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Abstract

The surface and atmospheric radiation budgets of the latest version of the Max—
Planck Institute GCM, the ECHAM4, differ considerably from the earlier version
ECHAM3 and other GCMs in both short— and longwave ranges. The absorbed short—
wave radiation at the surface is substantially smaller (147 Wm‘2) than typically found
in current GCMS, due to a larger atmospheric absorption of 90 Wm‘2. The enhanced
shortwave atmospheric absorption is related to an increase of both simulated clear—sky
and cloud absorption. Observational evidence is presented that this revised disposition
of shortwave absorption is more realistic than typically found in current GCMs. This
conclusion is based on a comparison of the model radiative fluxes with a large num-
ber of surface and collocated top-of—atmosphere observations, as well as stand—alone
validations of the radiation scheme. In contrast to other GCMS which show a smaller
atmospheric and a larger surface shortwave absorption, respectively, the ECHAM4
shortwave absorption is closer to the observations.
The clear-sky surface insolation of the ECHAM4 radiation scheme is shown to
be very accurately calculated in a stand-alone validation, compared to other schemes
which tend to overestimate these fluxes. This suggests that the global mean ECHAM4—
calculated clear-sky shortwave absorption of 72 Wm‘2 within the atmosphere and
214 Wm‘2 at the surface are realistic values. Further, the ECHAM4—calculated cloud
amount is in good agreement with surface-based observations. The above findings
imply that the increase in the cloud absorption in ECHAM4 (TOA—to-surface cloud
radiative forcing ratio R of 1.35) is consistent with the available observations on the
global scale.
Zonally, observational evidence for a necessity to increase cloud absorption in
GCMs is found in the low latitudes in agreement with other recent studies, but not
so in the higher latitudes: the comparisons favour a value of R near 1.3 - 1.4 in the
tropics but closer to 1 in the extratropics.
Overall, this study indicates that not only an increased solar absorption by clouds
but also by the cloud-free atmosphere is essential to reduce the discrepancies between
GCM—calculated atmospheric shortwave absorption and observations.
The smaller surface insolation and associated reduction of the available energy at
the surface is partly compensated for by an increased downward longwave flux at the
surface (344 Wm—2 in ECHAM4), which is considerably larger than in other GCMS.
The larger downward longwave flux is supported by surface measurements and by a
stand—alone valdidation of the radiation scheme for clear-sky conditions. The enhanced
downward longwaveflux allows to maintain the level of available energy at the surface
needed for a realistic intensity of the global hydrological cycle.