A Parameterization of the Absorption in the 15 μm CO2Spectral Region with Application to Climate Sensitivity Studies

Abstract

A parameterization of the absorption in the 15 μm CO2 spectral region has been developed based upon the wing scaling approximation of Chou and Arking (1980, 1981). The spectrum is divided into a band-wing region and a band-center region, and the CO2 amount in an inhomogeneous atmosphere is scaled separately for the two regions. The spectrally averaged transmittance over each region is then expressed as a simple function of the scaled amount of CO2. Compared to fine-by-line calculations, the error of the parameterization is <0.025 in the transmittance and <0.04°C day−1 in the tropospheric and lower stratospheric cooling rates. The cooling rate error in the upper stratosphere is generally ten than a few tenths of a degree per day except for the region above the 3 mb level where the error is too large to be acceptable for some studies on the phenomena in that region. The effect of the parameterization of absorption due to CO2 on climate studies has been investigated with the Multi-Layer Energy Balance Model (MLEBM) developed at GLAS (Peng et al, 1982). It is found that, compared to the accurate perturbation method, the parameterization introduces very small differences in the model temperatures and radiation budgets for both the normal and doubled CO2 concentrations. In addition, we have investigated the effect of including the CO2 absorption in the margins of the 15 μm spectral band on the CO2 climate sensitivity. It is found that the surface temperature sensitivity is enhanced by 20% for a doubled CO2 concentration and by 30% for a quadrupled CO2 concentration when the spectral range of CO2 absorption is extended from 580–760 to 540–800 cm−1.