Decoupling of a Douglas fir canopy: a look into the subcanopy with continuous vertical temperature profiles

Abstract

Abstract. Complex ecosystems such as forests make accurately measuring atmospheric energy and matter fluxes difficult. One of the issues that can arise is that parts of the canopy and overlying atmosphere can be turbulently decoupled from each other, meaning that the vertical exchange of energy and matter is reduced or hampered. This complicates flux measurements performed above the canopy. Wind above the canopy will induce vertical exchange. However, stable thermal stratification, when lower parts of the canopy are colder, will hamper vertical exchange. To study the effect of thermal stratification on decoupling, we analyze high-resolution (0.3 m) vertical temperature profiles measured in a Douglas fir stand in the Netherlands using distributed temperature sensing (DTS). The forest has an open understory (0–20 m) and a dense overstory (20–34 m). The understory was often colder than the atmosphere above (80 % of the time during the night, >99 % during the day). Based on the aerodynamic Richardson number the canopy was regularly decoupled from the atmosphere (50 % of the time at night). In particular, decoupling could occur when both u*<0.4 m s−1 and the canopy was able to cool down through radiative cooling. With these conditions the understory could become strongly stably stratified at night. At higher values of the friction velocity the canopy was always well mixed. While the understory was nearly always stably stratified, convection just above the forest floor was common. However, this convection was limited in its vertical extent, not rising higher than 5 m at night and 15 m during the day. This points towards the understory layer acting as a kind of mechanical “blocking layer” between the forest floor and overstory. With the DTS temperature profiles we were able to study decoupling and stratification of the canopy in more detail and study processes which otherwise might be missed. These types of measurements can aid in describing the canopy–atmosphere interaction at forest sites and help detect and understand the general drivers of decoupling in forests.

Aerosol deposition to the boreal forest in the vicinity of the Alberta Oil Sands

“…Gordon et al, 2011;Pryor et al, 2008) either by adding biogenic mass to anthropogenic aerosols or by aggregation of organic matter. Furthermore, the influence of mixing and coupling on deposition is often significant where stagnant air in the understory can act as a blocking layer between the canopy top and the surface (Schilperoort et al, 2020).…”

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

“…The below-canopy behaviour of L up and L do can be explained with a near-neutral stratification in the below-canopy air space and a stable layer close to the canopy height that might result from strong radiative cooling of the canopy and possibly concurrent heat release from the soil at the forest floor (Jacobs et al, 1994). Such temperature profiles were not uncommon during the two measurement campaigns, and are not uncommon in forests in general (Jacobs et al, 1994;Dupont and Patton, 2012;Everard et al, 2020;Schilperoort et al, 2020). In such conditions, the 𝜃 isolines fluctuate vertically over the whole neutrally stratified air layer and hence L do ≈ z and L up ≈ z inv − z below the stably stratified inversion layer (residing at height z inv ) close to the canopy height.…”

The NY-Ålesund TurbulencE Fiber Optic eXperiment (NYTEFOX): investigating the Arctic boundary layer, Svalbard

“…One goal of the NYTEFOX campaign was to provide a proof of concept for future applications of the horizontal fiber-optic distributed sensing technique in similarly chal-M.-L. Zeller et al: NYTEFOX -Ny-Ålesund Turbulence Fiber Optic Experiment lenging environments. While the feasibility of FODS has been proven for midlatitude boundary layers (Thomas et al, 2012;Sayde et al, 2015;Peltola et al, 2020;Schilperoort et al, 2020;Pfister et al, 2021a), the high-quality FODS data and their physical consistency with other more traditional near-surface meteorological observations underline the technical feasibility and the functionality of FODS deployments in extreme temperature and wind conditions of the polar regions. Note that temperatures during the measuring period dropped to −30 • C with an average of −17 • C, which is extraordinarily cold for NY-Ålesund and more representative of the higher Arctic.…”

“…The third main wind direction is southwest with wind speeds typically less than 5 m s −1 (Maturilli et al, 2013). Southwesterly winds are associated with katabatic flows down Zeppelin Mountain and the Brøgger Glacier and orographic channeling of the flow by the Brøgger Massif (Schulz, 2017). In wintertime, southwesterly winds are often accompanied by stable stratification and gravity waves excited at low wind speeds (Jocher et al, 2012).…”