I. T. Baker

Colorado State University, Fort Collins, Colorado, United States

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Publications (62)75.75 Total impact

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    ABSTRACT: According to current budget estimations the seasonal variation of carbonyl sulfide (COS) is governed by oceanic release and vegetation uptake. Its assimilation by plants is assumed to be similar to the photosynthetic uptake of CO2 but, contrary to the latter process, to be irreversible. Therefore, COS has been suggested as cotracer of the carbon cycle. Observations of COS, however, are sparse, especially in tropical regions. We use the comprehensive data set of spaceborne measurements of the Michelson Interferometer for Passive Atmospheric Sounding to analyze its global distribution. Two major features are observed in the tropical upper troposphere around 250 hPa: enhanced amounts over the western Pacific and the Maritime Continent, peaking around 550 parts per trillion by volume (pptv) in boreal summer, and a seasonally varying depletion of COS extending from tropical South America to Africa. The large-scale COS depletion, which in austral summer amounts up to -40 pptv as compared to the rest of the respective latitude band, has not been observed before and reveals the seasonality of COS uptake through tropical vegetation. The observations can only be reproduced by global models, when a large vegetation uptake and a corresponding increase in oceanic emissions as proposed in several recent publications are assumed.
    No preview · Article · Nov 2015 · Geophysical Research Letters
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    ABSTRACT: Understanding carbon dioxide (CO2) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and inter-annual variability of CO2 in the atmosphere. Atmospheric inversions based on CO2 concentration measurements alone can only determine net biosphere fluxes, but not differentiate between photosynthesis (uptake) and respiration (production). Carbonyl sulfide (OCS) could provide an important additional constraint: it is also taken up by plants during photosynthesis but not emitted during respiration, and therefore is a potential mean to differentiate between these processes. Solar absorption Fourier Transform InfraRed (FTIR) spectrometry allows for the retrievals of the atmospheric concentrations of both CO2 and OCS from measured solar absorption spectra. Here, we investigate co-located and quasi-simultaneous FTIR measurements of OCS and CO2 performed at three selected sites located in the Northern Hemisphere. These measurements are compared to simulations of OCS and CO2 using a chemical transport model (GEOS-Chem). The OCS simulations are driven by different land biospheric fluxes to reproduce the seasonality of the measurements. Increasing the plant uptake of Kettle et al. (2002a) by a factor of three resulted in the best comparison with FTIR measurements. However, there are still discrepancies in the latitudinal distribution when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres. The coupled biospheric fluxes of OCS and CO2 from the simple biosphere model (SiB) are used in the study and compared to measurements. The CO2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than FTIR measurements at selected sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen. Using OCS as a photosynthesis proxy can help to understand how the biospheric processes are reproduced in models and to further understand the carbon cycle in the real world.
    No preview · Article · Sep 2015 · Atmospheric Chemistry and Physics
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    ABSTRACT: Significant changes in the water cycle are expected under current global environmental change. Robust assessment of present-day water cycle dynamics at continental to global scales is confounded by shortcomings in the observed record. Modeled assessments also yield conflicting results which are linked to differences in model structure and simulation protocol. Here we compare simulated gridded (1° spatial resolution) runoff from six terrestrial biosphere models (TBMs), seven reanalysis products, and one gridded surface station product in the contiguous United States (CONUS) from 2001 to 2005. We evaluate the consistency of these 14 estimates with stream gauge data, both as depleted flow and corrected for net withdrawals (2005 only), at the CONUS and water resource region scale, as well as examining similarity across TBMs and reanalysis products at the grid cell scale. Mean runoff across all simulated products and regions varies widely (range: 71 to 356 mm yr−1) relative to observed continental-scale runoff (209 or 280 mm yr−1 when corrected for net withdrawals). Across all 14 products 8 exhibit Nash–Sutcliffe efficiency values in excess of 0.8 and three are within 10% of the observed value. Region-level mismatch exhibits a weak pattern of overestimation in western and underestimation in eastern regions—although two products are systematically biased across all regions—and largely scales with water use. Although gridded composite TBM and reanalysis runoff show some regional similarities, individual product values are highly variable. At the coarse scales used here we find that progress in better constraining simulated runoff requires standardized forcing data and the explicit incorporation of human effects (e.g., water withdrawals by source, fire, and land use change).
