Forests and regional-scale processes

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Abstract
In this paper the surface energy balance of forests is considered on a regional scale. The interaction between the surface fluxes of sensible and latent heat on the one hand and the temperature (and humidity) of the planetary boundary layer (PBL) on the other is accounted for, as are entrainment processes at the top of the PBL. For this purpose, the one-dimensional PBL model of Troen & Mahrt is coupled to the simple ‘single-leaf’ vegetation model of Penman-Monteith. The surface conductance is described in accordance with Stewart, whereas the aerodynamic conductance is corrected for stability. The integrated model applies in stable, unstable and neutral conditions. Two forest types (‘Thetford’ and ‘Les Landes’) are modelled, and compared with grass and bare soil. In summertime, calculated temperature, saturation deficit and depth of the PBL are greater over forest than over grass. The entrainment of warm, dry air at the top of the PBL appears to be important. It supplies the energy to maintain the evaporation of wet forest higher than net radiation. Well-known features, such as low transpiration and high evaporation rates (compared with grass) when the canopy is dry and wet respectively, are described correctly.