Line-by-line calculation of atmospheric fluxes and cooling rates: 2. Application to carbon dioxide, ozone, methane, nitrous oxide and the halocarbons


  • S. A. Clough,

  • M. J. Iacono


A line-by-line model (LBLRTM) has been applied to the calculation of clear-sky longwave fluxes and cooling rates for atmospheres including CO2, O3, CH4, N2O, CCl4, CFC-11, CFC-12, and CFC-22 in addition to water vapor. The present paper continues the approach developed in an earlier article in which the radiative properties of atmospheres with water vapor alone were reported. Tropospheric water vapor continues to be of principal importance for the longwave region due to the spectral extent of its absorbing properties, while the absorption bands of other trace species have influence over limited spectral domains. The principal effects of adding carbon dioxide are to reduce the role of the water vapor in the lower troposphere and to provide 72% of the 13.0 K d−1 cooling rate at the stratopause. In general, the introduction of uniformly mixed trace species into atmospheres with significant amounts of water vapor has the effect of reducing the cooling associated with water vapor, providing an apparent net atmospheric heating. The radiative consequences of doubling carbon dioxide from the present level are consistent with these results. For the midlatitude summer atmosphere the heating associated with ozone that occurs from 500 to 20 mbar reaches a maximum of 0.25 K d−1 at 50 mbar and partially offsets the cooling of 1.0 K d−1 contributed by H2O and CO2 at this level. In the stratosphere the 704 cm−1 band of ozone, not included in many radiation models, contributes 25% of the ozone cooling rate. Radiative effects associated with anticipated 10-year constituent profile changes, 1990–2000, are presented from both a spectral and spectrally integrated perspective. The effect of the trace gases has been studied for three atmospheres: tropical, midlatitude summer, and midlatitude winter. Using these results and making a reasonable approximation for the polar regions, we obtain a value for the longwave flux at the top of the atmosphere of 265.5 W m−2, in close agreement with the clear-sky Earth Radiation Budget Experiment (ERBE) observations. This agreement provides strong support for the present approach as a reference method for the study of radiative effects resulting from changes in the distributions of trace species on global radiative forcing. Many of the results from the spectral calculations reported here are archived at the Carbon Dioxide Information and Analysis Center for use by the community.