    Full-text · Article · May 2015 · Ecological Modelling
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    ABSTRACT: Significant changes in the water cycle are expected under current global environmental change. Robust assessment of these changes at global scales is confounded by shortcomings in the observed record. Modeled assessments yield conflicting results which are linked to differences in model structure and simulation protocol. Here we compare simulated runoff from six terrestrial biosphere models (TBMs), five reanalysis products, and one gridded surface station product with observations from a network of stream gauges in the contiguous United States (CONUS) from 2001 to 2005. We evaluate the consistency of simulated runoff with stream gauge data at the CONUS and water resource region scale, as well as examining similarity across TBMs and reanalysis products at the grid cell scale. Mean runoff across all simulated products and regions varies widely (range: 71-356 mm yr-1) relative to observed continental-scale runoff (209 mm yr-1). Across all 12 products only two are within 10% of the observed value and only four exhibit Nash-Sutcliffe efficiency values in excess of 0.8. Region-level mismatch exhibits a weak pattern of overestimation in western and underestimation in eastern regions; although two products are systematically biased across all regions. In contrast, bias in a temporal sense, within region by water year, is highly consistent. Although gridded composite TBM and reanalysis runoff show some regional similarities for 2001-2005 with CONUS means, individual product values are highly variable. To further constrain simulated runoff and to link model-observation mismatch to model structural characteristics would require watershed-level simulation studies coupled with river routing schemes, standardized forcing data, and explicit consideration of water cycle management.
    Full-text · Article · Dec 2013 · Biogeosciences Discussions
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    ABSTRACT: Surface ecophysiology at five sites in tropical South America across vegetation and moisture gradients is investigated. From the moist northwest (Manaus) to the relatively dry southeast (Pé de Gigante, state of São Paulo) simulated seasonal cycles of latent and sensible heat, and carbon flux produced with the Simple Biosphere Model (SiB3) are confronted with observational data. In the northwest, abundant moisture is available, suggesting that the ecosystem is light-limited. In these wettest regions, Bowen ratio is consistently low, with little or no annual cycle. Carbon flux shows little or no annual cycle as well; efflux and uptake are determined by high-frequency variability in light and moisture availability. Moving downgradient in annual precipitation amount, dry season length is more clearly defined. In these regions, a dry season sink of carbon is observed and simulated. This sink is the result of the combination of increased photosynthetic production due to higher light levels, and decreased respiratory efflux due to soil drying. The differential response time of photosynthetic and respiratory processes produce observed annual cycles of net carbon flux. In drier regions, moisture and carbon fluxes are in-phase; there is carbon uptake during seasonal rains and efflux during the dry season. At the driest site, there is also a large annual cycle in latent and sensible heat flux.
    Full-text · Article · Dec 2013 · Agricultural and Forest Meteorology
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    ABSTRACT: Earth system processes exhibit complex patterns across time, as do the models that seek to replicate these processes. Model output may or may not be significantly related to observations at different times and on different frequencies. Conventional model diagnostics provide an aggregate view of model-data agreement, but usually do not identify the time and frequency patterns of model-data disagreement, leaving unclear the steps required to improve model response to environmental drivers that vary on characteristic frequencies. Wavelet coherence can quantify the times and timescales at which two time series, for example time series of models and measurements, are significantly different. We applied wavelet coherence to interpret the predictions of 20 ecosystem models from the North American Carbon Program (NACP) Site-Level Interim Synthesis when confronted with eddy-covariance-measured net ecosystem exchange (NEE) from 10 ecosystems with multiple years of available data. Models were grouped into classes with similar approaches for incorporating phenology, the calculation of NEE, the inclusion of foliar nitrogen (N), and the use of model-data fusion. Models with prescribed, rather than prognostic, phenology often fit NEE observations better on annual to interannual timescales in grassland, wetland and agricultural ecosystems. Models that calculated NEE as net primary productivity (NPP) minus heterotrophic respiration (HR) rather than gross ecosystem productivity (GPP) minus ecosystem respiration (ER) fit better on annual timescales in grassland and wetland ecosystems, but models that calculated NEE as GPP minus ER were superior on monthly to seasonal timescales in two coniferous forests. Models that incorporated foliar nitrogen (N) data were successful at capturing NEE variability on interannual (multiple year) timescales at Howland Forest, Maine. The model that employed a model-data fusion approach often, but not always, resulted in improved fit to data, suggesting that improving model parameterization is important but not the only step for improving model performance. Combined with previous findings, our results suggest that the mechanisms driving daily and annual NEE variability tend to be correctly simulated, but the magnitude of these fluxes is often erroneous, suggesting that model parameterization must be improved. Few NACP models correctly predicted fluxes on seasonal and interannual timescales where spectral energy in NEE observations tends to be low, but where phenological events, multi-year oscillations in climatological drivers, and ecosystem succession are known to be important for determining ecosystem function. Mechanistic improvements to models must be made to replicate observed NEE variability on seasonal and interannual timescales.
