Comparison of a Simple 1-D Model to Describe Heat Transfer in a Sparse Vegetation Canopy with Experimental Data

We use data from a wind tunnel experiment to test a simple model of heat transfer, relating air temperature, radiative surface temperature and heat exchange between a sparse canopy and the overlying atmosphere. The model canopy, representing a homogeneous, sparse forest of trees with dense crowns consisted of small light globes mounted on a copper base that allowed the heat source to be distributed independently between the ground and canopy. Air temperature profiles and infrared surface temperatures were measured for source distributions ranging from entirely ground to entirely canopy. A simple one-dimensional heat transfer model was used to predict air temperature profiles, surface-air temperature differences for the ground and canopy and hence the Stanton number for the canopy, kB-1, for a range of source distributions. The model combined Localised Near Field theory of turbulent transfer and convective heat transfer theories for a flat plate and a sphere in a uniform free stream. Comparison of predictions with measurements indicates that the model describes heat transfer from the canopy well, but that ground/air temperature differences tend to be over-estimated by up to 50% when the heat source is concentrated at the ground. This would lead to an under-estimation of surface heat fluxes inferred from satellite observations of land surface temperature, particularly for sparsely vegetated areas, where ground heat fluxes are large.