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  • 1
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    Isentropic transport and the seasonal cycle amplitude of CO2 (2016)
    Wiley
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    Publication Date: 2016-06-26
    Description: Carbon-concentration feedbacks and carbon-climate feedbacks constitute one of the largest sources of uncertainty in future climate. Since the beginning of the modern atmospheric CO 2 record, seasonal variations in CO 2 have been recognized as a signal of the metabolism of land ecosystems and quantitative attribution of changes in the seasonal cycle amplitude (SCA) of CO 2 to ecosystem processes is critical for understanding and projecting carbon-climate feedbacks far into the 21st Century. Here, the impact of surface carbon fluxes on the SCA of CO 2 throughout the Northern Hemisphere troposphere is investigated, paying particular attention to isentropic transport across latitudes. The analysis includes both a chemical transport model (GEOS-Chem) and an idealized tracer in a gray-radiation aquaplanet. The results of the study can be summarized by two main conclusions: (1) the SCA of CO 2 roughly follows surfaces of constant potential temperature, which can explain the observed increase in SCA with latitude along pressure surfaces and (2) increasing seasonal fluxes in lower latitudes have a larger impact on the SCA of CO 2 throughout most of the troposphere compared to increasing seasonal fluxes in higher latitudes. These results provide strong evidence that recently observed changes in the SCA of CO 2 at high northern latitudes (poleward of 60° N) are likely driven by changes in mid-latitude surface fluxes, rather than changes in Arctic fluxes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Moist synoptic transport of CO2 along the mid-latitude storm track (2011)
    Wiley
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    Publication Date: 2011-05-12
    Description: Atmospheric mixing ratios of CO2 are strongly seasonal in the Arctic due to mid-latitude transport. Here we analyze the seasonal influence of moist synoptic storms by diagnosing CO2 transport from a global model on moist isentropes (to represent parcel trajectories through stormtracks) and parsing transport into eddy and mean components. During winter when northern plants respire, warm moist air, high in CO2, is swept poleward into the polar vortex, while cold dry air, low in CO2, that had been transported into the polar vortex earlier in the year is swept equatorward. Eddies reduce seasonality in mid-latitudes by ∼50% of NEE (∼100% of fossil fuel) while amplifying seasonality at high latitudes. Transport along stormtracks is correlated with rising, moist, cloudy air, which systematically hides this CO2 transport from satellites. We recommend that (1) regional inversions carefully account for meridional transport and (2) inversion models represent moist and frontal processes with high fidelity.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    A sampling method for improving the representation of spatially varying precipitation and soil moisture using the Simple Biosphere Model (2013)
    Wiley
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    Publication Date: 2013-12-13
    Description: Representing spatially varying precipitation for current grid length scales used in General Circulation Models (GCMs) is a continuing challenge. Furthermore, to fully capture the hydrologic effects of non-uniform precipitation, a representation of soil moisture heterogeneity and distribution of spatially varying precipitation must exist within the same framework. For this study, the explicit and sampling methods of Sellers et al . [2007] are tested off-line using the Simple Biosphere Model (SiB3) in an arid, semi-arid and wet site, and are numerically compared to the bulk method, which is currently used in GCMs. To carry out the numerical experiments, an arbitrary grid area was defined by (1) a single instance of SiB3 (bulk method), (2) 100 instances of SiB3 (explicit method) and (3) less than 100 instances of SiB3 (sampling method). Precipitation was randomly distributed over fractions of the grid area for the explicit and sampling methods, while the standard SiB3 exponential distribution relating precipitation intensity to the grid area wet fraction was used in the bulk method. Comparing the sampling and bulk method to the explicit method indicates that 10 instances of SiB3 in the sampling method better captures the spatial variability in soil moisture and grid area flux calculations produced by the explicit method, and deals realistically with spatially varying precipitation at little additional computational cost to the bulk method.
    Electronic ISSN: 1942-2466
    Topics: Geography , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    A global reanalysis of vegetation phenology (2011)
    Wiley
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    Publication Date: 2011-08-25
    Description: Simulations of the global water and carbon cycle are sensitive to the model representation of vegetation phenology. Current phenology models are empirical, and few predict both phenological timing and leaf state. Our previous study demonstrated how satellite data assimilation employing an Ensemble Kalman Filter yields realistic phenological model parameters for several ecosystem types. In this study the data assimilation framework is extended to global scales using a subgrid-scale representation of plant functional types (PFTs) and elevation classes. A reanalysis of vegetation phenology for 256 globally distributed regions is performed using 10 years of Moderate Resolution Imaging Spectroradiometer (MODIS) fraction of photosynthetically active radiation (FPAR) absorbed by vegetation and leaf area index (LAI) data. The 9 · 108 quality screened observations (corresponding to
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Reflections on a Symposium on Computation (2012)
    Oxford University Press
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    Publication Date: 2012-06-30
    Description: ACM Ubiquity hosted a symposium in 2010–2011 on Turing's question, ‘What is computation?’ The editor reflects on how the symposium was organized and what conclusions it reached. The authors showed strong consensus around the propositions that computation is a process, computational model matters, many computations are natural, many important computations are continuous, many important computations are nonterminating and computational thinking has emerged as a core practice of computing. They left open the questions of whether the Turing model is the best reference model, is computational necessarily a physical process, what is information and what is an algorithm.