    Full-text · Article · Nov 2013 · Biogeosciences
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    ABSTRACT: [1] Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO2 concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO2 in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO2 with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.
    Full-text · Article · Jun 2013 · Journal of Geophysical Research: Biogeosciences
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    Full-text · Article · Feb 2013 · Biogeosciences Discussions
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    ABSTRACT: There is a continued need for models to improve consistency and agreement with observations [Friedlingstein et al., 2006], both overall and under more frequent extreme climatic events related to global environmental change such as drought [Trenberth et al., 2007]. Past validation studies of terrestrial biosphere models have focused only on few models and sites, typically in close proximity and primarily in forested biomes [e.g., Amthor et al., 2001; Delpierre et al., 2009; Grant et al., 2005; Hanson et al., 2004; Granier et al., 2007; Ichii et al., 2009; Ito, 2008; Siqueira et al., 2006; Zhou et al., 2008]. Furthermore, assessing model‐data agreement relative to drought requires, in addition to high‐quality observedCO2 exchange data, a reliable drought metric as well as a natural experiment across sites and drought conditions.
    Full-text · Article · Feb 2013 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: Terrestrial carbon fluxes and pools cannot be measured directly on regional and global scales, thus land surface models are a vital tool in improving estimates of carbon sources, sinks, above and below ground biomass, and soil stocks. The Simple Biosphere Model (SiB4) is a self-consistent model that uses minimal input data to simulate carbon fluxes and pools in a fully prognostic system. Land surface models like SiB4 can be evaluated against a variety of data: carbon pools from field campaigns; carbon fluxes from chambers and flux towers; leaf out and senescence timing, as well as length of growing season from in situ observations; crop yields; and remotely sensed leaf area index (LAI), productivity, fluorescence, and biomass. To improve the simulation of the carbon cycle, this study evaluates SiB4 against these metrics. Preliminary results show that SiB4 has skill at predicting carbon pools and fluxes over forests and crops (maize, soybean, and wheat). Biomass, crop yield, LAI, fluorescence, and productivity are reasonable compared to data. Calculating the leaf pool explicitly improves the LAI over satellite-derived estimates, particularly in boreal, temperate, and agricultural ecosystems; and the improved LAI combined with the use of carbon pools to calculate autotrophic and heterotrophic respiration results in an improvement in carbon fluxes. Vegetation-specific comparisons highlight deficiencies in both grasslands and shrubs, which are addressed and will be further developed in the future. The resulting carbon pools and fluxes provide a realistic estimate of the current global terrestrial carbon cycle and are also realistic a priori fluxes for use by the NASA Carbon Monitoring System (CMS) Flux Project to help attribute CO2 climate forcing to spatially resolved emissions.
    No preview · Article · Jan 2013
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    ABSTRACT: Correct representations of root functioning, such as root water uptake and hydraulic redistribution, are critically important for modeling the responses of vegetation to droughts and seasonal changes in soil moisture content. However, these processes are poorly represented in global land surface models. In this study, we incorporated two root functions: a root water uptake function which assumes root water uptake efficiency varies with rooting depth, and a hydraulic redistribution function into a global land surface model, CABLE. The water uptake function developed by Lai and Katul (2000) was also compared with the default one (see Wang et al., 2010) that assumes that efficiency of water uptake per unit root length is constant. Using eddy flux measurements of CO2 and water vapor fluxes at three sites experiencing different patterns of seasonal changes in soil water content, we showed that the two root functions significantly improved the agreement between the simulated fluxes of net ecosystem exchange and latent heat flux and soil moisture dynamics with those observed during the dry season while having little impact on the model simulation during the wet seasons at all three sites. Sensitivity analysis showed that varying several model parameters influencing soil water dynamics in CABLE did not significantly affect the model's performance. We conclude that these root functions represent a valuable improvement for land surface modeling and should be implemented into CABLE and other land surface models for studying carbon and water dynamics where rainfall varies seasonally or interannually.
