OPUS 4 | Geowissenschaften

Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem (2004)

Thomas Wrzesinsky

Der nebelgebundene Eintrag von Wasser und Spurenstoffen kann in den Bergwäldern Mitteleuropas eine wichtige Rolle spielen. Die Quantifizierung dieses Eintrags stieß jedoch in der Vergangenheit auf messtechnische Grenzen. Nach der Entwicklung und Erprobung eines Systems aus einem Tropfenspektrometer zur schnellen Messung der Größenverteilung (40 Tropfengrößenklassen zwischen ø1,5 und 50 µm) im Nebel und einem Ultraschallanemometer zur Bestimmung des vertikalen Windes konnten an der Ökosystemmessstation „Waldstein“ von April 2001 bis März 2002 Messungen zur Nebelwasserdeposition durchgeführt werden. Zusätzlich wurden die Sichtweite und die chemische Zusammensetzung (pH, elektrische Leitfähigkeit, Na+, K+, NH4+, Mg2+, Ca2+, Cl–, NO3–, SO42– und PO43–) des Nebelwassers gemessen. Zur Sammlung von Nebelwasser wurde ein aktiver beheizbarer Nebelsammler entwickelt und parallel zu den Wasserflussmessungen eingesetzt. Die Proben wurden automatisch alle acht Stunden genommen. Die Sammelmengen betrugen im Median 249 ml und erlaubten die gewünschten chemischen Analysen. Im Untersuchungszeitraum waren 223 Nebeltage zu verzeichnen. Der Nebelanteil betrug 25,7 %. Für die Qualitätskontrolle der gemessenen Flüsse wurden die Daten auf Stationarität und Turbulenz überprüft und der Datensatz entsprechend angepasst. Die Messung der Nebeldeposition im Untersuchungszeitraum ergab einen Eintrag von 108 kg ha–1 a–1 Wasser für die turbulente Deposition und 17 kg ha–1 a–1 für den Eintrag über Sedimentation. Der turbulente Eintrag dominiert also mit ca. 86 % die Nebeldeposition. Die Summe aus beiden Eintragsarten entspricht einem Nebelniederschlag von 125 mm p. a. Eine klare Saisonalität der Nebelwasserflüsse ist erkennbar. Die höchsten Nebelniederschläge sind im Spätherbst und im Winter zu verzeichnen, monatlich bis zu 24 mm (Januar) wurden gemessen. Die geringste Nebeldeposition wurde im August mit ca. 1 mm gemessen. Die ermittelten Tropfenspektren zeigen bei der Anzahlverteilung Maxima bei 2, 6 und 9 µm sowie ein Maximum von 12 µm in der Massenverteilung. Für die Massengrößenverteilung sind Verteilungen mit Maxima bei 9, 12 und 15 µm häufig. Die gemessenen Flüssigwassergehalte lagen bei einem Median von 156 mg m–3 und erreichten Maxima von 2639 mg m–3 (5-min-Mittel). Den größten Anteil am Fluss hatte die Größenklasse von 14,5 bis 15,5 µm Tropfendurchmesser. Tropfen kleiner 7 µm wurden effektiv emittiert, die größeren deponiert. Die im Untersuchungszeitraum gefallene Menge an Regen und Schnee beträgt 1414 mm. Der Anteil des Nebels am atmosphärischen Eintrag von Wasser beträgt demzufolge etwa 8 %. Für insgesamt 253 Nebelereignisse wurden im Untersuchungszeitraum Proben gewonnen. Außerdem wurden zum Vergleich auf wöchentlicher Basis wet-only-Proben genommen. Die Konzentrationen in Nebel- und Regenwasser sind hoch variabel. Die Mediane liegen im Nebelwasser bei pH 4,14, 621 µeq l–1 für NH4+, 487 µeq l–1 für NO3– und 321 µeq l–1 für SO42–. Diese 3 Hauptionen machen ca. 87 % der Gesamtkonzentration aus. Die Konzentrationen im Nebelwasser sind deutlich gegenüber dem wet-only-Niederschlag erhöht. Die Anreicherungsfaktoren sind 18,1 (NH4+), 13,1 (NO3–) bzw. 11,5 (SO42–). Der nebelgebundene Eintrag der wichtigsten Ionen wurde aus der Konzentration und dem Nebelwasserfluss errechnet. Die eingetragenen Mengen sind 9,8 kg ha–1 für NH4+ (7,9 kg ha–1 für wet-only), 27,9 kg ha–1 für NO3– (25,1) bzw. 14,0 kg ha–1 für SO42– (15,0). Die durch feuchte oder okkulte Deposition eingetragene Menge ist für diese Ionen also im gleichen Größenbereich wie die Menge aus Regen und Schnee. Der Stickstoffeintrag beträgt insgesamt 13,9 kg N ha–1 a–1 (11,8 für wet-only). Der im Unterschungszeitraum durch den Bestandesniederschlag gemessene Eintrag von Stickstoff liegt bei 23,3 kg N ha–1 a–1. Die Differenz aus Bestandesniederschlag einerseits und wet-only und Nebel andererseits liegt mit –0,9 kg N ha–1 a–1 nahe Null. Zusätzliche Einträge sind durch die trockene Deposition (z. B. durch partikuläres Nitrat und Salpetersäure) zu erwarten. Der Umsatz von Stoffen im Kronenraum spielt dann eine wichtige Rolle in der Schließung der Ökosystembilanz für die verschiedenen Stoffe.

