Heat Transfer through Grass: A Diffusive Approach

Heat transport through short and closed vegetation, such as, grass is modelled by a

simple diffusion process. The grass is treated as a homogeneous ``sponge layer'' with

uniform thermal diffusivity and conductivity, placed on top of the soil. The temperature

and heat flux dynamics in both vegetation and soil are described using harmonic

analysis. All thermal properties have been determined by optimization against

observations from the Haarweg station in the Netherlands. Our results

indicate that both phase and amplitude of soil temperatures can be accurately

reproduced from the vegetation surface temperature. The diffusion approach requires

no specific tuning to, e.g., the daily cycle, but instead responds to all frequencies

present in the input data, including quick changes in cloud cover and day-night

transitions. The newly determined heat flux at the atmosphere-vegetation interface is

compared with the other components of the surface energy balance. The budget is

well-closed, particularly in the most challenging cases with varying cloud cover and

during transition periods. We conclude that the diffusion approach is a promising and

physically consistent alternative to more ad-hoc methods, like ``skin resistance''

approaches for vegetation and bulk correction methods for upper soil heat storage.

However, more work is needed to evaluate parameter variability and robustness under

different climatological conditions. From a numerical perspective, the multi-frequency

description allows for studying cases where the atmospheric boundary layer and the

top-surface interact on sub-hourly timescales. It would therefore be interesting to

couple the current land-surface description to turbulent resolving methods, such as,

large-eddy simulations.