    Full-text · Article · Oct 2012 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: Understanding of carbon exchange between terrestrial ecosystems and the atmosphere can be improved through direct observations and experiments, as well as through modeling activities. Terrestrial biosphere models (TBMs) have become an integral tool for extrapolating local observations and understanding to much larger terrestrial regions. Although models vary in their specific goals and approaches, their central role within carbon cycle science is to provide a better understanding of the mechanisms currently controlling carbon exchange. Recently, the North American Carbon Program (NACP) organized several interim-synthesis activities to evaluate and inter-compare models and observations at local to continental scales for the years 2000–2005. Here, we compare the results from the TBMs collected as part of the regional and continental interim-synthesis (RCIS) activities. The primary objective of this work is to synthesize and compare the 19 participating TBMs to assess current understanding of the terrestrial carbon cycle in North America. Thus, the RCIS focuses on model simulations available from analyses that have been completed by ongoing NACP projects and other recently published studies. The TBM flux estimates are compared and evaluated over different spatial (1◦ × 1◦ and spatially aggregated to different regions) and temporal (monthly and annually) scales. The range in model estimates of net ecosystem productivity (NEP) for North America is much narrower than estimates of productivity or respiration, with estimates of NEP varying between −0.7 and 2.2 PgC yr−1, while gross primary productivity and heterotrophic respiration vary between 12.2 and 32.9 PgC yr−1 and 5.6 and 13.2 PgC yr−1, respectively. The range in estimates from the models appears to be driven by a combination of factors, including the representation of photosynthesis, the source and of environmental driver data and the temporal
    Full-text · Article · Jul 2012 · Ecological Modelling
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    ABSTRACT: Interannual variability in biosphere-atmosphere exchange of CO2 is driven by a diverse range of biotic and abiotic factors. Replicating this variability thus represents the ‘acid test’ for terrestrial biosphere models. Although such models are commonly used to project responses to both normal and anomalous variability in climate, they are rarely tested explicitly against inter-annual variability in observations. Herein, using standardized data from the North American Carbon Program, we assess the performance of 16 terrestrial biosphere models and 3 remote sensing products against long-term measurements of biosphere-atmosphere CO2 exchange made with eddy-covariance flux towers at 11 forested sites in North America. Instead of focusing on model-data agreement we take a systematic, variability-oriented approach and show that although the models tend to reproduce the mean magnitude of the observed annual flux variability, they fail to reproduce the timing. Large biases in modeled annual means are evident for all models. Observed interannual variability is found to commonly be on the order of magnitude of the mean fluxes. None of the models consistently reproduce observed interannual variability within measurement uncertainty. Underrepresentation of variability in spring phenology, soil thaw and snowpack melting, and difficulties in reproducing the lagged response to extreme climatic events are identified as systematic errors, common to all models included in this study.
    Full-text · Article · Jun 2012 · Global Change Biology
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    ABSTRACT: Understanding of carbon exchange between terrestrial ecosystems and the atmosphere can be improved through direct observations and experiments, as well as through modeling activities. Terrestrial biosphere models (TBMs) have become an integral tool for extrapolating local observations and understanding to much larger terrestrial regions. Although models vary in their specific goals and approaches, their central role within carbon cycle science is to provide a better understanding of the mechanisms currently controlling carbon exchange. Recently, the North American Carbon Program (NACP) organized several interim-synthesis activities to evaluate and inter-compare models and observations at local to continental scales for the years 2000 to 2005. Here, we compare the results from the TBMs collected as part of the regional and continental interim-synthesis (RCIS) activities. The primary objective of this work is to synthesize and compare the 19 participating TBMs to assess current understanding of the terrestrial carbon cycle in North America.
