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The eddy-covariance method represents the only direct way to measure the turbulent fluxes of momentum, temperature, trace gases, and particles between the land surface and the atmosphere. It is a direct measurement of the net carbon-dioxide budget and dry deposition. For that purpose, it is widely used in networks of long-term ecosystem observatories around the world and is the centerpiece of intensive field campaigns investigating biosphere–atmosphere exchange processes. The instrumentation typically consists of a 3-D sonic anemometer/thermometer and one or more additional gas analyzers that are able to measure the high-frequency fluctuations of the scalar to be transported. These instruments are mounted on a meteorological mast to sample the turbulent field under the assumption that eddies are carried along with the mean wind. Further prerequisites of the method are horizontal homogeneity, steady-state conditions and well-developed turbulence. For successful application of the method, a series of quality tests and flux corrections is required, which will be presented together with commonly used instrumentation and postprocessing software. Moreover, we will provide a historical overview and provide guidelines for site selection and setup and the necessary maintenance procedures.

Actual and maximum rates of evaporation (E) and evapotranspiration (ET) are important to the operation of atmospheric process models and for hydrologic and agricultural modeling. Because rates of evapotranspiration are limited by both the available energy at the surface and the availability of water, a variety of techniques can be used for estimation. The near-maximum ET under nonlimiting water availability can be closely approximated by the reference ET concept using near-surface observations of air temperature, humidity, wind speed, and solar radiation via the Penman–Monteith or a similar method. The determination of actual rates of ET when water is limiting demands a more complex approach, and often requires daily (or even more frequent) water balance data for the upper soil layers. An alternative is to measure the actual ET using micrometeorological techniques such as the eddy-covariance and Bowen ratio methods. The application of standardized calculations for the reference ET is discussed, as are iterative surface energy balance–aerodynamic combinations, which are useful in conditions where water is limiting.

Measurements in the atmosphere differ from measurements in other media because of the thermodynamic and radiative properties of air (e. g., low density, low heat capacity, and transparency to a large part of the electromagnetic spectrum) and the fact that the air is almost always in (often turbulent) motion. The vertical and horizontal distributions of meteorological elements and the spatial and temporal scales of processes strongly influence the selection of appropriate measurement methods and devices. This chapter gives a brief overview of the different ways of classifying atmospheric measurements, the relevant states and processes that influence measurements in the atmosphere, and basic measurement techniques (including some fundamental aspects of the performance and interpretation of these techniques). It focuses in particular on the so-called atmospheric boundary layer, since most measurements are performed in this region and most human activities take place there.

Meteorological measurements are performed by various types of stations, including synoptic weather and climate monitoring stations, agrometeorological stations, and stations that require meteorological data for other purposes, such as to monitor traffic or air pollution or for private use. This chapter describes the history of stations that carry out atmospheric measurements, as well as recent developments in this field, mostly involving the use of new technologies to measure meteorological parameters. The specifics of the installation of stations for different purposes and the main parameters measured are summarized. The accuracy of the atmospheric measurements and the sensor system maintenance required vary depending on the intended application of the data. Therefore, the requirements for the measured meteorological parameters may differ from those defined in the corresponding parameter-specific chapters of this Handbook.

Quality assurance and control is fundamental to ensuring the scientific usefulness of atmospheric measurements. Quality is relevant to all stages of data generation, from site selection and system design to all physical components of a measurement system (including their calibration, operation, and maintenance) as well as data handling and processing. This chapter describes useful practices and techniques for a comprehensive quality management program that is designed to minimize problems and quantify quality along the entire data generation chain. Widely applicable methods of post-field data quality control for instrumented (in-situ), visual, and remotely sensed observations are presented. The chapter concludes with example applications of QA/QC in measurement networks, and a discussion of common data problems. Finally, future developments in quality assurance and quality control are presented.

While stationary measurements can be performed relatively frequently, they do not permit detailed spatial analyses of meteorological variables, particularly air temperature. This problem can be resolved by making the sensor mobile and then taking measurements at a variety of locations within the area of interest. Mobile measurements have therefore been used in climatological research since the early twentieth century. Due to improvements in and the miniaturization of the relevant measurement technologies, mobile measurements have undergone a renaissance since the 1970s. Urban climatology has become an important field of great scientific interest. The realization that the measurements of just one urban weather station—generally located (in line with the recommendations of the World Meteorological Organization) on short-cut lawn—is not sufficient to represent all of the climates present in an urban area has led to the acceptance of and even the need for mobile measurements taken on cars, bikes, or buses. New methodologies have been implemented, and advances in digital measurement and storage on data loggers have made mobile measurements an important tool for spatially distributed studies of air temperature, air humidity, and air pollution.

