Mathias Göckede

Sensitivity of a subregional scale atmospheric inverse CO2 modeling framework to boundary conditions

Received 4 May 2010; revised 14 September 2010; accepted 27 September 2010; published 23 December 2010. [1] We present an atmospheric inverse modeling framework to constrain terrestrial biosphere CO2 exchange processes at subregional scales. The model is operated at very high spatial and temporal resolution, using the state of Oregon in the northwestern United States as the model domain. The modeling framework includes mesoscale atmospheric simulations coupled to Lagrangian transport, a biosphere flux model that considers, e.g., the effects of drought stress and disturbance on photosynthesis and respiration CO2 fluxes, and a Bayesian optimization approach. This study focuses on the impact of uncertainties in advected background mixing ratios and fossil fuel emissions on simulated flux fields, both taken from external data sets. We found the simulations to be highly sensitive to systematic changes in advected background CO2, while shifts in fossil fuel emissions played a minor role. Correcting for offsets in the background mixing ratios shifted annual CO2 budgets by about 47% and improved the correspondence with the output produced by bottom‐up modeling frameworks. Inversion results were robust against shifts in fossil fuel emissions, which is likely a consequence of relatively low emission rates in Oregon.

Uncertainty estimates for 1-h averaged turbulence fluxes of carbon dioxide, latent heat and sensible heat

A new observational approach is presented to approximate the uncertainty (scatter or error variance) in 1-h averaged turbulence fluxes from eddy-covariance measurements. The uncertainty includes potential contributions from instrument problems, heterogeneity and non-stationarity in addition to classical random sampling error. The daytime relative flux uncertainty (RFE) is half as large (20%) at a simple maize site compared to two more complex forest sites (40%) for all scalars possibly due to the more homogeneous vegetation, flat terrain and especially the lower measurement height. The RFE is approximately the same for day and night periods for all scalars at the two mostly homogeneous sites (pine forest and maize field) except for latent heat over the forest, where the RFE doubles at night. Modest surface heterogeneity at the other forest site for nocturnal flux footprints approximately doubles the RFE compared to daytime conditions for all scalar fluxes. Compositing by atmospheric stability (instead of time of day) reveals a sharp increase in the RFE for the most strongly stable conditions. A theoretical prediction for the pure random sampling error based on the flux integral timescale is smaller by a factor of two compared to the observed variability.

Sensitivity of Lagrangian Stochastic footprints to turbulence statistics

This study tests the sensitivity of a Lagrangian Stochastic footprint model to the turbulence statistics describing the flow field, with a focus on the within canopy processes. Representative profiles of the input velocity statistics are taken from a long-term dataset of turbulence measurements within and above a tall spruce canopy. Based on a wavelet tool, which allows a detailed analysis of coherent structures along the vertical profile, we characterize several typical states of coupling and decoupling between surface, canopy and atmosphere. For each coupling regime, three flux footprints using different sources for turbulence statistics are compared: the first based on conditionally-averaged measurements, the second on a simple numerical solution and the third on measurements taken from literature. The effects of profile smoothing and connecting measured canopy data to parametrized atmospheric surface layer profiles are considered. Significant differences between footprints based on modelled and measured profiles were found for exchange regimes with the lower section of the profiles decoupled from the atmospheric surface layer. As such cases are likely to occur for tall canopies with moderate density, our results suggest that the accuracy of Lagrangian Stochastic footprint modelling could be improved by using better turbulence profiles for different exchange regimes.

Carbon budget of a spruce forest ecosystem

The investigation of carbon fluxes is of immense interest in ecosystem and climate research. Forest ecosystems may be a sink for anthropogenic carbon, if the assimilation is larger than the respiration. Alternatively, increasing temperatures due to climate change (IPCC 2001) may be a reason for increasing respiratory fluxes. While low-altitude spruce sites in Germany are significant carbon sinks (e.g. Bernhofer et al. 2003), sites above 600 m a.s.l. are only small sinks or may change their character by climate change. Therefore the Weidenbrunnen site in the Lehstenbach catchment was selected as a EUROFLUX site (Valentini et al. 2000) and was also used in the following CARBOEUROFLUX program for systematic investigations with respect to the data quality of turbulent fluxes. Overviews of the European carbon program and of the worldwide FLUXNET program are respectively given by Valentini (2003) and Baldocchi et al. (2001). All relevant references are also provided herein. Furthermore, the site was used for process studies to separate assimilation and respiration fluxes, and to study the exchange conditions between the forest and the atmosphere (Wichura et al., this Vol.). All of these studies were part of the ecosystem research of the Lehstenbach catchment, the main research area of the Bayreuth Institute of Terrestrial Ecosystem Research (BITOK). The main results for the carbon dioxide flux measurements since 1997 are discussed in this chapter.

