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 climatological
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, for example, 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 at this interface. 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 (either implemented
analytically or numerically) is a 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 present representation
of vegetation allows for both slow and rapid feedbacks between the atmosphere and
the surface. As such, it would be interesting to couple the present surface parametrization to
turbulence-resolving models, such as large-eddy simulations.