Lukas Siebicke

Aspects of CO2 advection measurements

Observations of vegetation–atmosphere exchange of carbon dioxide (CO2) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO2 concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity (

Description of the Waldstein Measuring Site

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Development of Flux Data Quality Tools

). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean

Coherent Structures and Flux Coupling

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Forest Climate in Vertical and Horizontal Scales

. Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE.

Quality Control Of Carboeurope Flux Data - Part 1: Coupling Footprint Analyses With Flux Data Quality Assessment To Evaluate Sites In Forest Ecosystems

The chapter gives a brief overview of the measuring sites of the Bayreuth Center of Ecology and Environmental Research at the Waldstein mountain, Germany, northeast Bavaria, with a special focus on the Waldstein-Weidenbrunnen site (FLUXNET site DE-Bay) as well as the Pflanzgarten and Kohlerloh sites. The climate of the area, the structure of the forest stands with plant area index profiles, and the soil properties are described. There is a special focus on the tower installations, the permanent program, and the special experimental campaigns. Pictures of all sites and all relevant data are given, while the instrumentation is listed in Appendix A. Due to the storm event “Kyrill” in January 2007, the southern part of the forest was destroyed. A trace gas flux footprint analysis is presented for conditions after a large wind throw.

Coupling processes and exchange of energy and reactive and non-reactive trace gases at a forest site - Results of the EGER experiment

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.

Vertical structure of evapotranspiration at a forest site (a case study)

This chapter summarizes the significant findings of the research on coherent structures contributed by investigations conducted at the Waldstein-Weidenbrunnen site from several field campaigns. The description of the quasi-online wavelet detection algorithm and of the coherent flux computation method using a triple decomposition is followed by a presentation of their application to define and diagnose vertical and horizontal couplings in forest canopies. It is demonstrated that these exchange regimes provide physically and biologically meaningful proxies for the communication of air and integration of the spatially separated sinks and sources as a result of the stratified canopy architecture. We continue by presenting two innovative applications of the coherent forest exchange that include the computation of daytime respiration fluxes directly from above-canopy eddy-covariance measurements and the explanation of stationary gradients in the sub-canopy CO2 field causing systematic advection as a result of the spatial heterogeneity of the forest architecture. Advantages and limitations of both are discussed. The chapter concludes by formulating directions for future research and indicating new observational techniques that may have the potential to improve understanding and quantifying the forest coherent exchange.

Quality control of CarboEurope flux data - Part I: Footprint analyses to evaluate sites in forest ecosystems

Microclimate was investigated within a heterogeneous spruce forest in Northern Bavaria, Germany, at the Waldstein–Weidenbrunnen site, especially during the EGER project in 2007, 2008 and 2011. Besides standard tower measurements, two innovative measuring techniques were used to investigate horizontal and vertical gradients. A particular focus was paid to advection within the homogeneous part and its effect on NEE, as well as gradients near a forest edge, measured by a mobile measuring system.