Adoption of footprint methods for the quality control of eddy-covariance measurements (2004)

Mathias Göckede

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.

Detection and Analysis of Coherent Structures within and above Tall-vegetated Canopies (2005)

Christoph Thomas

Coherent structures are an inherent phenomenon of the atmospheric turbulent flow in the proximity to tall-vegetated canopies. Although coherent structures have called increasing attention of the turbulence community during the past decades, the basic mechanisms of their emergence, their contributions to exchange processes and their importance for conventional flux determination methods remain poorly understood or even unknown. In particular, no studies have been published yet dealing with long-term observations rather than exemplary case studies using short-term data. This dissertation aims at the enhanced understanding of the driving mechanisms and statistical properties of coherent structures within and above tall-vegetated canopies through an extensive analysis using long-term observations. Thereby, it places a specific emphasis on the implications of coherent structures for exchange processes and assesses their impact on conventional flux determination methods such as the eddy covariance and relaxed eddy accumulation techniques. Data were mainly obtained using tower-based single-point turbulence measurements and acoustic remote sensing technique (Sodar-Rass) during extensive field campaigns conducted in summer 2003. The developed wavelet software tool for detection and analysis of coherent structures is verified to extract coherent structures objectively under varying environmental conditions and thus allows determining their statistics in long-term datasets. In the proximity to the plant canopy, the temporal scales of coherent structures typically range between 20 s and 35 s. The temporal scales of coherent structures in the horizontal wind velocity, the sonic temperature and the concentration of carbon dioxide and water vapour exceed those of the vertical wind velocity. Within the canopy, the temporal scales of all vector and scalar variables collapse at approx. 24 s to 28 s resulting in an enhanced symmetry. Besides this, coherent structures with temporal scales up to 220 s are evidenced well above the canopy with the aid of acoustic remote sensing. The application of the canopy mixing-layer analogy to the data partially yields departures of the ratio m between the streamwise structures spacing of coherent structures and the canopy shear scale from the prediction m = 7...10. The departures are due to the influence of the terrain affecting the shape of the canopy wind profile and therefore the vertical wind shear. An agreement is found for flows which are forced to reorganise downstream of flow obstacles. The vertical wind shear is identified as the main driving force from which coherent structures emerge close to the canopy. In the layer well above the canopy diabatic processes facilitate the generation of coherent structures of large temporal scales. Clearcuts in a fairly homogeneous canopy cause additional structures in the turbulent flow with large temporal scales. Coherent structures contribute about 16 % to total the momentum transfer and about 26 % to the total fluxes of buoyancy, carbon dioxide and latent heat. A scheme for the qualitative classification of exchange regimes between the atmosphere and the canopy is developed analysing the ejection and sweep phases of coherent structures along the vertical profile in the canopy. The presence of coherent structures causes flux errors in the eddy covariance method below 4 %. The effect of this flux error for long-term observations is negligible as individual flux errors average out. Coherent structures of large temporal scales significantly influence the scalar similarity required for the relaxed eddy accumulation technique. These flow structures are responsible for the diurnal changes of the scalar similarity observed in the traces of acoustic temperature and concentration of carbon dioxide and water vapour.

Quality Assurance for Eddy Covariance Measurements of Turbulent Fluxes and its Influence on the Energy Balance Closure Problem (2006)

