The role of greenhouse gases (GHGs) in causing global climate change is now well recognized (IPCC... more The role of greenhouse gases (GHGs) in causing global climate change is now well recognized (IPCC, 2013). Emissions of GHGs from terrestrial ecosystems play an important part in this, and the potential for feedback within the climate system which amplify the emissions of GHGs from natural ecosystems is substantial. Accurate estimates are therefore needed of the GHG balance of the land surface at regional and national scales, and over long time periods, if we are to understand the key driver of global change. Because of the large scale involved, in relation to the scale at which we can make observations, this presents a major challenge that spans the domains of biogeochemistry, ecology, remote sensing, and atmospheric science.
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
As part of the quality assurance and quality control activities within the CarboEurope-IP network... more As part of the quality assurance and quality control activities within the CarboEurope-IP network, a comparison of eddy-covariance software was conducted. For four five-day datasets, CO 2 flux estimates were calculated by seven commonly used software packages to assess the uncertainty of CO 2 flux estimates due to differences in postprocessing. The datasets originated from different sites representing different commonly applied instrumentation and different canopy structures to cover a wide range of realistic conditions. Data preparation, coordinate rotation and the implementation of the correction for high frequency spectral losses were identified as crucial processing steps leading to significant discrepancies in the CO 2 flux results. The overall comparison indicated a good although not yet perfect agreement among the different software within 5-10% difference for 30-min CO 2 flux values. Conceptually different ideas about the selection and application of processing steps were a main reason for the differences in the CO 2 flux estimates observed. A balance should be aspired between scientific freedom on the one hand, in order to advance methodical issues, and standardisation of procedures on the other hand, in order to obtain comparable fluxes for multi-site synthesis studies.
The version presented here may differ from the published version or from the version of the recor... more The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription.
Eddy covariance has been the de facto method of analyzing scalar turbulent transport data. To ref... more Eddy covariance has been the de facto method of analyzing scalar turbulent transport data. To refine the information available from these data, we derive a simplified version of the turbulent scalar-transport equation for the surface layer, which employs a more explicit form of signal decomposition and dispenses with Reynolds averaging in favour of an averaging operator based on the relevant scalar-flux driving variables. The resulting method, termed functional covariance, provides five areas of improvement in flux estimation: (i) Better representation of surface fluxes through closer correspondence of turbulent exchange with variations in the driving variables. (ii) An approximate 25% reduction in flux uncertainty resulting from improved independence of turbulent-flux samples. (iii) Improved data retention through less onerous quality control (stationarity) testing. (iv) Improved estimation of low-frequency flux contributions through reduced uncertainty and avoidance of driving-var...
Research infrastructures play a key role in launching a new generation of integrated long-term, g... more Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, sp...
Eddy covariance (EC) continues to provide invaluable insights into the dynamics of Earth's su... more Eddy covariance (EC) continues to provide invaluable insights into the dynamics of Earth's surface processes. However, despite its many strengths, spatial replication of EC at the ecosystem scale is rare. High equipment costs are likely to be partially responsible. This contributes to the low sampling, and even lower replication, of ecoregions in Africa, Oceania (excluding Australia) and South America. The level of replication matters as it directly affects statistical power. Whilst the ergodicity of turbulence and temporal replication allow an EC tower to provide statistically robust flux estimates for its footprint, these principles do not extend to larger ecosystem scales. Despite the challenge of spatially replicating EC, it is clearly of interest to be able to use EC to provide statistically robust flux estimates for larger areas. We ask: How much spatial replication of EC is required for statistical confidence in our flux estimates of an ecosystem? We provide the reader wi...
