Thomas Foken

Estimates of the annual net carbon and water exchange of forests: The EUROFLUX methodology

Publisher Summary The chapter has described the measurement system and the procedure followed for the computation of the fluxes and the procedure of flux summation, including data gap filling strategy, night flux corrections and error estimation. It begins with the introduction of estimates of the annual net carbon and water exchange of forests using the EUROFLUX methodology. The chapter then provides us with the theory and moves on to discuss the eddy covariance system and its sonic anemometer, temperature fluctuation measurements, infrared gas analyser, air transport system, and tower instrumentation. Additional measurements are also given in the chapter. Data acquisition and its computation and correction is discussed next in the chapter by giving its general procedure, half-hourly means (co-)variances and uncorrected fluxes, intercomparison of software, and correction for frequency response losses. The chapter has also discussed about quality control and four criteria are investigated here for the same. Spatial representativeness of measured fluxes and summation procedure are reviewed. The chapter then moves on to the discussion of data gap filling through interpolation and parameterization and neural networks. Corrections to night-time data and error estimation are also explored in the chapter. Finally, the chapter closes with conclusions.

The energy balance closure problem: An overview

This paper gives an overview of 20 years of research on the energy balance closure problem. It will be shown that former assumptions that measuring errors or storage terms are the reason for the unclosed energy balance do not stand up because even turbulent fluxes derived from documented methods and calibrated sensors, net radiation, and ground heat fluxes cannot close the energy balance. Instead, exchange processes on larger scales of the heterogeneous landscape have a significant influence. By including these fluxes, the energy balance can be approximately closed. Therefore, the problem is a scale problem and has important consequences to the measurement and modeling of turbulent fluxes.

Post-Field Data Quality Control

This Chapter summarizes the steps of quality assurance and quality control of flux measurements with the eddy covariance method. An important part is the di erent steps of the control for electronic, meteorological and statistical problems. The fulfillment of the theoretical assumptions of the measuring method and thenon-steady state test and the integral turbulence test are extensively discussed as well as an overall flagging for data quality and a site specific quality analysis using footprint models. Finally, problems are discussed which are not included yet in the control program, mainly connected with the complicated turbulence structure at a forest site.

New Equations For Sonic Temperature Variance And Buoyancy Heat Flux With An Omnidirectional Sonic Anemometer

Many new types of sonic anemometer obtain sonic temperature from an average value of temperature measured along three paths, unlike previous sonic anemometers that generally used one path. New equations are derived to calculate temperature variance from sonic temperature variance and sensible heat flux from buoyancy flux considering the influence of a crosswind. These equations can be applied to CSAT3, Solent R2, R3, R3A, HS, and USA-1 sonic anemometers with the corresponding correction factors given in this paper. The equations are verified by data measured by a CSAT3 sonic anemometer in the LITFASS-1998 field study.

Impact of post-field data processing on eddy covariance flux estimates and energy balance closure

This study evaluates the impact of post-field data processing methods on eddy covariance flux estimates and the resulting energy balance residual. To that end, a dataset from the LITFASS-2003 field campaign was analysed using an experimental software package. Widely discussed issues in data processing like an adequate flux averaging time, coordinate transformations and alternative approaches for the correction of density effects were examined. The impact of all the single processing steps on the turbulent flux estimates of sensible heat, latent heat and CO 2 as well as the impact on the resulting energy balance residual is demonstrated. The mean energy balance residual could be reduced by 17 % compared to 30-min covariances without any conversions or corrections if all necessary procedures were applied to the test dataset. Important procedures for the reduction of the experimental energy balance closure problem were the correction for spectral loss and the correction for density effects. Furthermore, the energy balance residual vanished almost completely if the covariance averaging time was extended from 30 minutes over 24 hours to five days. The low-frequency flux contributions could be explained through effects which were caused by the strong heterogeneity of the landscape surrounding the measurement site. The dependence of CO 2 flux estimates on the post-field data processing was stronger. Their mean value was changed by a factor of 2 through the correction for density effects.

Corrections and Data Quality Control

This chapter describes corrections that must be applied to measurements because practical instrumentation cannot fully meet the requirements of the underlying micrometeorological theory. Typically, measurements are made in a finite sampling volume rather than at a single point, and the maximum frequency response of the sensors is less than the highest frequencies of the turbulent eddies responsible for the heat and mass transport. Both of these cause a loss of the high-frequency component of the covariances used to calculate fluxes. Errors also arise in calculating fluxes of trace gas quantities using open-path analyzers because of spurious density fluctuations arising from the fluxes of heat and water vapor. This chapter gives the reader an overview of how these sources of error can be eliminated or reduced using some model assumptions and additional measurements. Corrections needed for some specific instruments are presented (Sect. 4.1), followed by a discussion of the generally observed lack of closure of the energy balance using the sum of latent and sensible heat fluxes (Sect. 4.2). The chapter closes with a discussion of measures needed to determine the quality of the final calculated fluxes (Sect. 4.3)

The Energy Balance Closure Problem

The equation representing the energy balance of the Earth’s surface is a fundamental component of all models of land-surface/atmosphere interaction. In its most common, simplest form, as applied over bare soil or short Vegetation, this equation states that the available energy at the surface (the net radiation (R n ) minus the ground heat flux (G)) is equal to the sum of the sensible heat flux (H) and the latent heat flux (γE, where γ is the latent heat of vaporisation)

The Eddy Covariance Method

R_n - G = H + \lambda E