Far-Infrared and Submillimeter Wave Attenuation by Clouds and Rain

Abstract

Newly determined optical constants for water at far-infrared and submillimeter wavelengths, as revealed by a recent survey, are used to estimate water cloud and rain attenuation over the wavelength range between λ12 μm and λ2 cm. For this purpose new analytic drop-size distribution models simulating fog, nimbostratus cloud and rain, corresponding to rainfall rates of 10 and 50 mm h⁻¹, are set up. The corresponding volume extinction and absorption coefficients as computed according to polydisperse Mie scattering theory at specific wavelengths are listed in tables and presented graphically in plots for purposes of interpolation. It is found that cloud extinction may exceed 50 nepers per kilometer in the λ≲100 μm region, whereas for wavelengths longer than λ200 μm, under near-saturated conditions, water vapor absorption should be the dominant attenuator. The greatest attenuation by heavy rain may be expected around λ5 mm with a value of about 5 nepers per kilometer. The results also suggest that in the presence of non-precipitating water clouds or fog there may be a relative transmission “window” centered around λ1.3 mm.