The eddy-covariance method is a tool that uses the high-frequency turbulent fluctuations within the atmosphere to determine energy and matter fluxes between the atmosphere and the underlying surface. The most important device is the sonic anemometer to measure the fluctuations of the three-component wind vectors, with the first instruments developed in the 1960s in Moscow and tested in the Volga steppe region. This paper presents a general overview on eddy-covariance methodology and instrumentation, including sophisticated updates based on wavelet analysis that help to overcome theoretical limitations with respect to investigating fluxes at high temporal resolution, and above large landscapes. As a first example for these updates, we present high-resolution methane fluxes in the Siberian permafrost area characterized by highly non-steady-state conditions. In a second example, we analyze aircraft measurements in the Lena region, which presents a horizontal distribution of the sensible heat fluxes from the surface to the boundary layer height, demonstrating the effect of the topography on the turbulent fluxes.

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.

Atmospheric waves and local wind phenomena in the atmospheric boundary layer are common forms of air motions observed above the forest canopy at night. Low-level jets with duration times of several hours and the gravity wave event were detected by SODAR-RASS and miniSODAR systems installed in the Fichtelgebirge Mountains in Germany.

One main topic of scientific and public interest regarding the Waldstein research site continues to be the high-quality observation of weather and air quality that captures the meteorology and climate of this particular site and the neighboring regions. Based on meteorological measurements made at the Waldstein facility from 1994 to the present, several studies—mainly of air temperature, precipitation, fog, wet deposition, and air pollutants—were conducted over the last 20 years. The expected strong global rise of earth’s basic air temperature will have a more moderated magnitude in Franconia, but the already continental location within the Central European climate zones, in combination with a heterogeneous landscape with imposed local orographic wind systems, will increase and reinforce diurnal and seasonal amplitudes and spatial variety of basic meteorological and air chemical elements and induce a higher risk of local extreme weather (climate) or smog (ozone) events. Forced by the change of macro- and mesoscale atmospheric circulation patterns across the northern hemisphere, the frequency and intensity of such weather-changing situations have increased during the last three or four decades in parallel with the span and the unpredictability of extreme weather conditions. That has, and will continue to have, an adjustment effect on air temperature and air humidity, sunshine duration and air pollution, wind (storm), date and duration of precipitation, and wet deposition of nitrogen, sulfur, salts, and metals and therefore a strong impact to the ecosystems at Waldstein.

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.

In this study we analyse eddy-covariance flux measurements of carbon dioxide and water vapour from 18 years at Waldstein–Weidenbrunnen (DE-Bay), a Norway spruce forest site in the Fichtelgebirge, Germany. Standard flux partitioning algorithms have been applied for separation of net ecosystem exchange NEE into gross primary production GPP and ecosystem respiration Reco, as well as gap-filling. The site has always been a carbon sink, and annual net uptake ( − NEE) shows a positive trend with values around 40 g C m−2 a−1 for 1997–1999 up to 615 ± 79 g C m−2 a−1 for 2011–2014. This is related to a strong increase in GPP, while Reco is slightly enhanced. Evapotranspiration increases coherently with NEE, while atmospheric demand, that is, potential evaporation, shows inter-annual variability, but no trend. Comparisons with studies from other warm-temperate coniferous forests show that our NEE estimates are at the upper range of the distribution, but still realistic. Also evapotranspiration estimates, evaluated in the Budyko framework, are in a similar range but with a large inter-annual variability. We identified instrumental problems and variability from different flux partitioning algorithms as a large source of uncertainty, but with only minor influence on the trends found. Warming and rising CO2-concentrations are consistent with the observed trend, but cannot be disentangled from site-specific changes such as the recovery from “Waldsterben” after liming and an increase in heterogeneity after a wind-throw, which likely plays the most important role in the observed dynamics. As such transitions from an “ideal” to a disturbed or heterogeneous site are likely more-often the case at FLUXNET stations built 10–20 years ago, a systematic bias in regional studies can only be avoided by taking each single site history into account.

This chapter summarizes the results of all 18 chapters and has a special focus on flux measurements in a changing environment, linking atmospheric turbulence and air chemistry, an optimal data quality protocol, and recommendations for the future of FLUXNET sites. It was found that windthrow and black beetle pests are responsible for the development of a larger heterogeneity of the forest site over the nearly 20-year observing period, with the consequence of an increase in carbon uptake. This heterogeneity, and not only the footprint and target area, should be included in a site description; however, such a heterogeneity index has yet to be developed. The developed coupling schema is extremely useful for interpreting chemical flux measurements, and it should be used together with the Damkohler number to characterize chemical reactions in a turbulent atmosphere in and above a forest. Furthermore, the possibility of characterizing the coupling should be implemented in each forest flux measurement program.

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.

