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https://doi.org/10.5194/bg-2020-216
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-216
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 24 Jun 2020

Submitted as: research article | 24 Jun 2020

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This preprint is currently under review for the journal BG.

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

Bart Schilperoort1, Miriam Coenders-Gerrits1, César Jiménez Rodríguez1,4, Christiaan van der Tol3, Bas van de Wiel2, and Hubert Savenije1 Bart Schilperoort et al.
  • 1Delft University of Technology, Water Management department, Stevinweg 1, 2628 CN Delft, the Netherlands
  • 2Delft University of Technology, Geoscience & Remote Sensing department, Stevinweg 1, 2628 CN Delft, the Netherlands
  • 3University of Twente, Faculty of Geo-Information Science and Earth Observation (ITC), Hengelosestraat 99, 7514 AE, Enschede, the Netherlands
  • 4Tecnológico de Costa Rica, Escuela de Ingeniería Forestal. 159-7050, Cartago, Costa Rica

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), and was regularly decoupled from the atmosphere (50 % of the time at night). The relationship between the temperature gradients and the friction velocity (u*) showed a clear threshold between coupling regimes. In particular, decoupling occurred when u* < 0.4 m s−1, where the understory could become strongly stably stratified at night. At higher values of the friction velocity the canopy was 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 mechanically 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. This type 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.

Bart Schilperoort et al.

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Speulderbos turbulence and temperature profile measurements B. Schilperoort, C. Jiménez Rodríguez, C. van der Tol, M. Ucer, and M. Coenders-Gerrits https://doi.org/10.4121/uuid:e0a3d8c9-cb3c-4029-bbe8-2b775c0b88ef

Bart Schilperoort et al.

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Short summary
With the DTS technology we measured a vertical temperature profile in a forest, from the forest floor to above the tree tops. Using this temperature profile we can see which parts of the forest canopy are colder (thus more dense) or warmer (and less dense), and study the effect this has on the suppression of turbulent mixing. This information can be used to improve our knowledge on the interaction between the atmosphere and forests, and improve carbon dioxide flux measurements over forests.
With the DTS technology we measured a vertical temperature profile in a forest, from the forest...
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