The influence of a plant canopy on scalar statistics and structure within the planetary boundary layer

Two large-eddy simulations (LESs) have been performed representing planetary boundary layer (PBL) flow. Nested-grids are utilized to reduce the influence of subgrid-scale (SGS) motions and therefore better resolve the exchanges of momentum, heat and scalars with the surface layer. The two simulations are identical with the exception being that one interacts interacting with a surface whose roughness is described solely by its roughness length, while the second interacts with a spatially distributed array of drag elements and scalar sources representing the presence of a plant canopy. Previous three-dimensional numerical studies have necessarily imposed limiting boundary conditions at approximately three times the canopy height, while the current simulations allow for the interaction between PBL and canopy scale eddies.

Of particular interest is the influence of a plant canopy on the mixing of passive scalars. We will explore the canopy-imposed modification of the scalar statistics and briefly of top-down/bottom-up relationships. In addition, through three-dimensional instantaneous scalar fields, an evaluation of the canopy imposed modification of atmospheric structure in the lowest reaches of the PBL will be presented.