PhD Thesis

Advection at a forest site - an updated approach

Lukas Siebicke (04/2007-01/2011)

Support: Thomas Foken

The exchange of carbon dioxide (CO2) across the vegetation-atmosphere interface of a spruce forest was investigated. Horizontal and vertical advection are recognized as important terms of the Net Ecosystem Exchange (NEE) budget in addition to the commonly measured turbulent ux and storage ux. Direct advection measurements are challenging because of the instrumental accuracy required to observe small concentration gradients and small wind velocities and because of the spatio-temporal measurement resolution required to observe complex 3-D ow phenomena. This work presents an experimental multi-analyzer setup for the observation of horizontal CO2 concentration gradients with high temporal resolution and good spatial resolution with no tradeo between the two. A statistical approach was developed to correct for inter-instrument bias by applying a conditional time dependent bias correction. This approach relies on properties of probability density distributions of concentration di erences between one sample point and the spatial average of the sample point eld. Sub-canopy CO2 concentration gradients observed with the above presented system showed a high spatial variability which was dependent on vegetation structure. Local concentration perturbations correlated with statistical properties of coherent structures and were explained by vertical exchange between CO2 enriched sub-canopy air and low concentration above-canopy air. The small-scale variability of CO2 concentration gradients brings into question the representativity of horizontal advection measurements for the control volume if observed with a low spatial resolution. Vertical advection estimates rely on accurate measurements of vertical wind velocity (w). Di erent procedures were applied during coordinate rotation to align the coordinate system of the sonic anemometer with the long-term stream lines. Spatial variability of the wind eld was addressed by a sector-wise coordinate rotation. An investigation of temporal aspects of vertical wind velocity showed signi cant contributions from low frequencies in the spectrum of w. The impact of the data set length used for coordinate rotation on w and on vertical advection was investigated and observed to be large. A sequential coordinate rotation with controlled window length was proposed. Advection contributed signi cantly to NEE during the night and during transition periods at the Waldstein-Weidenbrunnen (DEBay) FLUXNET site. Daily NEE budgets were more realistic, compared to NEE from turbulent ux and storage change alone, if direct advection measurements from continuous and bias corrected gradient sampling were included, reducing the estimated daily carbon sequestration of the forest by almost 50 %.

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last modified 2011-03-18