Structure of Turbulence in the Plant Canopy Derived from an Improved k-epsilon Turbulence Closure Model

A new turbulent closure model of the k-epsilon type is proposed for the simulation of the heat, water vapor and momentum transfer in the plant canopy. The governing equations of the model are derived by applying ensemble and spatial averaging procedures to the momentum equation, continuity equation, and equations of heat and water vapor transfer in the plant canopy. The transport of momentum, heat and water vapor in the vertical direction under density stratification is evaluated based on the assumption of a near-equilibrium shear flow where transport effects on the stresses and heat fluxes are negligible. With this closure procedure, effects of volume of stem and leaves, and density stratification on the transfer of momentum, heat and mass can be accounted for. The heating and transpiration processes in the plant canopy and at the ground surface are also simulated by a model coupling heat and mass transfer in the soil - plant - atmosphere system. This model includes a sub-model for the coupling of heat and water transfer in the soil, the exchange of water between root and the soil, the transfer of water in the stem, the transpiration at leaves, the radiative transfer inside the plant canopy, and the heating of the plant canopy.

Comparison between results of the model and available observational data on the air temperature, relative humidity and foliage temperature reveals that the model can satisfactorily simulate the momentum, heat and mass transfer processes in the plant canopy. Also, the model is validated by comparing results of the model on turbulent fluxes of momentum, heat and water vapor with those, computed by some other existing models. Results of the computations show that the model performs better than the standard k-epsilon model or other turbulence closure model, especially inside the plant canopy. The model is used to investigate the influence of various closure assumptions on the turbulent fluxes of momentum, heat and water vapor in and outside of the plant canopy. It is found from simulations that the density stratification and volume of branch and leaves significantly influence transport processes in the plant canopy. Numerical experiments with the model were performed to investigate various characteristics of turbulence inside and above the plant canopy under different canopy and stratification conditions.