Master Thesis

Modellierung des Energie – und Stoffaustausch im Grenzbereich Wald – Lichtung unter besonderer Berücksichtigung des Einflusses kohärenter Strukturen

Kathrin Gatzsche (02/2013-09/2013)

Support: Thomas Foken

In Kooperation mit der der Universität Leipzig, externe Betreuung

Simulating the exchange of energy and matter of horizontally heterogeneous terrain with forest – clearing transition utilizing a one – dimensional SVAT model (Soil –Vegetation – Atmosphere Transfer) represents a multi–faceted challenge. On the one hand, turbulent exchange processes of forest are characterized by counter – gradient fluxes caused by coherent structures. On the other hand, the edge of a forest represents heterogeneity, inducing secondary structures and leading to a possible increase of turbulent exchange. Additionally, clearings are characterized by alternating vegetation, which complicates adequate description by a one – dimensional model.

The present thesis utilizes the Advanced Canopy –Atmosphere – Soil Algorithm (ACASA) model to simulate the exchange processes of a spruce forest and a clearing. ACASA is an advanced SVAT model with an implemented third – order closure, capable of resolving the coherent flux contribution. This represents an advantage over K– theory and first – order closure most commonly utilized by models.

By means of extensive micrometeorological measurements, the EGER project (ExchanGE processes in mountainous Regions) offers the data base for the model input files as well as comparative values for the simulation results. Additionally, plant physiological measurements are used to adapt photosynthetic processes in the ACASA model to the Waldstein – Weidenbrunnen measuring area. The present thesis includes measurements of EGER projects third intensive observation period.

The turbulent exchange of the clearing is realized has also proven successful for the simulation of sensible and latent heat fluxes, because aerodynamic parameters can be incorporated. Model behavior of spruce forest and clearing are not solely evaluated by existing turbulent flux values. Detection software, based on wavelet analysis, determines the contribution of coherent structures to the turbulent flux as well their vertical coupling inside the forest and at its edge. Consequently, model deviations can be checked for their correlation to specific coupling conditions. Elevated model deviations during the night at decoupled subcanopy space cannot be evaluated without consideration of secondary structures. The present thesis’ model deviations are significantly lower than the simulation deficits for the spruce forest at vertically uncoupled coherent structures that occurred in a previous investigation which utilized a model with implemented first – order closure. In view of further investigations, the determination of secondary structures by means of specifically targeted measurement and analysis methods would be very useful in assessing their influence on the turbulent exchange at the Waldstein–Weidenbrunnen measuring area and corresponding model deviations.

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last modified 2013-10-01