Numerical investigation of Monin-Obukhov functions for CO₂ and H₂O in the Roughness Sublayer

The Monin-Obukhov Similarity Theory (MOST) is an extension of the logarithmic law of the wall to flows with buoyancy that allows the estimation of turbulent flow quantities in the Atmospheric Surface Layer. Its dimensionless functions are routinely used in field experiments and weather and climate models to estimate turbulent fluxes and provide lower boundary conditions to atmospheric circulation. Nonetheless, in the presence of tall vegetation MOST is known to break down in the Roughness Sublayer (RSL), a region that extends up to three times the canopy height. This poses a challenge to the use of MOST in forested ecosystems given that measurement towers usually lie inside the RSL. Many experimental studies have investigated the reasons behind this “failure”, the most common explanations being the mixed contribution (or “dissimilarity”) of sources and sinks of scalars inside the canopy; non-stationarity caused by external forcings; the active versus the passive role of scalars; and entrainment at the top of the Atmospheric Boundary Layer. Given the limitations of field measurements to study this problem in more detail, we designed a numerical study using Large Eddy Simulations. Our simulations replicate plant canopies with CO₂ and H₂O sinks and sources emitted (or absorbed) from soil (evaporation and respiration) and canopy (plant transpiration and photosynthesis). By simulating each component separately (canopy and soil), we can disentangle the role of boundary condition dissimilarity on the validity of MOST at various heights above the canopy. The impact of dissimilarity, buoyancy and stationarity are also addressed by investigating the budgets of scalar variances and covariances. Overall, the aim of this study is to examine the reasons behind MOST failure in the RSL, with focus on its nondimensional functions for scalars, as well as to provide a path to its improvement or the development of alternative theories.