Coupled LES and Lagrangian Particle Modeling of Canopy Dispersion over a Range of Stability During CHATS

A dispersion experiment conducted as part of the Canopy Horizontal Array Turbulence Study (CHATS) in a walnut orchard was aimed at determining the effects of canopy-induced stability on vertical dispersion. This aim was especially for night when the air in the lower canopy was unstable but air in the upper canopy and above it was stable. The study was intended to fill a void in canopy dispersion experiments, i.e., most have been conducted during near-neutral conditions. For the 6-day experiment, there were two daytime periods (of 4 - 6 hours duration) and five nighttime periods of similar duration. The experiments provided observations of the meteorology including turbulence, as well as scalar (SF6) concentrations for testing a coupled large eddy simulation (LES) and Lagrangian particle dispersion model (LPDM) of dispersion in and above a canopy.

In the LPDM, one tracks single particles to obtain the mean dispersion and concentration field using LES velocities to drive the model. The particle velocity is divided into "resolved" and "subfilter-scale" components consistent with the LES, where the SFS velocity is a random value from a stochastic model using the SFS turbulent kinetic energy. The coupled LES-canopy model (Patton et al., 2016) simulated a 512 x 512 x 256 m^3 domain with a 1-m grid size and a geostrophic wind of either 4 or 8 m/s. The LESs were conducted for a weakly-stable and stable boundary layer overlying a 10-m high walnut canopy and produced mean wind, temperature, and turbulence profiles similar to those in CHATS. In particular, the potential temperature profile for the stable case exhibited an inversion near the canopy top.

The LPDM-LES produced vertical concentration profiles within and above the canopy that were similar to those in CHATS with the weakly-stable or near-neutral case showing a monotonic decrease from the surface upwards--throughout and above the canopy; the modeled and observed concentrations were from a 1-m high line source. In contrast, the modeled concentrations in the stable case exhibited a scalar inversion near the canopy top that mimicked the temperature profile and the CHATS SF6 profiles. In addition, the LPDM surface-level concentration dependence on distance matched that in the near-neutral case well and suggested a strong effect of the canopy top wind shear. For the stable case, the surface concentration showed a slow decrease with distance, which was attributed to the temperature inversion temporally trapping the scalar beneath it. These and other results will be discussed.