absorption by water vapor is very weak. Although liquid water and ice are nearly transparent in the visible, they absorb weakly in the near-infrared and that absorption is amplified by multiple scattering in clouds. Reflectance measurements are simple to make, requiring neither high spectral resolution nor absolute detector response. Three distinct aspects of differences between absorption in liquid water and ice were used to infer phase: (a) Ratio of the signal at 1.65 μm to that at 2.2 μm; (b) Wavelength of peak signal in the 1.65 μm water vapor transmission window; (c) Half-bandwidth of the 2.1-2.3 μm feature. Representative spectra are presented and analyzed on the basis of the predicted behavior of liquid water and ice cloud absorption. The results are consistent with young cumuli rapidly glaciating as they reach cooler levels, well before evidence of anvil formation or fibrous structure, contrary to the notion that phase can be inferred from visible cloud features.