Connecting the failure of K theory inside and above vegetation canopies and ejection-sweep cycles by a large-eddy simulation

Abstract

Parameterizations of biosphere-atmosphere interaction processes in climate models and other hydrological applications require characterization of turbulent transport of momentum and scalars between vegetation canopies and the atmosphere, which is often modeled using a turbulent analogy to molecular diffusion processes. Simple flux-gradient approaches (K theory) fail for canopy turbulence, however. One cause is turbulent transport by large coherent eddies at the canopy scale, which can be linked to sweep-ejection events and bear signatures of nonlocal organized eddy motions. The K theory, which parameterizes the turbulent flux or stress proportional to the local concentration or velocity gradient, fails to account for these nonlocal organized motions. The connection to sweep-ejection cycles and the local turbulent flux can be traced back to the turbulence triple moment c'w'c'. In this work, large-eddy simulation is used to investigate the diagnostic connection between the failure of K theory and sweep-ejection motions. Analyzed schemes are quadrant analysis and complete and incomplete cumulant expansion methods. The latter approaches introduce a turbulence time scale in the modeling. Furthermore, it is found that the momentum flux and sensible heat flux need different formulations for the turbulence time scale. Accounting for buoyancy in stratified conditions is also deemed important in addition to accounting for nonlocal events to predict the correct momentum or scalar fluxes.