The Water Vapor Continuum and its Role in Remote Sensing

Abstract

The water vapor continuum plays an important role in atmospheric radiative transfer providing increased opacity between spectral lines over the full spectral region from the microwave to the visible. The continuum has a significant influence on atmospheric fluxes and cooling rates. Additionally the continuum is important to the physical solution of the inverse problem, the remote sensing of atmospheric state to retrieve temperature, water vapor, surface properties and other state parameters. There are two components to the continuum: the self-broadened continuum, dependent on the square of the partial pressure of water vapor, and the foreign-broadened continuum, dependent on the product of the water vapor partial pressure and the dry air pressure. As a consequence the self-broadened continuum tends to be more important in the lower atmosphere while the foreign-broadened continuum tends to be more important in the middle to upper troposphere. A comprehensive continuum model based on a single line shape for all transitions from the microwave to the visible has provided generally acceptable results (Clough et al., 1989 and Clough et al., 1980; hereafter CKD). The nadir and zenith spectral radiometric measurements with the University of Wisconsin HIS instrument have been particularly useful in establishing the general level of accuracy for the continuum. For most spectral regions the apparent error in the continuum is of the order of 10% or less. The term 'apparent' is used because it is difficult to unambiguously characterize the atmosphere, particularly with respect to aerosol loading and to sub-visual cloud effects. However, for special conditions of atmospheric state, measurements indicate that for specific and very limited spectral regions, errors in the CKD continuum may be significant. The modification for the current model has been developed in the context of two recent observation: (1) the measurement of the downwelling radiance at Kavieng, New Guinea by Westwater et al. (1994) which indicated that the self-broadened continuum gave rise to an 8K error in brightness temperature at 950 cm-1 for an atmosphere with ~6 precipitable cm of water vapor; and (2) a clear indication that the foreign continuum was in error in the wings of the 1600 cm-1 water vapor band as observed by Revercomb and colleagues at the U. of Wisconsin.