    No preview · Article · Jan 2012
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    ABSTRACT: Since terrestrial carbon fluxes and pools cannot be measured directly on regional and global scales, land surface models are an important tool in improving estimates of carbon sources and sinks. A variety of data sets now exist to evaluate land surface models against, which can help isolate shortcomings in the model and identify processes not being properly simulated. A variety of ecological properties are measured and hence can be investigated: above and below ground biomass and carbon pools from field campaigns; carbon fluxes from chambers and flux towers; and remotely sensed tree height, wood pools and leaf area index (LAI). In order to compare against these metrics, the Simple Biosphere Model (SiB4) has been restructured to include land cover heterogeneity, prognostic phenology, and carbon pools. This study presents results from SiB4, focusing on boreal North America and tropical South America. By utilizing data collected from field campaigns (the Boreal Ecosystem-Atmosphere Study, BOREAS, and the Large Scale Biosphere-Atmosphere Experiment in Amazonia, LBA), satellite data (remotely sensed LAI and IceSAT LIDAR measurements), and flux tower measurements, we analyze SiB4 results to help identify and resolve shortcomings and missing processes in the model. Land cover heterogeneity, prognostic phenology, and carbon pools have recently been combined in SiB4; and this study will investigate not only the impacts of these features, but also the overall performance of SiB4 in an effort to help improve terrestrial biogeochemical models and to advance our knowledge of the carbon cycle.
    No preview · Article · Jan 2012
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    ABSTRACT: Since terrestrial carbon fluxes and pools cannot be measured directly on regional and global scales, land surface models are an important tool in improving estimates of carbon sources and sinks. A variety of data sets now exist to evaluate land surface models against, which can help isolate shortcomings in the model and identify processes not being properly simulated. A variety of ecological properties are measured and hence can be investigated: above and below ground biomass and carbon pools from field campaigns; carbon fluxes from chambers and flux towers; and remotely sensed tree height, wood pools and leaf area index (LAI). In order to compare against these metrics, the Simple Biosphere Model (SiB4) has been restructured to include land cover heterogeneity, prognostic phenology, and carbon pools. This study presents results from SiB4, focusing on boreal North America and tropical South America. By utilizing data collected from field campaigns (the Boreal Ecosystem-Atmosphere Study, BOREAS, and the Large Scale Biosphere-Atmosphere Experiment in Amazonia, LBA), satellite data (remotely sensed LAI and IceSAT LIDAR measurements), and flux tower measurements, we analyze SiB4 results to help identify and resolve shortcomings and missing processes in the model. Land cover heterogeneity, prognostic phenology, and carbon pools have recently been combined in SiB4; and this study will investigate not only the impacts of these features, but also the overall performance of SiB4 in an effort to help improve terrestrial biogeochemical models and to advance our knowledge of the carbon cycle.
    No preview · Article · Jan 2012
  • I. T. Baker · A. B. Harper · J. A. Berry · A. Denning
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    ABSTRACT: It has been shown that there is a correlation between carbon uptake and diffuse fraction of radiation. This correlation can be direct, as higher diffuse fraction can result in greater penetration of radiation into the canopy. There are indirect effects as well, as greater diffuse fraction (and decreased overall insolation) may be associated with decreased temperature and vapor pressure deficit (VPD), both of which are associated with an increase in photosynthesis. In this study, we evaluate conditions at two eddy covariance flux towers, located within 20 km of each other, in the Tapajos River National Forest, Brazil. One tower is frequently underneath a regularly-occurring cloud band and the other is not, providing a situation where sites with similar vegetation experience different forcing regimes. While there are slight differences between the sites in terms of vegetation and topography, we do not find a systematic difference in ecosystem behavior at the sites on days when the meteorology is comparable at the 2 towers. We compare within- and between-site carbon flux during clear and cloudy days, and evaluate carbon flux relationship to variability in radiation, temperature, and moisture regimes. Using a mixture of observational data and model analysis, we describe the contrast in diurnal cycles of energy, moisture and carbon flux imposed by systematic differences in atmospheric forcing regime.
