Linking IMAGE and ECBILT

Several of the inadequacies of IMAGE 2 are related to the climate model, especially its inability to describe interdecadal variability. Lacking this aspect, IMAGE 2 is less suited to treat the interactions between climate and biosphere on these time scales. The simple climate model is uncapable to dynamically describe the 3-dimensional climate patterns and should therefore be replaced.

This document evaluates the requirements and desired aspects of an improved climate model for IMAGE 2 and introduces a new climate model, ECBILT, which will be used for in a new version of IMAGE. Relevant issues for linking ECBILT with the other components of IMAGE 2 are described. The first link established between IMAGE 2 and ECBILT is a radiation scheme, which determines long-wave radiation fluxes as a function of atmospheric composition. Calculated changes in radiative forcing form a boundary condition for ECBILT to dynamically simulate climatic change.

In the report the model is validated and additional tests are described. A state-of-the-art radiation model, like the ones used currently in the most advanced GCMs (General Circulation Models), is computationally too demanding for our purposes. Therefore, we have linearised such an advanced radiation model around reference profiles for the atmosphere. The new radiation scheme was found to perform well when compared to the original radiation scheme and to observations, especially considering the coarse vertical resolution of climate variables available from the ECBILT model. Of the two versions developed, a three-cloud-type version was shown to perform better than a one-cloud-type version, especially in the tropics. In our approach, input variables influence the radiative transfer independent of each other. This disadvantage is inherent in linear approximation.

As a result, for example, the effects of overlapping absorption bands of methane and nitrous oxide are not considered in the current version. A more process-based model would do better in capturing such features. Also, such a model is expected to perform less reliably when applied to a non-analogue climate state.  However, preliminary tests of implementing the new long-wave radiation scheme in ECBILT have shown that it does perform satisfactorily under such non-analogue conditions as well, supporting our confidence in the new model as an adequate link between IMAGE 2 and ECBILT.