137,086 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
Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus
We have undertaken an unprecedentedly large 1.1 millimeter continuum survey
of three nearby star forming clouds using Bolocam at the Caltech Submillimeter
Observatory. We mapped the largest areas in each cloud at millimeter or
submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq.
deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of
31", detecting 122, 44, and 35 cores, respectively. Here we report on results
of the Serpens survey and compare the three clouds. Average measured angular
core sizes and their dependence on resolution suggest that many of the observed
sources are consistent with power-law density profiles. Tests of the effects of
cloud distance reveal that linear resolution strongly affects measured source
sizes and densities, but not the shape of the mass distribution. Core mass
distribution slopes in Perseus and Ophiuchus (alpha=2.1+/-0.1 and
alpha=2.1+/-0.3) are consistent with recent measurements of the stellar IMF,
whereas the Serpens distribution is flatter (alpha=1.6+/-0.2). We also compare
the relative mass distribution shapes to predictions from turbulent
fragmentation simulations. Dense cores constitute less than 10% of the total
cloud mass in all three clouds, consistent with other measurements of low
star-formation efficiencies. Furthermore, most cores are found at high column
densities; more than 75% of 1.1 mm cores are associated with Av>8 mag in
Perseus, 15 mag in Serpens, and 20-23 mag in Ophiuchus.Comment: 32 pages, including 18 figures, accepted for publication in Ap
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: I. Limits and Detections
The DIRBE on the COBE spacecraft was designed primarily to conduct systematic
search for an isotropic CIB in ten photometric bands from 1.25 to 240 microns.
The results of that search are presented here. Conservative limits on the CIB
are obtained from the minimum observed brightness in all-sky maps at each
wavelength, with the faintest limits in the DIRBE spectral range being at 3.5
microns (\nu I_\nu < 64 nW/m^2/sr, 95% CL) and at 240 microns (\nu I_\nu < 28
nW/m^2/sr, 95% CL). The bright foregrounds from interplanetary dust scattering
and emission, stars, and interstellar dust emission are the principal
impediments to the DIRBE measurements of the CIB. These foregrounds have been
modeled and removed from the sky maps. Assessment of the random and systematic
uncertainties in the residuals and tests for isotropy show that only the 140
and 240 microns data provide candidate detections of the CIB. The residuals and
their uncertainties provide CIB upper limits more restrictive than the dark sky
limits at wavelengths from 1.25 to 100 microns. No plausible solar system or
Galactic source of the observed 140 and 240 microns residuals can be
identified, leading to the conclusion that the CIB has been detected at levels
of \nu I_\nu = 25+-7 and 14+-3 nW/m^2/sr at 140 and 240 microns respectively.
The integrated energy from 140 to 240 microns, 10.3 nW/m^2/sr, is about twice
the integrated optical light from the galaxies in the Hubble Deep Field,
suggesting that star formation might have been heavily enshrouded by dust at
high redshift. The detections and upper limits reported here provide new
constraints on models of the history of energy-releasing processes and dust
production since the decoupling of the cosmic microwave background from matter.Comment: 26 pages and 5 figures, accepted for publication in the Astrophyical
Journa
SPIRE Point Source Catalog Explanatory Supplement
The Spectral and Photometric Imaging Receiver (SPIRE) was launched as one of
the scientific instruments on board of the space observatory Herschel. The
SPIRE photometer opened up an entirely new window in the Submillimeter domain
for large scale mapping, that up to then was very difficult to observe. There
are already several catalogs that were produced by individual Herschel science
projects. Yet, we estimate that the objects of only a fraction of these maps
will ever be systematically extracted and published by the science teams that
originally proposed the observations. The SPIRE instrument performed its
standard photometric observations in an optically very stable configuration,
only moving the telescope across the sky, with variations in its configuration
parameters limited to scan speed and sampling rate. This and the scarcity of
features in the data that require special processing steps made this dataset
very attractive for producing an expert reduced catalog of point sources that
is being described in this document. The Catalog was extracted from a total of
6878 unmodified SPIRE scan map observations. The photometry was obtained by a
systematic and homogeneous source extraction procedure, followed by a rigorous
quality check that emphasized reliability over completeness. Having to exclude
regions affected by strong Galactic emission, that pushed the limits of the
four source extraction methods that were used, this catalog is aimed primarily
at the extragalactic community. The result can serve as a pathfinder for ALMA
and other Submillimeter and Far-Infrared facilities. 1,693,718 sources are
included in the final catalog, splitting into 950688, 524734, 218296 objects
for the 250\mu m, 350\mu m, and 500\mu m bands, respectively. The catalog comes
with well characterized environments, reliability, completeness, and
accuracies, that single programs typically cannot provide
Probes of Lorentz Violation in Neutrino Propagation
It has been suggested that the interactions of energetic particles with the
foamy structure of space-time thought to be generated by quantum-gravitational
(QG) effects might violate Lorentz invariance, so that they do not propagate at
a universal speed of light. We consider the limits that may be set on a linear
or quadratic violation of Lorentz invariance in the propagation of energetic
neutrinos, v/c=[1 +- (E/M_\nuQG1)] or [1 +- (E/M_\nu QG2}^2], using data from
supernova explosions and the OPERA long-baseline neutrino experiment. Using the
SN1987a neutrino data from the Kamioka II, IMB and Baksan experiments, we set
the limits M_\nuQG1 > 2.7(2.5)x10^10 GeV for subluminal (superluminal)
propagation, respectively, and M_\nuQG2 >4.6(4.1)x10^4 GeV at the 95%
confidence level. A future galactic supernova at a distance of 10 kpc would
have sensitivity to M_\nuQG1 > 2(4)x10^11 GeV for subluminal (superluminal)
propagation, respectively, and M_\nuQG2 > 2(4)x10^5 GeV. With the current CNGS
extraction spill length of 10.5 micro seconds and with standard clock
synchronization techniques, the sensitivity of the OPERA experiment would reach
M_\nuQG1 ~ 7x10^5 GeV (M_\nuQG2 ~ 8x10^3 GeV) after 5 years of nominal running.
If the time structure of the SPS RF bunches within the extracted CNGS spills
could be exploited, these figures would be significantly improved to M_\nuQG1 ~
5x10^7 GeV (M_\nuQG2 ~ 4x10^4 GeV). These results can be improved further if
similar time resolution can be achieved with neutrino events occurring in the
rock upstream of the OPERA detector: we find potential sensitivities to
M_\nuQG1 ~ 4x10^8 GeV and M_\nuQG2 ~ 7x10^5 GeV.Comment: 33 pages, 22 figures, version accepted for publication in Physical
Review
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