On the application of the hockey-stick transition hypothesis to characterize turbulence within and above a deciduous forest

Abstract

Turbulence governs many atmospheric processes including mixing, transport, and energy transfer. Consequently, there is a strong need for the examination and validation of existing turbulence theories. The HOckey-Stick Transition (HOST) hypothesis was proposed to challenge traditional understanding of near-surface turbulence processes derived from Monin-Obukhov Similarity Theory (MOST). Within the MOST framework, the momentum flux entirely depends upon $\partial \overline{U}/\partial z$ (i.e., the change in mean wind speed ($\overline{U}$) with height ($z$)), but this relationship is not as straightforward under HOST. Because HOST was developed using observations over relatively uniform, homogeneous terrain, questions arise regarding HOST's applicability within and above heterogeneous forest canopies where multi-level turbulence measurements are somewhat rare but are essential for developing a unified similarity scaling applicable over complex surfaces. To this end, we used one year (1 January 2016 through 31 December 2016) of turbulence measurements sampled at eight heights along a 60-m tower within and above a mixed deciduous forest at Chestnut Ridge in eastern Tennessee in the southeastern U.S. We examined the diurnal and seasonal variability of selected turbulence parameters (i.e., friction velocity ($u_{*}$) and turbulence velocity scale ($V_{TKE}$)) to detail the micrometeorological characteristics of the site during the study period. We then used these turbulence measurements to evaluate HOST by determining their relationship with $\overline{U}$ and to assess the dependencies of this relationship on time of day, season, wind direction, and atmospheric stability. We found that HOST is most applicable under very stable regimes, whereas the relationships between $u_{*}$ and $\overline{U}$, and between $V_{TKE}$ and $\overline{U}$, were more linear above the forest canopy than within the forest canopy and when the canopy was not foliated. Overall, this work builds upon previous studies that have described limitations in MOST and identifies scenarios when the HOST hypothesis may be more applicable than MOST for representing near-surface turbulence processes.