ABSTRACT Terrestrial carbon sinks are currently believed to be responsible for the sequestration ... more ABSTRACT Terrestrial carbon sinks are currently believed to be responsible for the sequestration of a significant fraction of anthropogenic carbon emissions from the atmosphere, and have become the focus of wide-ranging scientific studies in recent years. The ability to study the dynamics of carbon uptake in different ecosystems at scales meaningful to climatic forcing is essential for understanding the key processes involved and identifying crucial sensitivities and thresholds. Furthermore, the development of reliable algorithms to allow global estimates of carbon sequestration rates from satellite based observations (potentially providing both high spatial and temporal resolution) is a high priority for monitoring the behaviour of these sinks in a changing climate. Combined studies linking airborne and satellite hyperspectral observations of vegetation properties in combination with flux measurements provide one method to develop ecosystem process knowledge and suitable algorithms for carbon flux estimates. However significant challenges remain in obtaining direct flux measurements over larger areas than can be measured with traditional tower based eddy covariance. We present new techniques for the acquisition and analysis of airborne CO2 flux measurements capable of sampling extensive areas while resolving spatial patterns of surface carbon fluxes at scales suitable for analysis alongside hyperspectral vegetation products (such as PRI, NVDI and solar induced fluorescence signatures). We conducted a series of airborne CO2 flux measurement and remote sensing campaigns over a Scots Pine forest in and around the Hyytiälä forest research station in southern Finland in summer 2010. Airborne CO2 flux measurements using the University of Edinburgh's research aircraft were acquired by flying repeat transects (of up to 35km in length) each day over several days, from a height of 30m to 200m above canopy. We explore a number of techniques including wavelet analysis and flux fragmentation methods with varying combinations of averaging length, multi-pass averaging and allowance for flux divergence, in order to identify the most reliable approach to return relatively high spatial resolution measurements with acceptable levels of uncertainty. In each case direct comparisons with canopy tower-based measurements are made.
Modelisation du procede. Application du modele a la deshydratation d'azeotropes hydro-organiq... more Modelisation du procede. Application du modele a la deshydratation d'azeotropes hydro-organiques. Etat actuel de developpement, cout energetique
The relationships between CO2 flux and spectral reflectance indices are often examined across lim... more The relationships between CO2 flux and spectral reflectance indices are often examined across limited temporal and spatial scales. Micrometeorologists employing flux towers are constrained by the pre-determined spatial (
Quantifying landscape-scale methane (CH 4) fluxes from boreal and arctic regions, and determining... more Quantifying landscape-scale methane (CH 4) fluxes from boreal and arctic regions, and determining how they are controlled, is critical for predicting the magnitude of any CH 4 emission feedback to climate change. Furthermore, there remains uncertainty regarding the relative importance of small areas of strong methanogenic activity, vs. larger areas with net CH 4 uptake, in controlling landscape-level fluxes. We measured CH 4 fluxes from multiple microtopographical subunits (sedge-dominated lawns, interhummocks and hummocks) within an aapa mire in subarctic Finland, as well as in drier ecosystems present in the wider landscape, lichen heath and mountain birch forest. An intercomparison was carried out between fluxes measured using static chambers, up-scaled using a high-resolution landcover map derived from aerial photography and eddy covariance. Strong agreement was observed between the two methodologies, with emission rates greatest in lawns. CH 4 fluxes from lawns were strongly related to seasonal fluctuations in temperature, but their floating nature meant that water-table depth was not a key factor in controlling CH 4 release. In contrast, chamber measurements identified net CH 4 uptake in birch forest soils. An intercomparison between the aerial photography and satellite remote sensing demonstrated that quantifying the distribution of the key CH 4 emitting and consuming plant communities was possible from satellite, allowing fluxes to be scaled up to a 100 km 2 area. For the full growing season (May to October),~1.1-1.4 g CH 4 m À2 was released across the 100 km 2 area. This was based on up-scaled lawn emissions of 1.2-1.5 g CH 4 m À2 , vs. an up-scaled uptake of 0.07-0.15 g CH 4 m À2 by the wider landscape. Given the strong temperature sensitivity of the dominant lawn fluxes, and the fact that lawns are unlikely to dry out, climate warming may substantially increase CH 4 emissions in northern Finland, and in aapa mire regions in general.
