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D2.1 Report on improved process representation in TEMs

Significant progress has been made during the last twenty years in describing land-atmosphere fluxes of momentum, heat and vapour in climate and weather forecasting models. More recently, with interest in functioning of the global carbon cycle, such models have been enhanced to include CO2 fluxes, and are utilised at spatial scales appropriate to coupling within Global Circulation Models (GCMs). Central to most land-surface schemes is application of the Penman-Monteith energy combination equation for single leaves (Monteith, 1981), a description of the dependence of stomatal opening on environmental conditions, and an integration algorithm for scaling from leaf to canopy level.

Despite their more recent and novel uses, a fundamental requirement for land-surface models is that they can replicate fluxes measured at single points. In most cases, this involves comparison against data from eddy-covariance towers, using simultaneous weather measurements to drive the surface model. Unfortunately, it is frequently found in such studies that predictions of Net Ecosystem Productivity (NEP) saturate too rapidly during the diurnal cycle, and there is a growing consensus that this might be in part due to leaf-to-canopy scalings that are too simplistic. To address this, new process descriptions are being developed to give land-surface models a more explicit canopy structure. Some multi-source models already exist (Shuttleworth and Wallace, 1985, Huntingford et al., 1995) but these have concentrated on internal canopy energy fluxes. Here we examine the impact of a more sophisticated treatment of canopy light-interception on the simulation of the diurnal cycle in NEP.

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