Does the HOST Hypothesis Apply Within and Above a Forested Mountaintop Canopy?

Within the past ten years, researchers have proposed the HOckey-Stick Transition (HOST) hypothesis to explain near-surface turbulence regimes. In brief, HOST challenges classical understanding of turbulence derived from Monin-Obukhov Similarity Theory (MOST), namely that the momentum flux entirely depends on ∂V/∂z (i.e., the change in wind speed with height). As HOST was developed using observations over relatively uniform, homogeneous terrain, questions arise about HOST’s applicability in complex terrain. Whereas HOST is being applied to different regions to determine its robustness, HOST has not yet been evaluated within and above a forested mountaintop canopy. In this study, we use measurements obtained from Chestnut Ridge in eastern Tennessee. Chestnut Ridge is a site within NOAA’s Surface Energy Budget Network (SEBN) and is surrounded by a deciduous forest representative of the southeastern United States. Since 2005, meteorological measurements, as well as heat and carbon dioxide fluxes, have been sampled at the on-site 60-m tower. In 2016, turbulence quantities were sampled at multiple heights within (i.e., 5 m, 15 m, 20 m, and 25 m above ground level (AGL)) and above (i.e., 30 m, 35 m, 40 m, and 45 m AGL) the forest canopy. We use these measurements to evaluate HOST, which we do by determining the relationship between select turbulence variables (i.e., turbulent kinetic energy, friction velocity, and vertical velocity variance) and ∂V/∂z and how this relationship varies as a function of season and atmospheric stability. Results indicate that this relationship generally agrees well with previous work for measurements taken above the canopy, but the results show significant deviations from HOST within the canopy implying some modifications to this theory are necessary to better account for turbulence processes in forested regions.