Decoupling of a Douglas fir canopy

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

Schilperoort, B. (TU Delft Water Resources)
Coenders-Gerrits, Miriam (TU Delft Water Resources)
JIMENEZ RODRIGUEZ, C.D. (TU Delft Water Resources; Instituto Tecnologico de Costa Rica)
van der Tol, Christiaan (University of Twente)
van de Wiel, B.J.H. (TU Delft Atmospheric Remote Sensing)
Savenije, Hubert (TU Delft Water Resources)

2020

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 highresolution (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.

http://resolver.tudelft.nl/uuid:4eb40bdb-a7e0-4de8-a0f9-8dd7ccadfc97

https://doi.org/10.5194/bg-17-6423-2020

1726-4170

Biogeosciences, 17 (24), 6423-6439

Institutional Repository

journal article

© 2020 B. Schilperoort, Miriam Coenders-Gerrits, C.D. JIMENEZ RODRIGUEZ, Christiaan van der Tol, B.J.H. van de Wiel, Hubert Savenije