A new halocarbon absorption model based on HITRAN cross-section data and
new estimates of halocarbon instantaneous clear-sky radiative forcing
Abstract
The article describes a new practical model for the infrared absorption
of chlorofluorocarbons and other gases with dense spectra, based on
HITRAN absorption cross-sections. The model is very simple, consisting
of frequency-dependent polynomial coefficients describing the pressure
and temperature dependence of absorption. Currently it is implemented
for the halocarbon species required by the Radiative Forcing Model
Intercomparison Project (RFMIP). This approach offers practical
advantages compared to previously available options, and is traceable,
since the polynomial coefficients follow directly from the laboratory
spectra. The model is applied to the problem of computing instantaneous
clear-sky halocarbon radiative efficiencies and present day radiative
forcing. Results are in reasonable agreement with earlier assessments
that were carried out with the less explicit Pinnock method, and thus
broadly validate that method. Overall, halocarbons are responsible for a
substantial share of the present-day forcing, 0.573 Watt/squaremeter
(instantaneous clear-sky at the TOA), corresponding to approximately
20% of the total anthropogenic forcing, or 44% compared to
anthropogenic CO2 forcing alone.