Line-source diffusion in a walnut orchard canopy during CHATS

The Canopy Horizontal Array Turbulence Study (CHATS) occurred during spring 2007 in a walnut orchard near Dixon, California. The experiment was designed to measure the effects of tall vegetation on the turbulence field both within and above this 10-m high canopy, with the aim of improving subfilter-scale models for use in large-eddy simulations (LESs). CHATS used crosswind arrays of sonic anemometers as well as a 30-m tall tower instrumented at multiple levels with sonics and scalar sensing instruments (temperature, humidity, and chemical compounds). In addition, experiments on scalar (SF6) diffusion from a 1-m high line source were conducted on about six days. Measurements of the SF6 concentration were made at 7 heights on the tower, which was 40 m downwind of the source; they were complemented by a small array of surface monitors.

One of the key purposes of the dispersion experiments was to determine the effect of canopy-induced stability effects on diffusion, especially at night when the lower canopy layers were unstable but the upper and above-canopy layers were stable. A preliminary analysis shows that during daytime, the vertical concentration distribution tended towards an exponentially decreasing profile characteristic of a surface release in a convective boundary layer. However, at night the profiles exhibited much higher surface concentrations with a tendency towards fuller (convex shaped) profiles that fell to very low values near the canopy top. An analysis of the properly scaled concentration data including the surface monitor values will be presented. This will be complemented with a diffusion analysis from a Lagrangian particle dispersion model (Weil et al., 2004, 2009), driven by LES fields, and possibly other models.