1,127 research outputs found
The long-solved problem of the best-fit straight line: application to isotopic mixing lines
It has been almost 50 years since York published an exact
and general solution for the best-fit straight line to independent points
with normally distributed errors in both x and y. York's solution is highly
cited in the geophysical literature but almost unknown outside of it, so
that there has been no ebb in the tide of books and papers wrestling with
the problem. Much of the post-1969 literature on straight-line fitting has
sown confusion not merely by its content but by its very existence. The
optimal least-squares fit is already known; the problem is already solved.
Here we introduce the non-specialist reader to York's solution and
demonstrate its application in the interesting case of the isotopic mixing
line, an analytical tool widely used to determine the isotopic signature of
trace gas sources for the study of biogeochemical cycles. The most commonly
known linear regression methods – ordinary least-squares regression (OLS),
geometric mean regression (GMR), and orthogonal distance regression
(ODR) – have each been recommended as the best method for fitting isotopic
mixing lines. In fact, OLS, GMR, and ODR are all special cases of York's
solution that are valid only under particular measurement conditions, and
those conditions do not hold in general for isotopic mixing lines. Using
Monte Carlo simulations, we quantify the biases in OLS, GMR, and ODR under
various conditions and show that York's general – and convenient – solution
is always the least biased
Thermal noise suppression: how much does it cost?
In order to stabilize the behavior of noisy systems, confining it around a
desirable state, an effort is required to suppress the intrinsic noise. This
noise suppression task entails a cost. For the important case of thermal noise
in an overdamped system, we show that the minimum cost is achieved when the
system control parameters are held constant: any additional deterministic or
random modulation produces an increase of the cost. We discuss the implications
of this phenomenon for those overdamped systems whose control parameters are
intrinsically noisy, presenting a case study based on the example of a Brownian
particle optically trapped in an oscillating potential.Comment: 6 page
Accuracy of one-dimensional collision integral in the rigid spheres approximation
The accuracy of calculation of spectral line shapes in one-dimensional
approximation is studied analytically in several limiting cases for arbitrary
collision kernel and numerically in the rigid spheres model. It is shown that
the deviation of the line profile is maximal in the center of the line in case
of large perturber mass and intermediate values of collision frequency. For
moderate masses of buffer molecules the error of one-dimensional approximation
is found not to exceed 5%.Comment: LaTeX, 24 pages, 8 figure
Survey and Digital Documentation of Endangered Temple Wall Paintings in Shanxi Province, China
Shanxi Province is at the heart of China, and home to some of its richest architectural heritage. Covering an area of 156,000 square kilometres the Province is larger than England and Wales combined. Many earthen and timber buildings and temples contain wall paintings, witnessing Chinese folk religion, Buddhist and Daoist beliefs. Scattered over a large geographic area these remote village temples present a fairyland of Chinese traditional folklore. Many of these sites are endangered, and most are unrecorded.
Funded by Arcadia, a charitable trust fund of Lisbet Rausing and Peter Baldwin, the Shanxi Digital Documentation of Endangered Temple Wall Painting Project (SDDP) is a four-year programme (2018–2021) aimed at recording these historic temples and wall paintings using high-resolution photographic and three-dimensional photogrammetric techniques, and also including selective capture of multi-spectral imagery. These records will form an open-access digital archive of temple paintings and associated architecture, structured by a Chinese-English bilingual database.
The SDDP is a partnership between Zhejiang University, Shanxi Institute of Architecture Conservation and University College London (represented by two research centres within the Institute of Archaeology: the International Centre for Chinese Heritage and Archaeology and the Centre for Applied Archaeology). The programme of survey and research is guided by the Shanxi Provincial Bureau of Cultural Heritage (SBCH) and an international advisory board
Classical Spin Models with Broken Continuous Symmetry: Random Field Induced Order and Persistence of Spontaneous Magnetization
We consider a classical spin model, of two-dimensional spins, with continuous
symmetry, and investigate the effect of a symmetry breaking unidirectional
quenched disorder on the magnetization of the system. We work in the mean field
regime. We show, by numerical simulations and by perturbative calculations in
the low as well as in the high temperature limits, that although the continuous
symmetry of the magnetization is lost, the system still magnetizes, albeit with
a lower value as compared to the case without disorder. The critical
temperature at which the system starts magnetizing, also decreases with the
introduction of disorder. However, with the introduction of an additional
constant magnetic field, the component of magnetization in the direction that
is transverse to the disorder field increases with the introduction of the
quenched disorder. We discuss the same effects also for three-dimensional
spins.Comment: 12 pages, 12 figures, RevTeX
Does Amazon forest leaf phenology mediate transpiration seasonality and hence, ecoclimate teleconnections?