Matthias Mauder

The eddy covariance method enables direct measurements of turbulent fluxes at the earth’s surface. Such measurements are required to study the surface energy balance and the exchange of gaseous air constituents. Due to improvements in the construction of adequate sensors and the progress in computer technology during the last decades this method is now well-established. However, a general failure to close the energy balance equation has been reported for many micrometeorological field experiments. This unresolved problem motivated this dissertation, which aims at the quality assurance for eddy covariance measurements. The presented concept for quality assurance comprises investigations on the accuracy of the deployed sensors and on the impact of the data analysis for such measurements. A specific focus is set on possible implications for the determination of reliable CO2 flux estimates, since this issue gained importance during the last years for studies on the global carbon cycle related to global warming due to the green house effect. Data from several field experiments in Germany, California and Nigeria form the experimental basis for these investigations. A software package was developed to perform the necessary post processing for all eddy covariance measurements presented in this thesis. The results of sensor intercomparison experiments show a typical random error of eddy covariance measurements of 5% for the sensible heat flux and 10% for the latent heat flux, if sensors are well-calibrated and maintained and the assumptions for this method are fulfilled. The applicability of an objective quality assessment scheme of flux data was demonstrated for large datasets from a field campaign comprising 14 measuring systems. The energy balance closure problem has been studied at two exemplary sites. The energy balance could not be closed for measurements over an agricultural area in Germany. For this experiment the sum of turbulent heat fluxes was 30% smaller than the available energy at the surface. In contrast, no systematic bias of the energy balance could be found for measurements over fallow bush-land in Nigeria, although the measures of quality assurance were similar. Neither differences in instrumentation nor in the post-field data processing between both experiments can explain these findings. A further analysis of the dataset from the agricultural area in Germany showed that additional flux contributions can be found when extending the averaging time of covariances beyond the conventional 30 minute interval. The energy balance can even be closed for this site when applying an averaging time of 24 hours. Longwave flux contributions seem to be generated here by the much stronger heterogeneity of the surrounding terrain compared to the more or less homogeneous environment of the Nigerian site. The filtering of heterogeneity induced flux contributions from very low frequency covariances through the commonly used averaging times of less than 30 minutes is identified as major reason of the energy balance closure problem. To improve the understanding of the processes leading to low frequency flux contributions a more detailed analysis of further experiments in combination with large eddy simulation modelling are required. The impact of post-field data processing was not only evaluated for energy flux estimates but also for fluxes of CO2, which showed similar additional flux contributions for extended averaging times. Finally, it was demonstrated that the quality assessment scheme presented in this thesis provides a fundamental and robust rejection criterion for a successful gap-filling strategy to determine annual sums of CO2 net ecosystem exchange.

Modeling the exchange of energy and matter within and above a spruce forest with the higher-order closure model ACASA (2010)

Katharina Staudt

Multilayer SVAT-models that contain an advanced turbulence scheme are necessary for the detailed simulation of all relevant exchange processes above and within a forest canopy. The Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model incorporates such an advanced turbulence scheme, the third-order turbulence closure. This study presents the application of the ACASA model for a spruce forest at the Waldstein-Weidenbrunnen site in the Fichtelgebirge (Germany). The comprehensive micrometeorological and plant physiological measurements performed during the EGER project (ExchanGE processes in mountainous Regions) provided the necessary data base for this purpose, particularly eddy-covariance and sap flux measurements at several heights within the canopy. Thorough model tests were a main focus of this study and led to an improvement of the investigated model. This included both the exploration of the sensitivity and predictive uncertainty of the modeled fluxes and the analysis and correction of model errors that were encountered while working with the model. Furthermore, the ability of the ACASA model to reproduce measured quantities within and above the forest canopy was assessed, with an emphasis on the vertical structure of evapotranspiration and its components. To study the sensitivity and predictive uncertainty of the ACASA model, the Generalized Likelihood Uncertainty Estimation (GLUE) methodology was employed for two five day fair weather periods. Here, the sensitivity of the sensible heat flux, the latent heat flux and the net ecosystem exchange above the forest canopy was assessed. This analysis allowed the identification of influential parameters for the three fluxes. The fluxes were strongly sensitive to only a few parameters while the problem of equifinality was revealed for many parameters. Equifinality is a common problem for complex process-based SVAT-models. The calculated uncertainty bounds showed the ability of the ACASA model to well reproduce the fluxes for two periods with different meteorological conditions. Furthermore, the results of the GLUE analysis indicated weaknesses in the model structure concerning the soil respiration calculations. The latest ACASA version includes multiple improvements in comparison to older model versions which were introduced after a comparison of modeled within- and above-canopy fluxes and turbulence statistics with measurements. The former version of the ACASA model did not explicitly close the energy balance. Rather, an error was included in the model output. This modeled error, however, did not agree with the measured residual at our site and was shown to reach substantial magnitudes depending on the value of the leaf area index. Thus, a method to ensure a closed energy balance for all layers in ACASA was introduced. Measured third-order velocity statistics were largely underestimated by the former ACASA version, which required correcting the calculation algorithms for the third-order moments in the latest ACASA version. Comparisons of third- and second-order velocity statistics showed that simulations of the latest ACASA version were improved but only partly reproduced measurements. Sap flux and eddy-covariance measurements at several heights within the profile provided estimates of all components of evapotranspiration of the forest and its vertical distribution. Canopy transpiration of the stand measured with the eddy-covariance technique delivered larger estimates than measured with the sap flux technique. Possible reasons for this mismatch are discussed, such as a contribution of evaporation from intercepted water that was still present at the beginning of the study period and differences between the eddy-covariance footprint and the area to scale up sap flux measurements. The modeled evapotranspiration components by ACASA compared well to these measurements when taking the uncertainties of these measurements into account. Also, modeled in-canopy profiles of canopy (evapo-) transpiration agreed well with measurements, with a better agreement of mean profiles for daytime, a partly and completely coupled canopy than for nighttime and a decoupled canopy. Largest contributions to canopy (evapo-) transpiration stem from the upper half of the canopy at daytime, whereas during nighttime, the contribution shifted towards lower parts of the canopy. Additionally, model simulations of the 3D model STANDFLUX were included in this study. This study revealed that the ACASA model is a powerful tool to simulate in detail a large range of the relevant exchange processes within and above a spruce forest site. At the same time existing weaknesses in the model code were identified that should be improved in future ACASA versions.