With the widespread application of eddy covariance technology, long-term records of hourly ecosys... more With the widespread application of eddy covariance technology, long-term records of hourly ecosystem mass and energy exchange are becoming available for forests around the world. These data sets hold great promise for testing and validation of models of forest function. However, model validation is not a straightforward task. The goals of this paper were to: (1) review some of the problems inherent in model validation; and (2) survey the tools available to modelers to improve validation procedures, with particular reference to eddy covariance data. A simple set of models applied to a data set of ecosystem CO 2 exchange is used to illustrate our points. The major problems discussed are equifinality, insensitivity and uncertainty. Equifinality is the problem that different models, or different parameterizations of the same model, may yield similar results, making it difficult to distinguish which is correct. Insensitivity arises because the major sources of variation in eddy covariance data are the annual and diurnal cycles, which are represented by even the most basic models, and the size of the response to these cycles can mask effects of other driving variables. Uncertainty arises from three main sources: parameters, model structure and data, each of which is discussed in turn. Uncertainty is a particular issue with eddy covariance data because of the lack of replicated measurements and the potential for unquantified systematic errors such as flux loss due to advection. We surveyed several tools that improve model validation, including sensitivity analysis, uncertainty analysis, residual analysis and model comparison. Illustrative examples are used to demonstrate the use of each tool. We show that simplistic comparisons of model outputs with eddy covariance data are problematic, but use of these tools can greatly improve our confidence in model predictions.
Atmospheric and Oceanographic Sciences Library, 2005
Surface exchange fluxes measured by eddy covariance tend to be underestimated as a result of limi... more Surface exchange fluxes measured by eddy covariance tend to be underestimated as a result of limitations in sensor design, signal processing methods, and finite flux-averaging periods. But, careful system design, modern instrumentation, and appropriate data processing algorithms can minimize these losses, which, if not too large, can be estimated and corrected using any of several different approaches. However, no flux-correction method is perfect so methodological uncertainties are inevitable. This study addresses the uncertainties in surface flux measurements that have been corrected for spectral attenuation with the transfer function approach. The sources of the errors in the estimates of flux attenuation examined here include the (flux-averaging) periodto-period variablity of cospectra, the departure of real cospectra from presumed smooth curves, the inherent variability in maximum frequency (fx) of the frequency weighted cospectra, and possible imprecision in instrument related time constants. A method is proposed to estimate the uncertainty resulting from combining these effects. Also included in this study are a general mathematical relationship to describe spectra or cospectra, comparisons of observed cospectra for cospectral similarity, and discussions about including high-pass filters (associated with the flux-averaging procedures) when accounting for low frequency losses.
The role of greenhouse gases (GHGs) in causing global climate change is now well recognized (IPCC... more The role of greenhouse gases (GHGs) in causing global climate change is now well recognized (IPCC, 2013). Emissions of GHGs from terrestrial ecosystems play an important part in this, and the potential for feedback within the climate system which amplify the emissions of GHGs from natural ecosystems is substantial. Accurate estimates are therefore needed of the GHG balance of the land surface at regional and national scales, and over long time periods, if we are to understand the key driver of global change. Because of the large scale involved, in relation to the scale at which we can make observations, this presents a major challenge that spans the domains of biogeochemistry, ecology, remote sensing, and atmospheric science.
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
As part of the quality assurance and quality control activities within the CarboEurope-IP network... more As part of the quality assurance and quality control activities within the CarboEurope-IP network, a comparison of eddy-covariance software was conducted. For four five-day datasets, CO 2 flux estimates were calculated by seven commonly used software packages to assess the uncertainty of CO 2 flux estimates due to differences in postprocessing. The datasets originated from different sites representing different commonly applied instrumentation and different canopy structures to cover a wide range of realistic conditions. Data preparation, coordinate rotation and the implementation of the correction for high frequency spectral losses were identified as crucial processing steps leading to significant discrepancies in the CO 2 flux results. The overall comparison indicated a good although not yet perfect agreement among the different software within 5-10% difference for 30-min CO 2 flux values. Conceptually different ideas about the selection and application of processing steps were a main reason for the differences in the CO 2 flux estimates observed. A balance should be aspired between scientific freedom on the one hand, in order to advance methodical issues, and standardisation of procedures on the other hand, in order to obtain comparable fluxes for multi-site synthesis studies.
The version presented here may differ from the published version or from the version of the recor... more The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription.
