Stability Dependence of Canopy Flows over a Flat Larch Forest

Abstract

The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra. This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence. The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised at nearly half its value under stable conditions.

Momentum and scalar transport within a vegetation canopy following atmospheric stability and seasonal canopy changes: the CHATS experiment

“…Although, the mixing-layer structures dominating exchange during the foliated period transport these quantities more efficiently. This superposition of turbulent structures within canopy was suggested by Poggi et al (2004) and Kobayashi and Hiyama (2011), and deduced in the orchard canopy from the spectral analysis of the wind velocity components (Dupont and Patton, 2012) .…”

“…This speculation is consistent with Li and Bou-Zeid's (2011) recent study over natural surfaces (a lake and a vineyard), who also suggested that with increasing instability the transport dissimilarity between momentum and scalars could be explained through modification of the near-neutral surface atmospheric boundary layer's hairpin vortices and hairpin packets and their evolution into upward-and downward-moving thermal plumes. In sparse canopies, Poggi et al (2004) and Kobayashi and Hiyama (2011) suggested that mixing-layer type structures might also coexist with traditional atmospheric surface layer (ASL) turbulence. Forest heterogeneities at scales similar to canopyheight may further modify the efficiency of those turbulent structures at exchanging momentum (Bohrer et al, 2009).…”