Johannes Ruppert

Sensitivity of Lagrangian Stochastic footprints to turbulence statistics

This study tests the sensitivity of a Lagrangian Stochastic footprint model to the turbulence statistics describing the flow field, with a focus on the within canopy processes. Representative profiles of the input velocity statistics are taken from a long-term dataset of turbulence measurements within and above a tall spruce canopy. Based on a wavelet tool, which allows a detailed analysis of coherent structures along the vertical profile, we characterize several typical states of coupling and decoupling between surface, canopy and atmosphere. For each coupling regime, three flux footprints using different sources for turbulence statistics are compared: the first based on conditionally-averaged measurements, the second on a simple numerical solution and the third on measurements taken from literature. The effects of profile smoothing and connecting measured canopy data to parametrized atmospheric surface layer profiles are considered. Significant differences between footprints based on modelled and measured profiles were found for exchange regimes with the lower section of the profiles decoupled from the atmospheric surface layer. As such cases are likely to occur for tall canopies with moderate density, our results suggest that the accuracy of Lagrangian Stochastic footprint modelling could be improved by using better turbulence profiles for different exchange regimes.

Structure of Carbon Dioxide Exchange Processes Above a Spruce Forest

Several micrometeorological techniques,’such as the flux-gradient method or the eddy covariance technique, offer the potential to measure net fluxes of water vapor, CO2 and other trace gases exchanged between ecosystems and the atmosphere (e.g., Baldocchi and Meyers 1998). Subsequent data analyses allow the calculation of net ecosystem CO2 exchange. These net fluxes, however, reflect the balance between different component fluxes. In the case of CO2, two opposing fluxes contribute to this net flux: CO2 uptake during photosynthesis and CO2 release during respiration from above- and belowground organisms. Distinguishing among these components is critical to obtain insights into the processes underlying ecosystem responses to climate forcing (Buchmann 2002). This is because environmental parameters, such as temperature and soil moisture, differentially affect biological activities (e.g., Baldocchi et al. 2001).

Development of Flux Data Quality Tools

At the Waldstein-Weidenbrunnen site, several techniques for data quality control were developed and tested and later on applied at European FLUXNET sites. The history of this development and the specific results for the site form the subject of this chapter. These data quality criteria include integral turbulence characteristics, which are dependent on heterogeneities in the footprint area and inside the canopy. Furthermore, footprint models were applied to determine the footprint climatology and to link these models with the data quality of eddy covariance data. This tool was also applied to find the optimal period for the application of the planar-fit rotation method. The energy balance closure was found to be about 80 % in all periods. These findings were summarized as a schema for data quality control and characterization of FLUXNET sites.