Some Improvements and Complements to the Infrared Emissivity Algorithm Including a Parameterization of the Absorption in the Continuum Region

Louis Garand Recherche en Prévision Numérique, Dorval, Québec, Canada

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Abstract

Infrared heating rate calculations with the emissivity formulation are improved by developing simple methods to minimize the effects of the two main sources of error 1) the use of the strong-line approximation; 2) the neglect of temperature variations along the optical path. When compared to band-by-hand calculations, the resulting new scheme maintains a 5% accuracy on cooling rates, and errors generally less than 2 W m−2 on fluxes. The most recent band parameters of MeClatchey et al. (1973) are used in the band-by-band calculations, including measured data for the vibration-rotation bands.

Another novel aspect of the new scheme is the treatment of “e-type” absorption, which is a very important effect in hot humid air near the ground. An implicit parameterization is developed which allows for “e-type” absorption in the pre-tabulated transmission functions, and so requires no computational overhead. This is based on the derivation of an approximate formula which expresses the optical depth due to “e-type” absorption in terms of the optical depth due to water vapor.

A 10-level version requires 1.7 ms on a CDC 7600 computer. Carbon dioxide and clouds are included, and provision is made for the inclusion of ozone.

Abstract

Infrared heating rate calculations with the emissivity formulation are improved by developing simple methods to minimize the effects of the two main sources of error 1) the use of the strong-line approximation; 2) the neglect of temperature variations along the optical path. When compared to band-by-hand calculations, the resulting new scheme maintains a 5% accuracy on cooling rates, and errors generally less than 2 W m−2 on fluxes. The most recent band parameters of MeClatchey et al. (1973) are used in the band-by-band calculations, including measured data for the vibration-rotation bands.

Another novel aspect of the new scheme is the treatment of “e-type” absorption, which is a very important effect in hot humid air near the ground. An implicit parameterization is developed which allows for “e-type” absorption in the pre-tabulated transmission functions, and so requires no computational overhead. This is based on the derivation of an approximate formula which expresses the optical depth due to “e-type” absorption in terms of the optical depth due to water vapor.

A 10-level version requires 1.7 ms on a CDC 7600 computer. Carbon dioxide and clouds are included, and provision is made for the inclusion of ozone.

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