The Effect of Source Distribution on Bulk Scalar Transfer between a Rough Land Surface and the Atmosphere

We investigate the effect of source distribution on the bulk transfer of passive scalars between rough, vegetated land surfaces and the atmosphere, using data from a wind-tunnel experiment in which passive heat was emitted from both the underlying surface and canopy elements of a three-dimensional regular bluff-body array. The experimental results are compared with a simple one-dimensional, two-source model for scalar transfer. We find that: (1) the observed scalar transfer resistance across the boundary layer at the underlying surface is simply related to flat-plate theory by a constant of 0.62, despite the complexity of the turbulent flow within the wind-tunnel canopy; (2) one-dimensional gradient-transfer theory, even with extensions to account for the non-local nature of turbulent transfer within the canopy, does not describe the observed details of scalar concentration gradients in the highly three-dimensional canopy flow, but does provide a reasonable framework for bulk scalar transfer between the composite ground-canopy surface and the flow above the canopy; (3) the kB ⁻¹ parameter (which accounts for bulk excess resistance to scalar transfer over momentum transfer) is highly sensitive to scalar source partition between ground and canopy.