Eddy covariance has been the de facto method of analyzing scalar turbulent transport data. To ref... more Eddy covariance has been the de facto method of analyzing scalar turbulent transport data. To refine the information available from these data, we derive a simplified version of the turbulent scalar-transport equation for the surface layer, which employs a more explicit form of signal decomposition and dispenses with Reynolds averaging in favour of an averaging operator based on the relevant scalar-flux driving variables. The resulting method, termed functional covariance, provides five areas of improvement in flux estimation: (i) Better representation of surface fluxes through closer correspondence of turbulent exchange with variations in the driving variables. (ii) An approximate 25% reduction in flux uncertainty resulting from improved independence of turbulent-flux samples. (iii) Improved data retention through less onerous quality control (stationarity) testing. (iv) Improved estimation of low-frequency flux contributions through reduced uncertainty and avoidance of driving-var...
Research infrastructures play a key role in launching a new generation of integrated long-term, g... more Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, sp...
Eddy covariance (EC) continues to provide invaluable insights into the dynamics of Earth's su... more Eddy covariance (EC) continues to provide invaluable insights into the dynamics of Earth's surface processes. However, despite its many strengths, spatial replication of EC at the ecosystem scale is rare. High equipment costs are likely to be partially responsible. This contributes to the low sampling, and even lower replication, of ecoregions in Africa, Oceania (excluding Australia) and South America. The level of replication matters as it directly affects statistical power. Whilst the ergodicity of turbulence and temporal replication allow an EC tower to provide statistically robust flux estimates for its footprint, these principles do not extend to larger ecosystem scales. Despite the challenge of spatially replicating EC, it is clearly of interest to be able to use EC to provide statistically robust flux estimates for larger areas. We ask: How much spatial replication of EC is required for statistical confidence in our flux estimates of an ecosystem? We provide the reader wi...
ABSTRACT Terrestrial carbon sinks are currently believed to be responsible for the sequestration ... more ABSTRACT Terrestrial carbon sinks are currently believed to be responsible for the sequestration of a significant fraction of anthropogenic carbon emissions from the atmosphere, and have become the focus of wide-ranging scientific studies in recent years. The ability to study the dynamics of carbon uptake in different ecosystems at scales meaningful to climatic forcing is essential for understanding the key processes involved and identifying crucial sensitivities and thresholds. Furthermore, the development of reliable algorithms to allow global estimates of carbon sequestration rates from satellite based observations (potentially providing both high spatial and temporal resolution) is a high priority for monitoring the behaviour of these sinks in a changing climate. Combined studies linking airborne and satellite hyperspectral observations of vegetation properties in combination with flux measurements provide one method to develop ecosystem process knowledge and suitable algorithms for carbon flux estimates. However significant challenges remain in obtaining direct flux measurements over larger areas than can be measured with traditional tower based eddy covariance. We present new techniques for the acquisition and analysis of airborne CO2 flux measurements capable of sampling extensive areas while resolving spatial patterns of surface carbon fluxes at scales suitable for analysis alongside hyperspectral vegetation products (such as PRI, NVDI and solar induced fluorescence signatures). We conducted a series of airborne CO2 flux measurement and remote sensing campaigns over a Scots Pine forest in and around the Hyytiälä forest research station in southern Finland in summer 2010. Airborne CO2 flux measurements using the University of Edinburgh's research aircraft were acquired by flying repeat transects (of up to 35km in length) each day over several days, from a height of 30m to 200m above canopy. We explore a number of techniques including wavelet analysis and flux fragmentation methods with varying combinations of averaging length, multi-pass averaging and allowance for flux divergence, in order to identify the most reliable approach to return relatively high spatial resolution measurements with acceptable levels of uncertainty. In each case direct comparisons with canopy tower-based measurements are made.
