A New Narrowband Radiation Model for Water Vapor Absorption

Abstract

The accuracy of radiation models is a critical issue in climate studies. However, calculations from different radiation models used in climate calculations disagree with one another, and with more detailed models, at levels significant to many climate problems. With several new advances in the field of radiation modeling, it is possible to develop more accurate band models and validate them against radiation observations of known high accuracy. In this paper, a new accurate narrowband longwave radiative transfer model for clear-sky conditions is developed. In the first part of this study, only water vapor effects are included, and the model results are tested against line-by-line radiative transfer model (LBLRTM) calculations. In the model development, it is first shown that traditional techniques for estimating Malkmus statistical model parameters from the line compilation and line-by-line models cannot be trusted to give accurate transmittance function. A new technique is then described that calculates water vapor line transmittances with good agreement with LBLRTM calculations (i.e., with rms errors less than 0.01 for more than 97% of the intervals). The water vapor continuum is included in a manner consistent with the water vapor line absorption. Fluxes calculated with the model agree with LBLRTM to about 1 W m?2 for the entire vertical range of the atmosphere for several test cases. The heating rate errors are reduced by as much as 0.25°C day?1 below the tropopause for the test cases compared with the original narrowband model.