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Principles of remote sensing
of atmospheric parameters from space
February 1998
By R. Rizzi and updated by
R. Saunders
European Centre for Medium-Range Weather
Forecasts, Shinfield Park, Reading RG2 9AX, U.K.
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9 . Continuum Absorption
In addition to discrete molecular interactions with radiation there is also another important, but less understood, mechanism for interacting with the incident radiation field called continuum absorption/emission by some atmospheric molecules, most notably water vapour. Continuum absorption manifests itself in the infrared and millimetre wave window regions of the atmospheric spectrum causing the windows to be less transparent than predicted by discrete molecular absorption alone. For channels designed to sense the surface this is obviously an important factor. This continuum absorption/emission has a smooth frequency dependence making it impossible to ascribe to specific molecular transitions.
There are two main theories for continuum absorption/emission. One is that it is due to the residual effects of the far wings of strong lines which are not accurately modelled. If there are many strong lines some distance away (in frequency) the effect of the individual lines can accumulate. A second theory is that the interaction is caused by molecular polymers (e.g. water vapour dimer) which being large floppy molecules might be expected to have broad transitions and hence broad spectral features. The exact mechanism continues to be a matter of debate.
Laboratory and field measurements have measured the excess absorption and empirical relationships have been developed to provide an estimate of the continuum absorption for any atmospheric path. One problem is that most of the measurements are for warm paths (300 K) whereas most atmospheric paths are colder than this. Nevertheless empirical relationships are widely used to include this source of absorption/emission in the estimation of the atmospheric transmittance. The gases which have significant continuum absorption are H2O, CO2, N2, and O2 and the continuum is included for all window channel transmittance calculations from the visible to microwaves.
As an example, for the Advanced Very High Resolution Radiometer (AVHRR) infrared window channel at 925 cm-1, the mean transmittance due to molecular line effects is 0.97 whereas the transmittance due to the continuum is only 0.93 for a mid-latitude atmosphere.
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