295 research outputs found
Estimating causal networks in biosphere–atmosphere interaction with the PCMCI approach
Local meteorological conditions and biospheric activity are tightly coupled. Understanding these links is an essential prerequisite for predicting the Earth system under climate change conditions. However, many empirical studies on the interaction between the biosphere and the atmosphere are based on correlative approaches that are not able to deduce causal paths, and only very few studies apply causal discovery methods. Here, we use a recently proposed causal graph discovery algorithm, which aims to reconstruct the causal dependency structure underlying a set of time series. We explore the potential of this method to infer temporal dependencies in biosphere-atmosphere interactions. Specifically we address the following questions: How do periodicity and heteroscedasticity influence causal detection rates, i.e. the detection of existing and non-existing links? How consistent are results for noise-contaminated data? Do results exhibit an increased information content that justifies the use of this causal-inference method? We explore the first question using artificial time series with well known dependencies that mimic real-world biosphere-atmosphere interactions. The two remaining questions are addressed jointly in two case studies utilizing observational data. Firstly, we analyse three replicated eddy covariance datasets from a Mediterranean ecosystem at half hourly time resolution allowing us to understand the impact of measurement uncertainties. Secondly, we analyse global NDVI time series (GIMMS 3g) along with gridded climate data to study large-scale climatic drivers of vegetation greenness. Overall, the results confirm the capacity of the causal discovery method to extract time-lagged linear dependencies under realistic settings. The violation of the method's assumptions increases the likelihood to detect false links. Nevertheless, we consistently identify interaction patterns in observational data. Our findings suggest that estimating a directed biosphere-atmosphere network at the ecosystem level can offer novel possibilities to unravel complex multi-directional interactions. Other than classical correlative approaches, our findings are constrained to a few meaningful set of relations which can be powerful insights for the evaluation of terrestrial ecosystem models
Bead, Hoop, and Spring as a Classical Spontaneous Symmetry Breaking Problem
We describe a simple mechanical system that involves Spontaneous Symmetry
Breaking. The system consists of two beads constrained to slide along a hoop
and attached each other through a spring. When the hoop rotates about a fixed
axis, the spring-beads system will change its equilibrium position as a
function of the angular velocity. The system shows two different regions of
symmetry separated by a critical point analogous to a second order transition.
The competitive balance between the rotational diynamics and the interaction of
the spring causes an Spontaneous Symmetry Breaking just as the balance between
temperature and the spin interaction causes a transition in a ferromagnetic
system. In addition, the gravitational potential act as an external force that
causes explicit symmetry breaking and a feature of first-order transition. Near
the transition point, the system exhibits a universal critical behavior where
the changes of the parameter of order is described by the critical exponent
beta =1/2 and the susceptibility by gamma =1. We also found a chaotic behavior
near the critical point. Through a demostrative device we perform some
qualitative observations that describe important features of the system.Comment: 7 pages, 2 tables, 30 figures, LaTeX2
Quantum Phase Transition and Berry Phase in an Extended Dicke Model
We investigate quantum phase transitions, quantum criticality, and Berry
phase for the ground state of an ensemble of non-interacting two-level atoms
embedded in a non-linear optical medium, coupled to a single-mode quantized
electromagnetic field. The optical medium is pumped externally through a
classical electric field, so that there is a degenerate parametric
amplification effect, which strongly modifies the field dynamics without
affecting the atomic sector. Through a semiclassical description the different
phases of this extended Dicke model are described. The quantum phase transition
is characterized with the expectation values of some observables of the system
as well as the Berry phase and its first derivative, where such quantities
serve as order parameters. It is remarkable that the model allows the control
of the quantum criticality through a suitable choice of the parameters of the
non-linear optical medium, which could make possible the use of a low intensity
laser to access the superradiant region experimentally.Comment: 7 pages, 4 figures, submitted to The European Physical Journal
