Intercomparison of Radiation Codes in Climate Models

Project Description
The fourth intercomparison is actually one of the earliest
conducted by DOE. ICRCCM is a program co-sponsored by DOE, the
World Meteorological Organization (WMO), and the International
Radiation Commission (IRC). The late Fred Luther gave the best
description of the rationale for the program: "Since the
transfer of solar and longwave radiation is the prime physical
process that drives the circulation of the atmosphere and its
temperature structure, it is natural that an evaluation of the
modeling of physical processes important to climate begin with
radiation." (Luther 1984)

Principal Findings:

1. Line-by-line models are in good agreement with each other to within
a few W/m2 (usually within 1%) when arbitrary line width cutoffs
are universally applied. The ICRCCM concluded that: "Uncertainties
in the physics of line wings and in the proper treatment of the
continuum make it impossible for line-by-line models to provide an
absolute reference . . ." (Luther et al. 1988). Thus, no
present-day model furnishes a reliable standard by which to judge
other models, nor are appropriate data available.

2. There is no systematic difference between wide-band and narrow-band
model results. However, there is a large variation among the band
models. While average differences from line-by-line results range
from 5 to 10%, the spread among the band models is several times

3. Band model calculations of sensitivities to changes in absorbing
constituents show poorer agreement with line-by-line results, and a
much larger spread, than calculations of flux components. For
example, when is doubled, the median band model sensitivities
differ by up to 18% from line-by-line values, while their spread is
an order of magnitude larger.

4. In cases of only and only, the spread in results among band models
increases considerably compared to the case when all absorbing
gases are included; this indicates that the success in the latter
case is partly fortuitous because of the way absorbing bands
overlap in the Earth's atmosphere.

5. For the longwave clear cases, with about 40 participants
representing almost all the world's major modeling groups, ICRCCM
revealed intermodel disagreements in fluxes and flux sensitivity to
constituent changes ranging from 30 to 70% (Luther et
al. 1988). The disagreements are worst for single absorbing gas
atmospheres, indicating that the better agreement found in the
all-gas cases is partly accidental. Subsequent ICRCCM calculations,
involving cloudy longwave cases, and clear and cloudy shortwave
cases, have revealed equally large or larger disagreements, ranging
up to 20 to 30% in fluxes and up to 70% in flux sensitivity to
constituent changes.

6. Comparisons are still in progress for vertical profiles of
radiative heating rates. Disagreements in radiative heating rates
are expected to be larger than for fluxes, because heating rate is
the derivative of flux and taking derivatives magnifies errors

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