Explaining the convector effect in canopy turbulence by means of large-eddy simulation

Semi-arid forests are found to sustain a massive sensible heat flux in spite of having a low surface to air temperature difference by lowering the aerodynamic resistance to heat transfer (r(H)) - a property called the "canopy convector effect" (CCE). In this work large-eddy simulations are used to demonstrate that the CCE appears more generally in canopy turbulence. It is indeed a generic feature of canopy turbulence: r(H) of a canopy is found to reduce with increasing unstable stratification, which effectively increases the aerodynamic roughness for the same physical roughness of the canopy. This relation offers a sufficient condition to construct a general description of the CCE. In addition, we review existing parameterizations for r(H) from the evapotranspiration literature and test to what extent they are able to capture the CCE, thereby exploring the possibility of an improved parameterization.