Abstract OOS 11-5
Canopy-Atmosphere Exchange of Carbon, Water and Energy at Harvard Forest EMS Tower since 1991
Stomatal conductance influences both photosynthesis and transpiration,
thereby coupling the carbon and water cycles and affecting surface–atmosphere
energy exchange. The environmental response of stomatal conductance has been
measured mainly on the leaf scale, and theoretical canopy models are relied
on to upscale stomatal conductance for application in terrestrial ecosystem
models and climate prediction. Here we estimate stomatal conductance and
associated transpiration in a temperate deciduous forest directly on the canopy scale via two independent approaches: (i)Â from heat and water vapor
exchange and (ii)Â from carbonyl sulfide (OCS) uptake. We use the eddy
covariance method to measure the net ecosystem–atmosphere exchange of OCS,
and we use a flux-gradient approach to separate canopy OCS uptake from soil
OCS uptake. We find that the seasonal and diurnal patterns of canopy stomatal
conductance obtained by the two approaches agree (to within ±6 %
diurnally), validating both methods. Canopy stomatal conductance increases
linearly with above-canopy light intensity (in contrast to the leaf scale,
where stomatal conductance shows declining marginal increases) and otherwise
depends only on the diffuse light fraction, the canopy-average leaf-to-air
water vapor gradient, and the total leaf area. Based on stomatal conductance,
we partition evapotranspiration (ET) and find that evaporation increases from
0 to 40 % of ET as the growing season progresses, driven primarily by
rising soil temperature and secondarily by rainfall. Counterintuitively,
evaporation peaks at the time of year when the soil is dry and the air is
moist. Our method of ET partitioning avoids
concerns about mismatched scales or measurement types because both ET and
transpiration are derived from eddy covariance data. Neither of the two ecosystem
models tested predicts the observed dynamics of evaporation or transpiration,
indicating that ET partitioning such as that provided here is needed to
further model development and improve our understanding of carbon and water
cycling
Impact of FTO genotypes on BMI and weight in polycystic ovary syndrome : a systematic review and meta-analysis
Aims/hypothesis
FTO gene single nucleotide polymorphisms (SNPs) have been shown to be associated with obesity-related traits and type 2 diabetes. Several small studies have suggested a greater than expected effect of the FTO rs9939609 SNP on weight in polycystic ovary syndrome (PCOS). We therefore aimed to examine the impact of FTO genotype on BMI and weight in PCOS.
Methods
A systematic search of medical databases (PubMed, EMBASE and Cochrane CENTRAL) was conducted up to the end of April 2011. Seven studies describing eight distinct PCOS cohorts were retrieved; seven were genotyped for SNP rs9939609 and one for SNP rs1421085. The per allele effect on BMI and body weight increase was calculated and subjected to meta-analysis.
Results
A total of 2,548 women with PCOS were included in the study; 762 were TT homozygotes, 1,253 had an AT/CT genotype, and 533 were AA/CC homozygotes. Each additional copy of the effect allele (A/C) increased the BMI by a mean of 0.19 z score units (95% CI 0.13, 0.24; p = 2.26 × 10−11) and body weight by a mean of 0.20 z score units (95% CI 0.14, 0.26; p = 1.02 × 10−10). This translated into an approximately 3.3 kg/m2 increase in BMI and an approximately 9.6 kg gain in body weight between TT and AA/CC homozygotes. The association between FTO genotypes and BMI was stronger in the cohorts with PCOS than in the general female populations from large genome-wide association studies. Deviation from an additive genetic model was observed in heavier populations.
Conclusions/interpretation
The effect of FTO SNPs on obesity-related traits in PCOS seems to be more than two times greater than the effect found in large population-based studies. This suggests an interaction between FTO and the metabolic context or polygenic background of PCOS
Atmospheric gas absorption knowledge in the submillimeter: Modeling, field measurements, and uncertainty quantification
Members of the atmospheric and astronomical science communities met to review the current state of the art of the submillimeter spectral region. Knowledge of gas spectroscopy is still questionable at these frequencies but is important to fully exploit upcoming
meteorological satellite measurements
- …