HIRS Sensitivity to C02 Mixing Ratio and a Pragmatic Correction Term

Abstract

The effect of changes in the mixing ratio of C02 on the uniformly mixed gases component of the TIROS-N Observational Vertical Sounder radiances in High-Resolution Infrared Sounder (HIRS) channels is examined using a line-by-line radiative transfer model. A 9% increase in C02 concentration is found to cause up to a 3.5% change in the clear-sky radiance in some channels. When simple black clouds are introduced in the simulations, the magnitude of such radiance changes decreases with increasing cloud-top height. A simple correction term to account for C02 mixing ratio changes is developed that can be applied to the current uniformly mixed gases component of the National Environmental Satellite Data and Information Service fast transmittance model. The correction term is a linear function of the difference between the C02 mixing ratio and a reference mixing ratio and is applied as an exponent to the uniformly mixed gas transmittance obtained from the fast transmittance model. The method requires the determination of a single coefficient, ??, for each of the 19 HIRS channels. These coefficients are tabulated for the NOAA-9 through NOAA-12 satellites.