Land-surface characterization in climate models: biome-based parameter inference is not equivalent to local direct estimation

Abstract

Soil-Vegetation-Atmosphere Transfer (SVAT) schemes in atmospheric general circulation models (AGCMs) require land-surface information. SVATs need this information to simulate the interactions between the atmosphere and the biosphere, in general, and to determine how the radiation absorbed by the surface is partitionned into sensible and latent heat, in particular. When investigating future climates, parameters can either be inferred from the tabulated, average characteristics of predicted biomes or computed locally, i.e. cell by cell. A null hypothesis is formulated to test if AGCMs can distinguish between the two approaches: “For AGCMs, biome-based parameter inference is equivalent to cell by cell estimation”. The hypothesis is tested with a terrestrial biosphere model (TBM), the Ecological ModUle (EMU). EMU uses a set of primary and composite generic vegetation types to calculate local vegetation characteristics and to identify dominant assemblages. For the test, EMU is run with two different climate scenarios: a present climate and the Climate Change scenario for an equivalent doubling of the atmospheric concentration of carbon dioxide generated with the United Kingdom Meteorological Office AGCM. Results indicate that the two approaches are not equivalent. Consequently, the null hypothesis is rejected. Moreover, the differences in vegetation characteristics are of a magnitude which should have noticeable impacts on exchanges of energy, water, and momentum between the atmosphere and the surface. This suggests that estimating vegetation locally should increase the credibility of AGCM investigations of Global Change.