The interplay between precipitation and evapotranspiration determines the input into the hydrological system of a catchment. Annual values of precipitation, evapotranspiration, and runoff measured at the catchment outlet for the 2002–2009 period were available. Annual precipitation clearly surmounted the sum of evapotranspiration and runoff. Part of the observed discrepancy might be due to the heterogeneity of precipitation and evapotranspiration within the catchment which has not been studied in sufficient detail. Annual evapotranspiration fluxes were remarkably constant during this period, whereas precipitation and runoff exhibited much larger interannual variability.

In 2011, a special experiment was conducted to investigate turbulent structures at the edge between the Waldstein–Weidenbrunnen forest site and the Kohlerloh clearing. A horizontal moving measuring system was used to detect significant gradients of the radiation fluxes, temperature, moisture, ozone, and carbon dioxide concentrations for different situations at day and night. In agreement with other studies, an increase of the turbulent fluxes and ejections at the forest edge could be found. This means that the energy balance closure was also better than that obtained directly at the Weidenbrunnen site. The vertical coupling by coherent structures was often—mainly at daytime—very good. In contrast, the horizontal coupling between the forest and the clearing at the edge was, in most cases, not apparent. For wind directions coming from the forest, the coherent structures did not touch down at the surface of the clearing. These investigations were made with a wavelet tool. A clear indication of secondary circulations between the forest and the clearing was not possible.

Exchange conditions at the forest floor are complex due to the heterogeneity of sources and sinks and the inhomogeneous radiation but are important for linking soil respiration to measurements in the trunk space or above canopy. Far more attention has therefore been paid to above and within canopy flows, but even studies that addressed forest floor exchange do not present measurements below 1 m or 2 m. We used a multilayer model that explicitly resolves the laminar layer, the buffer layer, and the turbulent layer to calculate fluxes from the measured profiles in the lowest meter above ground and to calculate effective surface concentrations from given fluxes. The calculated fluxes were compared to measured eddy covariance fluxes of sensible heat and O3 and to chamber derived soil fluxes of CO2 and 222Rn. Sensible heat fluxes agreed surprisingly well given the heterogeneity of radiative heating and the generally low fluxes (max. 25 W m−2). The chamber fluxes turned out to be not comparable as the chamber fluxes were too low, probably due to one of the well-known problems of enclosures such as pressure differences, disturbed gradients and exclusion of naturally occurring turbulence events and surface cooling. The O3 fluxes agreed well for high O3 values reaching down to the forest floor during full coupling of the canopy by coherent structures. During most of the time, the model overestimated the fluxes as chemical reactions were dominating within the profile. One new approach was to calculate the effective surface concentration from a given flux and compare this to measured surface concentrations. This allowed the identification of situations with a coupled and decoupled forest floor layer, which has important consequences for respiration measurements in the trunk space or above canopy and should be considered in upcoming studies.

Within micrometeorology the term modeling is not uniquely defined. It refers to various methods covering a range of complexity extending from simple regressions up to complicated numerical models . In applied meteorology (agro meteorology and hydro meteorology) simple analytical models are very common. Modeling of evaporation is particularly important but sophisticated numerical methods are not yet widely used in this research area. The following chapter describes different types of models and their limitations beginning with simple analytical methods up to numerical models of near-surface energy and matter transport. The application of models in heterogeneous terrain receives special attention and related flux averaging approaches are addressed in a separate subchapter.

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.

Measuring energy and matter fluxes between the atmosphere and vegetation using the Eddy Covariance (EC) technique is the state-of-the-art method to quantify carbon exchange between terrestrial ecosystems and their surrounding. The EC equipment is usually mounted onto a flux tower reaching higher than the local canopy. Today, more than 600 flux towers are in operation worldwide. The methodological requirements lead to high sampling frequency (20 Hz) and thus to the production of very long time series. These are related to temperature, wind components, water vapour, heat and gas exchange, and others. In this chapter, the potential of Recurrence Analysis (RA) to investigate the dynamics of this atmosphere-vegetation boundary system is elucidated. In particular, the effect of temporal resolution, the identification of periods particular suitable for reliable EC flux calculations, and the detection of transitions between dynamical regimes will be highlighted.

International audience; Instrumentation for the continuous measurement of stable isotopes in atmospheric trace substances such as carbon dioxide (CO2), methane (CH4), and also water vapor (H2O), has grown tremendously in the last decade. This is due to the develop- ment of new highly sensitive infrared spectroscopic techniques such as Fourier transform infrared spectroscopy, or laser-based systems like quan- tum cascade laser absorption spectroscopy and cavity ring-down spectroscopy. These systems are not only applied for precise continuous concentra- tion measurements, but have also recently been used for high-frequency isotope measurements of net ecosystem-atmosphere exchange fluxes. Similar development of isotope ratio mass spec- trometry systems led to successful application of the spectrometers in quasicontinuous mode at field stations. The currently available continuous measurement systems for CO2, CH4, and H2O sta- ble isotopic compositions are described here and information about calibration, maintenance, and quality assurance requirements of these systems is given.