Modelisation du procede. Application du modele a la deshydratation d'azeotropes hydro-organiq... more Modelisation du procede. Application du modele a la deshydratation d'azeotropes hydro-organiques. Etat actuel de developpement, cout energetique
The relationships between CO2 flux and spectral reflectance indices are often examined across lim... more The relationships between CO2 flux and spectral reflectance indices are often examined across limited temporal and spatial scales. Micrometeorologists employing flux towers are constrained by the pre-determined spatial (
Quantifying landscape-scale methane (CH 4) fluxes from boreal and arctic regions, and determining... more Quantifying landscape-scale methane (CH 4) fluxes from boreal and arctic regions, and determining how they are controlled, is critical for predicting the magnitude of any CH 4 emission feedback to climate change. Furthermore, there remains uncertainty regarding the relative importance of small areas of strong methanogenic activity, vs. larger areas with net CH 4 uptake, in controlling landscape-level fluxes. We measured CH 4 fluxes from multiple microtopographical subunits (sedge-dominated lawns, interhummocks and hummocks) within an aapa mire in subarctic Finland, as well as in drier ecosystems present in the wider landscape, lichen heath and mountain birch forest. An intercomparison was carried out between fluxes measured using static chambers, up-scaled using a high-resolution landcover map derived from aerial photography and eddy covariance. Strong agreement was observed between the two methodologies, with emission rates greatest in lawns. CH 4 fluxes from lawns were strongly related to seasonal fluctuations in temperature, but their floating nature meant that water-table depth was not a key factor in controlling CH 4 release. In contrast, chamber measurements identified net CH 4 uptake in birch forest soils. An intercomparison between the aerial photography and satellite remote sensing demonstrated that quantifying the distribution of the key CH 4 emitting and consuming plant communities was possible from satellite, allowing fluxes to be scaled up to a 100 km 2 area. For the full growing season (May to October),~1.1-1.4 g CH 4 m À2 was released across the 100 km 2 area. This was based on up-scaled lawn emissions of 1.2-1.5 g CH 4 m À2 , vs. an up-scaled uptake of 0.07-0.15 g CH 4 m À2 by the wider landscape. Given the strong temperature sensitivity of the dominant lawn fluxes, and the fact that lawns are unlikely to dry out, climate warming may substantially increase CH 4 emissions in northern Finland, and in aapa mire regions in general.
With the widespread application of eddy covariance technology, long-term records of hourly ecosys... more With the widespread application of eddy covariance technology, long-term records of hourly ecosystem mass and energy exchange are becoming available for forests around the world. These data sets hold great promise for testing and validation of models of forest function. However, model validation is not a straightforward task. The goals of this paper were to: (1) review some of the problems inherent in model validation; and (2) survey the tools available to modelers to improve validation procedures, with particular reference to eddy covariance data. A simple set of models applied to a data set of ecosystem CO 2 exchange is used to illustrate our points. The major problems discussed are equifinality, insensitivity and uncertainty. Equifinality is the problem that different models, or different parameterizations of the same model, may yield similar results, making it difficult to distinguish which is correct. Insensitivity arises because the major sources of variation in eddy covariance data are the annual and diurnal cycles, which are represented by even the most basic models, and the size of the response to these cycles can mask effects of other driving variables. Uncertainty arises from three main sources: parameters, model structure and data, each of which is discussed in turn. Uncertainty is a particular issue with eddy covariance data because of the lack of replicated measurements and the potential for unquantified systematic errors such as flux loss due to advection. We surveyed several tools that improve model validation, including sensitivity analysis, uncertainty analysis, residual analysis and model comparison. Illustrative examples are used to demonstrate the use of each tool. We show that simplistic comparisons of model outputs with eddy covariance data are problematic, but use of these tools can greatly improve our confidence in model predictions.
Atmospheric and Oceanographic Sciences Library, 2005
Surface exchange fluxes measured by eddy covariance tend to be underestimated as a result of limi... more Surface exchange fluxes measured by eddy covariance tend to be underestimated as a result of limitations in sensor design, signal processing methods, and finite flux-averaging periods. But, careful system design, modern instrumentation, and appropriate data processing algorithms can minimize these losses, which, if not too large, can be estimated and corrected using any of several different approaches. However, no flux-correction method is perfect so methodological uncertainties are inevitable. This study addresses the uncertainties in surface flux measurements that have been corrected for spectral attenuation with the transfer function approach. The sources of the errors in the estimates of flux attenuation examined here include the (flux-averaging) periodto-period variablity of cospectra, the departure of real cospectra from presumed smooth curves, the inherent variability in maximum frequency (fx) of the frequency weighted cospectra, and possible imprecision in instrument related time constants. A method is proposed to estimate the uncertainty resulting from combining these effects. Also included in this study are a general mathematical relationship to describe spectra or cospectra, comparisons of observed cospectra for cospectral similarity, and discussions about including high-pass filters (associated with the flux-averaging procedures) when accounting for low frequency losses.
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Papers by Robert Clement