Introduction

Plant canopies modify their own microclimate. The heat and vapour released into the atmosphere at plant surfaces changes the temperature and humidity of the air in contact with those surfaces. These changes in temperature and humidity, in their turn, modulate the fluxes of heat and vapour from the vegetation. The importance of this ‘atmospheric feedback’ depends, amongst other things, on the area of the plant canopy (Jarvis & McNaughton, 1986). Small areas of vegetation modify shallow layers of the atmosphere, and local changes in microclimate are small. The influence of a single field extends upwards for perhaps 10 metres. The gradients of temperature and humidity through this layer have been studied in detail by canopy meteorologists.

If a uniform canopy covers an area of some hundreds of square kilometres then the effect of the vegetation will be felt throughout the whole of the turbulent planetary boundary layer, up to a kilometre or so above the ground. On this regional scale, processes affecting the surface energy balance have received very little scientific attention. This situation is now changing under pressure from hydrologists, who want methods for estimating regional evaporation, and climatologists, who must model the surface energy balance to improve predictions from their models of the global circulation of the atmosphere.

The purpose of this chapter is to review efforts to extend canopy energy balance models to the regional scale. First is a brief descriptive account of atmospheric transport processes in the whole planetary boundary layer, to set the scene.