Master Thesis

Influence of a clear cut on properties and vertical coupling of coherent structures

Fabian Eder (08/2011-01/2012)

Support: Andrei Serafimovich, Thomas Foken

Little is known about the importance of coherent structures on the exchange process in forest ecosystems, especially around forest edges. Thus, this thesis tries to describe fundamental characteristics of coherent structures between a forest site and an adjacent clear cutting. Therefore, a methodological approach using wavelet analysis developed for homogeneous forest conditions was applied to a disturbed forest ecosystem. Within the frame of the EGER (ExchanGE processes in mountainous Regions) project, eddy covariance systems were set up in different heights along a forest-to-clearing transect. From these high-frequency turbulence data, the dominant large-scale transport scales were extracted which were assumed to be caused by coherent structures. Then, the time and length scales, coherent flux contributions to total fluxes and finally, vertical coupling regimes at the clear cutting and the forest were analyzed.

The time scales revealed that there are considerably larger structures within the trunk space of the forest suggesting intermittency of coherent structures within the forest and fully developed turbulence at the clear cutting and the forest edge. This supports the assertion of better vertical coupling at the forest edge. Contrariwise, coherent structures contribute less to total turbulent flux at the forest edge and above the clear cutting than within the nearby forest. Accordingly, the canopy exchange regimes which base upon coherent flux contributions do not show that vertical coupling is better at the forest edge. But there might be additional structures forming above the clear cutting which the applied wavelet routine is not able to detect. Directly at the forest edge, downward-directed sweeps dominate during nighttime hours whereas strong upward-moving ejections occur during the daytime. It is hypothesized that there are even larger structures above the clear cutting induced by thermal differences, which cause this systematic inflow and outflow at the roughness heterogeneity, depending on time of the day. Furthermore, additional sweep and ejection motions are created by the wind field at the forest edge, depending on whether the wind hits a leading or a trailing edge of the forest. All in all, these observations show that there could be more transport mechanisms at the forest edge which are not included into the methodological approach which was applied.

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last modified 2012-01-31