Time Scale Analysis of a Fluidized-Bed Catalytic Reactor Based on a Generalized Dynamic Model
A fluidized-bed reactor model is implemented to simulate a Maleic Anhydride (MA) reactor with special emphasis on its dynamic behaviour. The dynamic model is general enough that it can treat a wide range of catalytic systems, subject to mass and energy balances within the phases. The model represents multiple phases and regions (low-density phase, high-density phase, freeboard region) and can account for heat and mass axial and radial anisotropic dispersion, change in molar/volumetric flow due to reaction, temperature and pressure profiles, hydrodynamic regime variation, catalyst deactivation, energy options, and multiple membranes of various geometries for introduction/extraction of any compound. The model reduces, as special cases, to most fluidized bed reactor models reported in the literature, allowing the influence of simplifying assumptions to be investigated. Introduction of different assumptions for a MA fluidized-bed reactor of industrial scale reveal quite different predicted time scales for key dynamic phenomena inherent to the process. A mass transfer/reaction time scale was found to be close to the residence time of the gas molecules in the reactor. The heat transfer time scale is several orders of magnitude larger for the current system. This type of time-scale analysis may be a useful tool to identify the appropriate degree of sophistication to predict the dynamics of complex reacting systems
Seguimiento del proceso de maduración del tomate (licopersicum esculentum mili) y desarrollo de normas preliminares de calidad
El presente trabajo describe los cambios de algunos parámetros fisicoquímicos, durante la maduración del tomate a temperatura ambiente, en las dos variedades más consumidas en Colombia como son el Chonto y el Milano. Se encontraron coeficientes de correlación significativos entre el tiempo de madurez y los siguientes parámetros: Pérdida de peso, viscosidad de la pulpa, dureza, índice de madurez, acidez, y vitamina C en las dos variedades. El pH sólo presentó correlación significativa para el tomate Chonto. Los grados Brix no presentaron correlación con el tiempo en las dos variedades. Se realizó una caracterización química en los tres estados de madurez no encontrándose diferencias en el pH del Milano entre los estados verde y pintón, como tampoco en los grados Brix entre pintón y maduro, ni en la vitamina C entre los estados verde y pintón, pintón y maduro
Femtosecond pulses and dynamics of molecular photoexcitation: RbCs example
We investigate the dynamics of molecular photoexcitation by unchirped
femtosecond laser pulses using RbCs as a model system. This study is motivated
by a goal of optimizing a two-color scheme of transferring
vibrationally-excited ultracold molecules to their absolute ground state. In
this scheme the molecules are initially produced by photoassociation or
magnetoassociation in bound vibrational levels close to the first dissociation
threshold. We analyze here the first step of the two-color path as a function
of pulse intensity from the low-field to the high-field regime. We use two
different approaches, a global one, the 'Wavepacket' method, and a restricted
one, the 'Level by Level' method where the number of vibrational levels is
limited to a small subset. The comparison between the results of the two
approaches allows one to gain qualitative insights into the complex dynamics of
the high-field regime. In particular, we emphasize the non-trivial and
important role of far-from-resonance levels which are adiabatically excited
through 'vertical' transitions with a large Franck-Condon factor. We also point
out spectacular excitation blockade due to the presence of a quasi-degenerate
level in the lower electronic state. We conclude that selective transfer with
femtosecond pulses is possible in the low-field regime only. Finally, we extend
our single-pulse analysis and examine population transfer induced by coherent
trains of low-intensity femtosecond pulses.Comment: 25 pages, 12 figure
Characterizing Ecosystem-Atmosphere Interactions from Short to Interannual Time Scales
Characterizing ecosystem-atmosphere interactions in terms of carbon and water exchange on different time scales is considered a major challenge in terrestrial biogeochemical cycle research. The respective time series are now partly comprising an observation 5 period of one decade. In this study, we explored whether the observation period is already sufficient to detect cross relationships of the variables beyond the annual cycle as they are expected from comparable studies in climatology. We explored the potential of Singular System Analysis (SSA) to extract arbitrary kinds of oscillatory patterns. The method is completely data adaptive and performs an 10 effective signal to noise separation.
