RADIATIVE-TRANSFER FOR INHOMOGENEOUS ATMOSPHERES - RRTM, A VALIDATED CORRELATED-K MODEL FOR THE LONGWAVE

A rapid and accurate radiative transfer model (RRTM) for climate appli cations has been developed and the results extensively evaluated. The current version of RRTM calculates fluxes and cooling rates for the lo ngwave spectral region (10-3000 cm(-1)) for an arbitrary clear atmosph ere. The molecular species treated in the model are water vapor, carbo n dioxide, ozone, methane, nitrous oxide, and the common halocarbons. The radiative transfer in RRTM is performed using the correlated-k met hod: the k distributions are attained directly from the LBLRTM line-by -line model, which connects the absorption coefficients used by RRTM t o high-resolution radiance validations done with observations, Refined methods have been developed for treating bands containing gases with overlapping absorption, for the determination of values of the Planck function appropriate for use in the correlated-k approach, and for the inclusion of minor absorbing species in a band. The flux and cooling rate results of RRTM are linked to measurement through the use of LBLR TM, which has been substantially validated with observations. Validati ons of RRTM using LBLRTM have been performed for the midlatitude summe r, tropical, midlatitude winter, subarctic winter, and four atmosphere s from the Spectral Radiance Experiment campaign. On the basis of thes e validations the longwave accuracy of RRTM for any atmosphere is as f ollows: 0.6 W m(-2) (relative to LBLRTM) for net flux in each band at all altitudes, with a total (10-3000 cm(-1)) error of less than 1.0 W m(-2) any altitude; 0.07 K d(-1) for total cooling rate error in the t roposphere and lower stratosphere, and 0.75 K d(-1) in the upper strat osphere and above. Other comparisons have been performed on RRTM using LBLRTM to gauge its sensitivity to changes in the abundance of specif ic species, including the halocarbons and carbon dioxide. The radiativ e forcing due to doubling the concentration of carbon dioxide is attai ned with an accuracy of 0.24 W m(-2), an error of less than 5%. The sp eed of execution of RRTM compares favorably with that of other rapid r adiation models, indicating that the model is suitable for use in gene ral circulation models.