Multiple constraint analysis of regional land–surface carbon flux

We applied and compared bottom-up (process model-based) and top-down (atmospheric inversion-based) scaling approaches to evaluate the spatial and temporal patterns of net ecosystem production (NEP) over a 2.5 × 10 5 km 2 area (the state of Oregon) in the western United States. Both approaches indicated a carbon sink over this heterogeneous region in 2003 (a relatively warm, dry year in western Oregon) and 2007 (near normal), with carbon uptake primarily in forested and agricultural areas. The statewide mean NEP for 2007 using the bottom-up approach was 80 gC m -2 yr -1 , which compares with 145 gC m -2 yr -1 for the top-down approach. Seasonality of daily NEP at the ecoregion scale showed similar patterns across the two approaches, but with less sensitivity to seasonal drought in the top-down model. In 2003, simulated annual NEP was lower than in 2007 for both scaling approaches, but the reduction was stronger with the bottom-up approach. Estimates of mean NEP on forested lands from a forest inventory approach, and from the CarbonTracker inversion scheme, bracketed that of our bottom-up approach (ratios to bottom-up estimates were 1.3 and 0.3, respectively). These results support the need for a multiple constraint approach to evaluation of regional trace gas budgets. DOI: 10.1111/j.1600-0889.2011.00525.x

STINHO - Structure of turbulent transport under inhomogeneous surface conditions. Part 1: The micro-α scale field experiment

A micrometeorological field experiment was performed within the frame of the STINHO-project (structure of turbulent transport under inhomogeneous conditions) at the boundary layer field site of the Meteorological Observatory Lindenberg of the German Meteorological Service (Deutscher Wetterdienst) in the summer of 2002 in order to investigate the interaction of thermally heterogeneous surfaces with the turbulent atmosphere. The intention was to compare conventional meteorological point and vertically integrated measurements with area-covering air flow observations and numerical simulations. To observe horizontally variable flow and temperature fields above a heterogeneous land surface, simultaneous acoustic methods (travel time tomography), optical observation methods (IR-camera and line-integrated scintillometer-measurements), as well as the airborne measurement system Helipod were used. The data set will be applied in future to validate large-eddy simulations adjusted to the area of investigation.

Bayesian optimization of the community land model simulated biosphere-atmosphere exchange using CO2 observations from a dense tower network and aircraft campaigns over Oregon

AbstractThe vast forests and natural areas of the Pacific Northwest compose one of the most productive ecosystems in the Northern Hemisphere. The heterogeneous landscape of Oregon poses a particular challenge to ecosystem models. This study presents a framework using a scaling factor Bayesian inversion to improve the modeled atmosphere–biosphere exchange of CO2. Observations from five CO/CO2 towers, eddy covariance towers, and airborne campaigns were used to constrain the Community Land Model, version 4.5 (CLM4.5), simulated terrestrial CO2 exchange at a high spatial and temporal resolution (1/24°; 3 hourly). To balance aggregation errors and the degrees of freedom in the inverse modeling system, the authors applied an unsupervised clustering approach for the spatial structuring of the model domain. Data from flight campaigns were used to quantify the uncertainty introduced by the Lagrangian particle dispersion model that was applied for the inversions. The average annual statewide net ecosystem productiv...

High-quality eddy-covariance CO2 budgets under cold climate conditions

This study aimed at quantifying potential negative effects of instrument heating to improve eddy covariance flux data quality in cold environments. Our overarching objective was to minimize heating-related bias in annual CO2 budgets from an Arctic permafrost system. We used continuous eddy-covariance measurements covering three full years within an Arctic permafrost ecosystem with parallel sonic anemometers operation with activated heating and without heating as well as parallel operation of open- and closed-path gas analyzers, the latter serving as a reference. Our results demonstrate that the sonic anemometer heating has a direct effect on temperature measurements while the turbulent wind field is not affected. As a consequence, fluxes of sensible heat are increased by an average 5 W m-2 with activated heating, while no direct effect on other scalar fluxes was observed. However, the biased measurements in sensible heat fluxes can have an indirect effect on the CO2 fluxes in case they are used as input for a density-flux WPL correction of an open-path gas analyzer. Evaluating the self-heating effect of the open-path gas analyzer by comparing CO2 flux measurements between open- and closed-path gas analyzers we found systematically higher CO2 uptake recorded with the open-path sensor, leading to a cumulative annual offset of 96 gC m-2, which was not only the result of the cold winter season but also due to substantial self-heating effects during summer. With an inclined sensor mounting, only a fraction of the self-heating correction for vertically mounted instruments is required.

Development of Flux Data Quality Tools

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.

Accurate measurements of atmospheric carbon dioxide and methane mole fractions atthe Siberian coastal site Ambarchik

Sparse data coverage in the Arctic hampers our understanding of its carbon cycle dynamics and our predictions of the fate of its vast carbon reservoirs in a changing climate. In this paper, we present accurate measurements of atmospheric carbon dioxide (CO2) and methane (CH4) dry air mole fractions at the new atmospheric carbon observation station Ambarchik, which closes a large gap in the atmospheric trace gas monitoring network in northeastern Siberia. The site, which has been operational since August 2014, is located near the delta of the Kolyma River at the coast of the Arctic Ocean. Data quality control of CO2 and CH4 measurements includes frequent calibrations traced to World Meteorological Organization (WMO) scales, employment of a novel water vapor correction, an algorithm to detect the influence of local polluters, and meteorological measurements that enable data selection. The available CO2 and CH4 record was characterized in comparison with in situ data from Barrow, Alaska. A footprint analysis reveals that the station is sensitive to signals from the East Siberian Sea, as well as the northeast Siberian tundra and taiga regions. This makes data from Ambarchik highly valuable for inverse modeling studies aimed at constraining carbon budgets within the panArctic domain, as well as for regional studies focusing on Siberia and the adjacent shelf areas of the Arctic Ocean.