We found that most observations (NEE, GP P , Reco, V P D, LE, H, u, P ) were influenced significantly by low frequency components (interannual variability). Furthermore we extracted a set of nonlinear relationships and found clear annual hysteresis effects except for the NEE-Rg relationship which turned out to be the sole linear relationship 15 in the observation space. SSA provides a new tool to investigate these phenomena explicitly on different time scales. Furthermore, we showed that SSA has great potential for eddy covariance data processing since it can be applied as novel gap fillingapproach relying on the temporal time series structure only.JRC.H.2-Climate chang
Characterizing ecosystem-atmosphere interactions from short to interannual time scales
International audienceCharacterizing ecosystem-atmosphere interactions in terms of carbon and water exchange on different time scales is considered a major challenge in terrestrial biogeochemical cycle research. The respective time series currently comprise an observation period of up to one decade. In this study, we explored whether the observation period is already sufficient to detect cross-relationships between the variables beyond the annual cycle, as they are expected from comparable studies in climatology. We investigated the potential of Singular System Analysis (SSA) to extract arbitrary kinds of oscillatory patterns. The method is completely data adaptive and performs an effective signal to noise separation. We found that most observations (Net Ecosystem Exchange, NEE, Gross Primary Productivity, GPP, Ecosystem Respiration, Reco, Vapor Pressure Deficit, VPD, Latent Heat, LE, Sensible Heat, H, Wind Speed, u, and Precipitation, P) were influenced significantly by low-frequency components (interannual variability). Furthermore, we extracted a set of nontrivial relationships and found clear seasonal hysteresis effects except for the interrelation of NEE with Global Radiation (Rg). SSA provides a new tool for the investigation of these phenomena explicitly on different time scales. Furthermore, we showed that SSA has great potential for eddy covariance data processing, since it can be applied as a novel gap filling approach relying on the temporal correlation structure of the time series structure only
Desempeño agronómico y valor nutritivo de Tithonia diversifolia (Hemsl.) A Gray para la alimentación de bovinos
The traditional livestock, due to the pressure exerted on the resources, must undergo a transformation that includes elements that allow the sustainability of the activity in the productive environment. The implementation of silvopastoral systems constitute a viable alternative for the improvement of production through the inclusion of woody species in the pasture for the formation of different plant stratum, which brings multiple benefits to the bovine productive cycle. The selection of these species should be carried out taking into account agronomic and compositional aspects to optimize animal response. This paper presents a review of the Mexican Sunflower (Tithonia diversifolia - (Asteraceae)), in feeding cattle. Main ways of establishment, nutrient content, biomass production, effect on the soil due to its use as a green manure and their productive effects in the diet of ruminants are described. Mexican Sunflower (Tithonia diversifolia) can be established by sexual or vegetative seed. Its nutritional composition shows high levels of protein and soluble carbohydrates, low neutral detergent fiber content (NDF), and balanced levels of minerals. The establishment of this specie and its use as green manure can improve soil fertility to increase production and yield of the pasture. These characteristics give it a high potential in bovine feeding, either as a resource for browsing, processed in flour or silage, to reduce production costs in livestock farms.La producción ganadera tradicional debido a la presión ejercida sobre los recursos, debe sufrir una transformación que incluya elementos que permitan la sostenibilidad de la actividad en el entorno productivo. La implementación de sistemas silvopastoriles se constituye en una alternativa viable para el mejoramiento de la producción a través de la inclusión de especies leñosas en el potrero para la formación de diferentes estratos, que generen múltiples beneficios a todos los elementos del ciclo bovino. La selección de las especies silvopastoriles debe ser realizada teniendo en cuenta aspectos agronómicos y composicionales para optimizar la respuesta animal. Este documento presenta una revisión sobre el potencial agronómico, la calidad nutricional y el uso del Botón de oro (Tithonia diversifolia – Asteraceae) en la alimentación de bovinos. Se describen las principales formas de establecimiento, contenido de nutrientes, producción de biomasa, acción en el suelo debido a su uso como abono verde y sus efectos productivos en la dieta de los rumiantes. El establecimiento del Botón de oro (Tithonia diversifolia), puede ser realizado a través de semilla sexual y vegetativa. Su composición nutricional muestra niveles elevados de proteína y carbohidratos solubles, bajo contenido de FDN, y niveles adecuados de minerales, su establecimiento y uso como abono verde puede mejorar las condiciones de fertilidad del suelo para aumentar la producción y rendimiento del pasto. Estas características le confieren una alto potencial en la alimentación bovina, ya sea como recurso para ramoneo, procesado en harina o ensilado, para reducir los costos de producción en explotaciones ganaderas
- …