CO2 and Isotope Flux Measurements above a Spruce Forest (2008)

Johannes Ruppert

The measurement of the turbulent carbon dioxide (CO2) exchange by the eddy covariance (EC) method has become a fundamental tool for the quantitative determination of the atmospheric CO2 net ecosystem exchange (NEE) and the investigation of the carbon mass balances of ecosystems. Such measurements require a high degree of quality control in order to prevent systematic errors. The determination of the annual sum of NEE and filling of data gaps is complicated by characteristic diurnal and seasonal variation in the governing gross flux components of assimilation, i.e. photosynthetic uptake of CO2, and respiration. In this dissertation, a set of criteria is suggested for the identification of high quality NEE data. They are applied to data obtained above a spruce forest in the Fichtelgebirge Mountains in Germany. The application of the quality criteria resulted in less systematic distribution of data gaps compared to a commonly applied criterion based on the friction velocity u-star measured above the canopy. The suggested method is therefore able to reduce the risk of double accounting of nighttime respiration fluxes and systematic error in the annual sum of NEE. The isotopic flux partitioning method can be applied to quantify the assimilation and respiration flux components. Especially above forest ecosystems, it requires isotope flux measurements with high analytical precision in order to resolve small gradients in the isotopic signature of the turbulent exchange. A conditional sampling instrument was developed and tested in laboratory and field experiments. By combining the hyperbolic relaxed eddy accumulation method (HREA), whole-air sampling and high precision isotope ratio mass spectrometry (IRMS), 13CO2 and CO18O isotopic flux densities (isofluxes) could be measured with an estimated uncertainty of 10-20% during a three day intensive measuring campaign of the field experiment WALDATEM-2003 (Wavelet Detection and Atmospheric Turbulent Exchange Measurements 2003). Thorough quality control was applied at all stages of the experiment, including the data evaluation. The sampling process and the assumption of similarity in the turbulent exchange characteristics of different scalars (scalar similarity) were assessed by simulation of HREA sampling based on high temporal resolution data of the turbulent energy and gas exchange. Above three different vegetation types, distinct diurnal changes of scalar similarity were observed and attributed to events on time scales longer than 60 s, which most likely represent changes in the source/sink strength or convective or advective processes. Poor scalar-scalar correlations indicate the risk of systematic underestimation of fluxes measured by HREA. There is some evidence for good scalar similarity and a generally linear relation between bulk CO2 mixing ratios and its isotopic signatures in the turbulent exchange. However, the slope of that relation was observed to change temporarily so that especially for the EC/flask method temporal and spatial scales represented in flask samples must carefully be considered. HREA isoflux measurements have a footprint similar to the footprint of EC measurements and are therefore able to integrate small-scale heterogeneity in ecosystems. CO2 mixing ratios and delta-13C and delta-18O isotopic signatures measured in updraft and downdraft whole-air samples allowed determining ecosystem integrated and truly flux weighted isotopic signatures of the atmospheric ecosystem gas exchange and ecosystem isotope discrimination Delta-e and Delta-E on half-hourly timescales. The observed diurnal variability demonstrates the need for their repeated high precision measurement at ecosystem scale for the evaluation of isotopic mass balances. For the isotopic flux partitioning method, additional data on the integrated canopy isotope discrimination Delta´-canopy from independent measurements or validated models is indispensable. An observed fast equilibration of isotopic disequilibria D13C and D18O between the assimilation and respiration fluxes may indicate that the successful application of the isotopic flux partitioning method is limited to short periods after significant environmental changes on the scale of few days.

Documentation and Instruction Manual of the Eddy Covariance Software Package TK2 (2004)

Matthias Mauder Thomas Foken

no abstract

ATEM Software for Athmospheric Turbulence Exchange Measurements using Eddy Covariance and Relaxed Eddy Accumulation Systems and Bayreuth whole-air REA system setup (2005)

Johannes Ruppert

no abstract

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