Improved representation of surface and atmospheric radiation budgets in the ECHAM4 General Circulation Model

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