About the three-dimensional behavior of the flow within a forest under unstable conditions

Wavelet spectra within a forest subcanopy under different wind directions and stabilities

“…Holford and Linden [] observed that stratification aids in maintaining the vorticity in the wakes but dampens turbulence, resulting in apparent periodic motions within the frequency space that may not translate to increased turbulence. This is consistent with the idea of inactive turbulence where observable motions exist but do not contribute to turbulent mixing [ Högström , ] and may aid in the explanation of why “sloshing” is observed near the ground within forest canopies [ Finnigan , ; Boldes et al ., ].…”

“…However, this would require the generation mechanisms to overcome buoyant suppression and dissipation [ Stull , ]. If the horizontal motions were organized [ Boldes et al ., ], the resulting vorticity could cascade energy to larger scales [ Holford and Linden , ; Riley and deBruynKops , ]. The inability for larger eddies to penetrate through the canopy under stable/downslope flow conditions, which allows the generation of a secondary maximum as the more “traditional” spectra structure, is not imposed on top of the canopy effects.…”

“…Boldes et al . [] used the Mexican Hat wavelet to calculate the local and global wavelet transforms concluding that the foliage region acts like a high‐frequency filter for horizontal motions but not for vertical motions. They also noted similarities between the cross‐wind and along‐wing wavelet spectra indicating organization in the horizontal flow.…”

“…Wavelet analysis has been used in studying turbulence structures in forest canopies, but the focus is often on identifying coherent structures, events, and turbulent transport [Collineau and Brunet, 1993;Raupach et al, 1996;Thomas andFoken, 2005, 2007;Eder et al, 2013;Starkenburg et al, 2013]. Although some studies have used wavelet transforms [Boldes et al, 2007;Prabha et al, 2008], global spectral analysis is still most often accomplished via Fourier transforms [Liu et al, 2001;Villani et al, 2003;Su et al, 2004;Cava and Katul, 2008;Lee et al, 2011;Dupont et al, 2012]. The assumptions surrounding the use of a Fourier transform (stationarity and lack of intermittent processes) can be a detriment to its use in studying turbulence in forests where the wind speeds are weaker, nonstationary conditions, or intermittent processes are observed [Collineau and Brunet, 1993;Mi et al, 2005;Huang and Wu, 2008].…”

“…Wavelets describe the data in both time and frequency components through a scalable window while Fourier transforms operate over a fixed timewindow [Terradellas et al, 2001]. Boldes et al [2007] used the Mexican Hat wavelet to calculate the local and global wavelet transforms concluding that the foliage region acts like a high-frequency filter for horizontal motions but not for vertical motions. They also noted similarities between the cross-wind and along-wing wavelet spectra indicating organization in the horizontal flow.…”