Large-Eddy Simulations of Turbulent Flow over Vegetated Hills – the Effects of Topography on in-Canopy Residence Times

The exchange of passive and reactive gases and particles across the top of a vegetated canopy is of significance to a plethora of applications in air quality, atmospheric, climate, ecological, hydrological sciences. In particular, the fraction of gases and particles emitted inside a canopy that escape into the overlying planetary boundary layer (PBL) remains a subject of inquiry and active research. This “escape fraction” results from a competition between turbulent transport and in-canopy sinks (such as deposition and chemical loss). In this context, controls on residence times of air parcels inside the canopy volume are necessary (but not sufficient). The few studies that address this question are based on idealized conditions, including the assumption of flat topography. However, large-eddy simulation (LES) studies suggest that even moderate topographical features can introduce substantial changes in the structure of the turbulence within the canopy, likely altering residence times and escape fractions. This question is addressed here using a suite of idealized LES simulations for a neutral PBL. The vegetation canopy is represented using a standard drag model and idealized sinusoidal topographic features are modeled using the immersed boundary method (IBM). Air parcels are tracked using an offline Lagrangian model driven by the turbulence fields derived from the LES. A series of simulations in which the amplitudes of the topographical features are systematically increased relative to the canopy height are then used to quantify the modulations in the turbulence and their impacts on residence times. The importance of these results to modeling escape fraction in large scale models is discussed.