Showing papers by "Thomas Foken published in 2021"

Selected breakpoints of net forest carbon uptake at four eddy-covariance sites

Abstract: Extensive studies are available that analyse time series of carbon dioxide and water flux measurements of FLUXNET sites over many years and link these results to climate change such as changes in a...

Estimating immediate post-fire carbon fluxes using the eddy-covariance technique

Abstract: . Wildfires typically affect multiple forest ecosystem services, with carbon sequestration being affected both directly, through the combustion of vegetation, litter and soil organic matter, and indirectly, through perturbation of the energy and matter balances. Post-fire carbon fluxes continue poorly studied at the ecosystem scale, especially during the initial window-of-disturbance when changes in environmental conditions can be very pronounced due to the deposition and subsequent mobilization of a wildfire ash layer and the recovery of the vegetation. Therefore, an eddy-covariance system was installed in a burnt area as soon as possible after a wildfire that had occurred on 13 August 2017, and has been operating from the 43rd post-fire day onwards. The study site was specifically selected in a Mediterranean woodland area dominated by Maritime Pine stands with a low stature that had burnt at high severity. The carbon fluxes recorded during the first post-fire hydrological year tended to be very low, so that a specific procedure for the analysis and, in particular, gap filling of the eddy covariance data had to be developed. Still, the carbon fluxes varied noticeably during the first post-fire year, broadly revealing five consecutive periods. During the rainless period after the wildfire, fluxes were reduced but, somewhat surprisingly, indicated a net assimilation. With the onset of the autumn rainfall, fluxes increased and corresponded to a net emission, while they became insignificant with the start of the winter. From the mid winter onwards, net fluxes became negative, indicating a weak carbon update during spring followed by a strong uptake during summer. Over the first post-fire year as a whole, the cumulative net ecosystem exchange was −347 g C m−2, revealing a relatively fast recovery of the carbon sink function of the ecosystem. This recovery was mainly due to understory species, both resprouter and seeder species, since pine recruitment was reduced. Specific periods during the first post-fire year were analyzed in detail for improving process understanding. Perhaps most surprisingly, dew formation and, more specifically, its subsequent evaporation was found to play a role in carbon emissions during the rainless period immediately after fire, involving a mechanism distinct from de-gassing of the ash/soil pores by infiltrating water. The use of a special wavelet technique was fundamental for this inference.

High Levels of CO 2 Exchange During Synoptic Scale Events Introduce Large Uncertainty Into the Arctic Carbon Budget

Abstract: CO2 release from thawing permafrost is both a consequence of, and a driver for, global warming, making accurate information on the Arctic carbon cycle essential for climate predictions. Eddy covariance data obtained from Bayelva (Svalbard) in 2015, using well‐established processing and quality control techniques, indicate that most of the annual net CO2 uptake is due to high CO2 flux events in winter that are associated with strong winds and probably relate to technical limitations of the gas analyzer. Emission events may relate to either (unidentified) instrumental limitations or to physical processes such as CO2 advection. Excluding the high winter uptake events yields an annual CO2 budget close to zero; whether or not these events are included can, therefore, have a considerable effect on carbon budget calculations. Further investigation will be crucial to pinpoint the factors causing these high CO2 flux events and to derive scientifically substantiated flux processing standards.

Eddy-Covariance Measurements

Abstract: 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.

Lettau’s Contribution to the Obukhov Length Scale: A Scientific Historical Study

Abstract: It has been repeatedly assumed that Heinz Lettau found the Obukhov length in 1949 independently of Obukhov in 1946 However, it was not the characteristic length scale, the Obukhov length L, but the ratio of height and the Obukhov length (z/L), the Obukhov stability parameter, that he analyzed Whether Lettau described the parameter z/L independently of Obukhov is investigated herein Regardless of speculation about this, the significant contributions made by Lettau in the application of z/L merit this term being called the Obukhov–Lettau stability parameter in the future

Evapotranspiration Measurements and Calculations

Abstract: 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.

Importance of the Webb, Pearman, and Leuning (WPL) correction for the measurement of small CO2 fluxes

Abstract: . The WPL (Webb, Pearman, and Leuning) correction is fully accepted to correct trace gas fluxes like CO 2 for density fluctuations due to water vapour and temperature fluctuations for open-path gas analysers. It is known that this additive correction can be on the order of magnitude of the actual flux. However, this is hardly ever included in the analysis of data quality. An example from the Arctic shows the problems, because the size of the correction is a multiple of the actual flux. As a general result, we examined and tabulated the magnitude of the WPL correction for carbon dioxide flux as a function of sensible and latent heat flux. Furthermore, we propose a parameter to better estimate possible deficits in data quality and recommend integrating the quality flag derived with this parameter into the general study of small carbon dioxide fluxes.

Introduction to Atmospheric Measurements

Abstract: 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.

Ground-based Mobile Measurement Systems

Abstract: 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.

Quality Assurance and Control

Abstract: 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.

Atmospheric Measurements for Different Purposes

Abstract: 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.