Canopy dynamics and the surface energy balance

Many numerical weather prediction models implicitly use standard flux-gradient relationships within their surface exchange schemes to relate the fluxes of mass, momentum and energy to the predicted mean values. However it has long been known that these relationships fail over and within canopies; this failure is now thought to be a consequence of an additional instability mechanism in the wind speed profile and the resulting dominance of the turbulent transport of mass, momentum and energy by coherent structures within the turbulence.

Here we introduce a revised formulation for the flux-gradient relationships which incorporates these coherent structures. The associated predictions for the profiles of mean wind speed, temperature and water vapour mixing ratio agree with observations across a range of canopies and diabatic stabilities. By including the revised relationships within a simple coupled energy balance for a canopy and the boundary layer above it, we investigate the impact of the relationships on the diurnal evolution of the canopy and boundary layer states. We will show that the revised relationships can lead to a significantly different diurnal evolution of the energy balance and boundary layer, particularly in the partitioning between sensible and latent heat fluxes (the Bowen ratio) during the day time. We also highlight some implications of this work for the formulation of surface exchange schemes (for canopies) within more complex numerical weather prediction models.