- Quellgebiet (1) (remove)
- Adoption of footprint methods for the quality control of eddy-covariance measurements (2004)
- Footprint models determine the spatial context of a measurement by defining a transfer function between sources or sinks of the signal and the sensor position. The resulting source area provides an important quality control tool to improve the interpretation of micrometeorological data sets. However, to date no approaches have been presented in the literature that provide a standardised footprint-based methodology that allows observers to include terrain characteristics into quality assessment and quality control strategies. One problem in this context is the small number of studies that concentrate on the validation of footprint models under the non-ideal conditions in which they are frequently being used. Therefore, for many applications, the accuracy of the source areas computed by the footprint models cannot be evaluated. To further increase the acceptance of footprint-based studies, a stronger focus on footprint validation studies for a wide variety of experimental designs is needed. This dissertation focuses on the development of a footprint-based evaluation tool for complex measurement sites that allows the combination of quality assessment results for micrometeorological measurements with characteristics of the surrounding terrain. The standardised method is easy-to-use in order to encourage its application on a large number of sites. To improve the interpretation of the obtained results, a second objective of this thesis was to develop and test approaches to validation experiments for footprint models. Göckede et al. (2004) presented an approach for the evaluation of micrometeorological measurement sites in complex terrain, which combined a method for quality assessment of eddy-covariance measurements with an analytic footprint model. Their software package provided micrometeorologists a practical tool for determining the average flux contributions from the land use type intended to observe at a specific site, or to identify footprint areas for which a high data quality could be assumed. Rebmann et al. (2005) proved the efficiency of this evaluation approach for extensive studies on a large number of sites organised in a network. Their results may serve as a tool for an improved determination of yearly sums of the net ecosystem exchange, because fluxes originating from sectors of minor quality could be excluded from the analysis. Because of these important contributions to quality control, Foken et al. (2004) integrated the site evaluation approach into a comprehensive survey on micrometeorological post-field data quality control techniques. The experiences obtained during the extensive study by Rebmann et al. (2005) allowed us identification of the major weak points of the approach, which we were able to improve in subsequent studies. Using remote sensing methods Reithmaier et al. (2005) studied the influence of the characteristics of the land use maps and different roughness length assignment schemes on the performance of the site evaluation approach. Finally, Göckede et al. (2005a) developed an updated version of the site evaluation approach, which improved the basic method by replacing the analytic footprint model with a Lagrangian stochastic footprint model that is more suitable for studies above high vegetation, and by applying a more sophisticated microscale flux aggregation method for the determination of areally-averaged roughness lengths. Although the implemented models are far more sophisticated than in the original version, the approach by Göckede et al. (2005a) still permits a practical application that allows for comparative studies of a large number of sites. With respect to the development of validation methods for footprint models using natural tracer measurements from field scale experiments, Göckede et al. (2005b) presented two different experimental approaches. Firstly, a comparison of measured flux differences and modelled land use differences for pairs of measurement positions revealed general correlations between measurement data and model results. Secondly, Göckede et al. (2005b) tested a correlation analysis between measured and modelled parameters using reference measurements and footprint results. This approach resulted in an objective quantitative evaluation of the accuracy of the footprint model. The study by Reth et al. (2005) could not be employed for footprint validation purposes because of a large systemic scatter between these measurement systems. Overall, both the paper by Göckede et al. (2005b) and by Reth et al. (2005) provided successful methods to testing the suitability of natural tracer experiments in the validation of footprint models. Although experimental deficits prevented the working out of significant differences between the results of the employed footprint models, their studies developed an improved design for natural tracer experiments that are especially designed for footprint validation purposes.