    Print ISSN: 0010-4620
    Electronic ISSN: 1460-2067
    Topics: Computer Science
    Published by Oxford University Press
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  • 6
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    Introduction to What is Computation (2012)
    Oxford University Press
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    Publication Date: 2012-06-30
    Print ISSN: 0010-4620
    Electronic ISSN: 1460-2067
    Topics: Computer Science
    Published by Oxford University Press
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  • 7
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    Opening Statement: What is Computation? (2012)
    Oxford University Press
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    Publication Date: 2012-06-30
    Description: Most people understand a computation as a process evoked when a computational agent acts on its inputs under the control of an algorithm. The classical Turing machine model has long served as the fundamental reference model because an appropriate Turing machine can simulate every other computational model known. The Turing model is a good abstraction for most digital computers because the number of steps to execute a Turing machine algorithm is predictive of the running time of the computation on a digital computer. However, the Turing model is not as well matched for the natural, interactive, and continuous information processes frequently encountered today. Other models whose structures more closely match the information processes involved give better predictions of running time and space. Models based on transforming representations may be useful.
    Print ISSN: 0010-4620
    Electronic ISSN: 1460-2067
    Topics: Computer Science
    Published by Oxford University Press
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  • 8
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    Closing Statement: What Have We Said About Computation? (2012)
    Oxford University Press
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    Publication Date: 2012-06-30
    Description: The "computation" symposium presents the reflections of thinkers from many sectors of computing on the fundamental question in the background of everything we do as computing professionals. While many of us have too many immediate tasks to allow us time for our own deep reflection, we do appreciate when others have done this for us. Peter Freeman points out, by analogy, that as citizens of democracies we do not spend a lot of time reflecting on the question, "What is a democracy." But from time to time we find it helpful to see what philosophers and political scientists are saying about the context in which we act as citizens.
    Print ISSN: 0010-4620
    Electronic ISSN: 1460-2067
    Topics: Computer Science
    Published by Oxford University Press
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  • 9
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    Estimates of North American summertime planetary boundary layer depths derived from space-borne lidar (2012)
    Wiley
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    Publication Date: 2012-08-01
    Description: The planetary boundary layer (PBL) mediates exchanges of energy, moisture, momentum, carbon, and pollutants between the surface and the atmosphere. This paper is a first step in producing a space-based estimate of PBL depth that can be used to compare with and evaluate model-based PBL depth retrievals, inform boundary layer studies, and improve understanding of the above processes. In clear sky conditions, space-borne lidar backscatter is frequently affected by atmospheric properties near the PBL top. Spatial patterns of 5-year mean mid-day summertime PBL depths over North America were estimated from the CALIPSO lidar backscatter and are generally consistent with model reanalyses and AMDAR (Aircraft Meteorological DAta Reporting) estimates. The rate of retrieval is greatest over the subtropical oceans (near 100%) where overlying subsidence limits optically thick clouds from growing and attenuating the lidar signal. The general retrieval rate over land is around 50% with decreased rates over the Southwestern United States and regions with high rates of convection. The lidar-based estimates of PBL depth tend to be shallower than aircraft estimates in coastal areas. Compared to reanalysis products, lidar PBL depths are greater over the oceans and areas of the boreal forest and shallower over the arid and semiarid regions of North America.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    Biosphere model simulations of interannual variability in terrestrial 13C/12C exchange (2013)
    Wiley
    Details
    Publication Date: 2013-05-22
    Description: [1]  Previous studies suggest that a large part of the variability in the atmospheric ratio of 13 CO 2 / 12 Co 2 originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric 13 C variations. We use the SiBCASA biogeochemical model, including a detailed isotopic fractionation scheme, separate 12 C and 13 C biogeochemical pools, and satellite-observed fire disturbances. This model of 12 CO 2 and 13 CO 2 thus also produces return fluxes of 13 CO 2 from its differently aged pools, contributing to the so-called disequilibrium flux. Our simulated terrestrial 13 C budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes, and similarly suggests that variations in C 3 discrimination and year-to-year variations in C 3 and C 4 productivity are the main drivers of their IAV. But the year-to-year variability in the isotopic disequilibrium flux is much lower (1 σ  = ± 1.5 PgC ‰ yr –1 ) than required (± 12.5 PgC ‰ yr –1 ) to match atmospheric observations, under the common assumption of low variability in net ocean CO 2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SIBCASA still misses processes that enhance variability in plant discrimination and relative C 3 /C 4 productivity. Alternatively, 13 C budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly more IAV in order to close the atmospheric 13 C budget on a year-to-year basis.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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