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Karlsruher Instituts fur Technologie (KIT) 2018

Interactions between a solitary semi-arid forest and the Atmospheric Boundary Layer

Brugger Peter

Titre : Interactions between a solitary semi-arid forest and the Atmospheric Boundary Layer

Auteur : Brugger Peter

Université de soutenance : Karlsruher Instituts fur Technologie (KIT)


Résumé partiel
Semi-arid forests are expected to become more important in the future due to predicted warming and drying trends for parts of the global land area and due to afforestation efforts in the Mediterranean region. To model the climate impact of these ecosystems correctly, it is important to understand their surface-atmosphere interactions. Therefore, the underlying driving mechanisms of the surfaceatmosphere exchange for heat and momentum of semi-arid forests and their impact on the atmospheric boundary layer are explored within this PhD thesis. Meteorological field measurements at the isolated, semi-arid Yatir forest (length of 7 km in main wind direction, area 2800 ha) in Israel are used to achieve this goal. The field campaign entailed three measurement sites : one located in the shrubland upwind of the forest, one at the center of the forest, and one downwind of the forest. The upwind and forest site were equipped with an ultrasonic anemometer and a Doppler lidar each. A ceilometer was deployed at the downwind site. Information about the turbulence structure, the turbulent transport efficiency and energetics, the wind speed and direction, and the boundary-layer height were extracted from those measurements. The results showed that the canopy sublayer of the forest has a more isotropic turbulence structure compared to the surface layer of the upwind shrubland at the large turbulence-producing scales. While isotropic turbulence is expected to be less efficient at transporting momentum, the forest was observed to have a more energetic and more efficient momentum transport compared to the shrubland. This was explained with higher vorticity due to the larger roughness of the canopy leading to a larger velocity gradient. These turbulence properties of the forest lead to a sensible heat flux which is driven by stronger turbulence, but not a more efficient transport process of heat. This separation of processes contributing to the sustainment of a larger turbulent flux for a given mean gradient in the canopy sublayer compared to the surface layer (which was already known previously in literature) allows to identify the important processes for modelling purposes. In case of the sensible heat fluxes at the Yatir forest the effect of the modified turbulence structure was more than five times smaller than the effect of the more energetic turbulence.


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Page publiée le 12 mars 2020, mise à jour le 12 avril 2023