Computations of Transmittance and Radiance in Infrared Water Vapor Sounding Channels

Abstract

The method originally developed by Chou and Arking (1981) for computing the absorption of solar radiation by water vapor has been extended to the computations of transmittance and radiance in infrared water vapor sounding channels. It utilizes the wing-scaling approximation and the k-distribution approach. The effects of the instrument response function, zenith angle and the variation of the Planck radiance with wavenumber can be easily and accurately included in computing transmittance and radiance. The method can be effectively applied to any tropospheric water vapor sounding channels in the infrared. Compared to line-by-line calculations, which can be considered as the most accurate but too time consuming to be operational, the rms error in transmittance of the method at any pressure level is less than 0.009 for the three operational HIRS/2 water vapor sounding channels. The rms error in brightness temperature is less than 0.2°C which is about a factor of 2?5 smaller than the instrument noise.