    No preview · Article · Dec 2011
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    ABSTRACT: Understanding biospheric CO2 fluxes is paramount if climate studies are to be able to analyze the response of terrestrial ecosystems to climate change and monitor fossil fuel emissions reductions. Carbonyl sulfide (COS) may be a useful tracer to provide a constraint on photosynthesis [gross primary production (GPP)]. Here we simulate both COS and CO2 using the Stochastic Time-Inverted Lagrangian Transport (STILT) model coupled with various biospheric fluxes, such as fluxes estimated from the Vegetation Photosynthesis and Respiration Model (VPRM), CarbonTracker, and from the Carnegie-Ames-Stanford Approach (CASA) model. The STILT model is driven by Weather Research and Forecast (WRF) meteorological fields. The WRF-STILT system is compared with the STILT driven by the ECMWF (European Center for Medium range Weather Forecasting) meteorology for the North American domain. This study uses measurements of COS and CO2 in 2008 from the NOAA/ESRL tall tower and aircraft air sampling networks, with ~ 6,000 observations in total. Biospheric COS fluxes will be estimated from a GPP-based model coupled with the GPP estimates from above mentioned biosphere models. Soil uptakes of COS are derived from a biosphere model (SiB) that assimilates the soil moisture and temperature. Estimation of other COS fluxes, such as anthropogenic, biomass burning are based on existing analyses of temporal and spatial variations. Empirical boundary curtains are built based on observations at the NOAA/ESRL marine boundary layer stations and from aircraft vertical profiles, and are utilized as the lateral boundary conditions for COS and CO2 for North America. Comparison of the simulations for both COS and CO2 using different biospheric fluxes provides an opportunity to assess the performance of both the biospheric models and the representation of atmospheric transport. In addition, we will estimate the carbon fluxes for North America from a joint inversion for COS and CO2 in a Bayesian synthesis framework, in which the GPP and Respiration are separately optimized for each vegetation type.
    No preview · Article · Dec 2011
  • B. Orescanin · A. Denning · I. T. Baker
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    ABSTRACT: The exchange of the trace gases between the land and atmosphere is highly influenced by vegetation. Therefore, the representation of phenological properties in global carbon models plays a key role in understanding and predicting the global carbon cycle. Phenological parameters such as Leaf Area Index (LAI) and fraction of photosynthetically active radiation absorbed (fPAR) are often calculated or estimated based on remote sensing measurements, which can be biased by clouds, aerosols, or snow. Alternatively, we can prognose vegetation phenology through the use of models that predict vegetation status based on meteorological conditions. Here our goal is to provide better understanding of carbon dynamics as a function of phenological parameters and their dependence on meteorological forcing. We evaluate phenological characteristics and their influence on carbon dynamics at several grassland sites. Modeled carbon flux, as a function of both diagnosed (from satellite) and prognosed phenological state are confronted with data from flux towers. Remotely-sensed phenology has a tendency to expand the growing season, and does not reflect the rapid response of vegetation in rain-green biomes as well as the prognostic phenology model does. These differences in phenology are reflected in modeled fluxes of energy, moisture, and carbon.
    No preview · Article · Dec 2011
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    ABSTRACT: Carbonyl sulfide (OCS) is an atmospheric trace gas that is taken up by leaves in parallel with photosynthetic uptake of CO2 (GPP). The latter is an important component of carbon cycle models and accurate measurements of photosynthesis are needed to test these models. However, it is difficult to obtain this information from atmospheric measurements of CO2 as air depleted of CO2 by GPP mixes very near the surface with air enriched in CO2 by respiration. Gradients in CO2, thus reflect the net sum of these two processes. In contrast, there is essentially no source of OCS in terrestrial ecosystems. The primary source of OCS is the oceans, thus mixing between this source and the photosynthetic sink occurs in the more remote atmosphere. This has led several researchers to postulate that gradients or time dependent changes in the concentration of OCS in air masses in contact with continental surfaces might be used to infer the rate of GPP of these surfaces. Several recent studies support this inference. However, it is important to note that these observations occur in a global context. The magnitude, spatial distribution and temporal dynamics of OCS sources and sinks are still poorly defined. This presentation will review recent efforts to improve our representation of the global cycle of OCS. We summarize new physiological measurements of exchange of OCS and CO2 by leaves including a wide range of species and environmental conditions, and report a new parameterization for OCS uptake for SiB3. We use these new simulations of surface exchanges in an atmospheric transport model, PCTM to simulate the concentration of OCS at points and times corresponding to flask measurements. While there remain important uncertainties about soil uptake and the ocean source, these simulations agree fairly well with observations.
    No preview · Article · Dec 2011