Absorptances for the 15-μm band system of CO2 are calculated from three models: the Goody model, the Malkmus model, and a wide band formulation. The wide band formulation used in this study explicitly accounts for hot and isotopic bands of CO2. Comparison is made between these calculated absorptances and measured absorptances. The band models are in good agreement with the measured absorptances. The sensitivity of these models to increased CO2 is investigated by intercomparing flux and heating rate changes computed by the three models. It is concluded that little difference exists between the narrow band and the wide band absorptance models, provided the wide band models account explicitly for the various hot and isotopic bands of CO2. We show that significant errors result if the Goody or Malkmus model is applied to spectral intervals larger than 10 cm−1. Thus, our results suggest that climate models that employ Goody or Malkmus type models with coarser spectral resolution (>10 cm−1) may be subject to errors (in CO2 fluxes and heating rates) larger than 10%. We also show the differences between the narrow and the wide band model treatment of H2O-CO2 overlap in the 15-μm region. The H2O continuum absorption in the 15-μm region alters significantly the vertical distribution of CO2 heating in the lower troposphere.
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