6,127 research outputs found
Hydrological connectivity inferred from diatom transport through the riparian-stream system
Funding for this research was provided by the Luxembourg National Research Fund (FNR) in the framework of the BIGSTREAM (C09/SR/14), ECSTREAM (C12/SR/40/8854) and CAOS (INTER/DFG/11/01) projects. We are most grateful to the Administration des Services Techniques de lâAgriculture (ASTA) for providing meteorological data. We also acknowledge Delphine Collard for technical assistance in diatom sample treatment and preparation, François Barnich for the water chemistry analyses, and Jean-François Iffly, Christophe Hissler, JĂ©rĂŽme Juilleret, Laurent Gourdol and Julian Klaus for their constructive comments on the project and technical assistance in the field.Peer reviewedPublisher PD
Noncontact atomic force microscopy simulator with phase-locked-loop controlled frequency detection and excitation
A simulation of an atomic force microscope operating in the constant
amplitude dynamic mode is described. The implementation mimics the electronics
of a real setup including a digital phase-locked loop (PLL). The PLL is not
only used as a very sensitive frequency detector, but also to generate the
time-dependent phase shifted signal driving the cantilever. The optimum
adjustments of individual functional blocks and their joint performance in
typical experiments are determined in detail. Prior to testing the complete
setup, the performances of the numerical PLL and of the amplitude controller
were ascertained to be satisfactory compared to those of the real components.
Attention is also focused on the issue of apparent dissipation, that is, of
spurious variations in the driving amplitude caused by the nonlinear
interaction occurring between the tip and the surface and by the finite
response times of the various controllers. To do so, an estimate of the minimum
dissipated energy that is detectable by the instrument upon operating
conditions is given. This allows us to discuss the relevance of apparent
dissipation that can be conditionally generated with the simulator in
comparison to values reported experimentally. The analysis emphasizes that
apparent dissipation can contribute to the measured dissipation up to 15% of
the intrinsic dissipated energy of the cantilever interacting with the surface,
but can be made negligible when properly adjusting the controllers, the PLL
gains and the scan speed. It is inferred that the experimental values of
dissipation usually reported in the literature cannot only originate in
apparent dissipation, which favors the hypothesis of "physical" channels of
dissipation
Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system
The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered
Heterocyst placement strategies to maximize growth of cyanobacterial filaments
Under conditions of limited fixed-nitrogen, some filamentous cyanobacteria
develop a regular pattern of heterocyst cells that fix nitrogen for the
remaining vegetative cells. We examine three different heterocyst placement
strategies by quantitatively modelling filament growth while varying both
external fixed-nitrogen and leakage from the filament. We find that there is an
optimum heterocyst frequency which maximizes the growth rate of the filament;
the optimum frequency decreases as the external fixed-nitrogen concentration
increases but increases as the leakage increases. In the presence of leakage,
filaments implementing a local heterocyst placement strategy grow significantly
faster than filaments implementing random heterocyst placement strategies. With
no extracellular fixed-nitrogen, consistent with recent experimental studies of
Anabaena sp. PCC 7120, the modelled heterocyst spacing distribution using our
local heterocyst placement strategy is qualitatively similar to experimentally
observed patterns. As external fixed-nitrogen is increased, the spacing
distribution for our local placement strategy retains the same shape while the
average spacing between heterocysts continuously increases.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Physical Biology. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The definitive publisher-authenticated version
will be available onlin
Reproducing the CO-to-Hâ conversion factor in cosmological simulations of Milky-Way-mass galaxies
We present models of CO(1â0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1â0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given Hâ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of âŒ3 pc in cold gas (T < 300 K) for both CO and Hâ, is able to reproduce both the observed CO(1â0) luminosity and inferred CO-to-Hâ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds
The Clustering of Massive Halos
The clustering properties of dark matter halos are a firm prediction of
modern theories of structure formation. We use two large volume,
high-resolution N-body simulations to study how the correlation function of
massive dark matter halos depends upon their mass and formation history. We
find that halos with the lowest concentrations are presently more clustered
than those of higher concentration, the size of the effect increasing with halo
mass; this agrees with trends found in studies of lower mass halos. The
clustering dependence on other characterizations of the full mass accretion
history appears weaker than the effect with concentration. Using the integrated
correlation function, marked correlation functions, and a power-law fit to the
correlation function, we find evidence that halos which have recently undergone
a major merger or a large mass gain have slightly enhanced clustering relative
to a randomly chosen population with the same mass distribution.Comment: 10 pages, 8 figures; text improved, references and one figure added;
accepted for publication in Ap
Nitrogen and phosphorus loads to temperate seepage lakes associated with allochthonous dissolved organic carbon loads
Terrestrial loads of dissolved organic matter (DOM) have increased in recent years in many north temperate lakes. While much of the focus on the âbrowningâ phenomena has been on its consequences for carbon cycling, much less is known about how it influences nutrient loading to lakes. We characterize potential loads of nitrogen and phosphorus to seepage lakes in northern Wisconsin, USA, based on a laboratory soil leaching experiment and a model that includes landscape cover and watershed area. In these seepage lakes, nutrient concentrations are positively correlated with dissolved organic carbon concentrations (nitrogen: r = 0.68, phosphorus: r = 0.54). Using longâterm records of browning, we found that dissolved organic matterâassociated nutrient loadings may have resulted in substantial increases in nitrogen and phosphorus in seepage lakes and could account for currently observed nutrient concentrations in the lake. âSilentâ nutrient loadings to brownâwater lakes may lead to future waterâquality concerns. PLAIN LANGUAGE SUMMARY: The color of many temperate lakes is changing; some lakes are becoming more darkly stained brown. The teaâcolored stain is due to dissolved organic matter from the surrounding landscape. Much of the research related to the causes and consequences of increased staining, or âbrownification,â relate to its connection to the carbon cycle. However, by examining longâterm lake chemical records, analyzing the properties of the organic compounds, and modeling potential flows of the compounds, we find that carbon is not the only element that is influenced by browning. Nitrogen and phosphorus, two nutrients important to growth of organisms at the base of the food web, may also be increasing in lakes due to brownification."Funding for this research was supported by the Northern Research Station and the Chequamegon-Nicolet National Forest (CNNF) of the United States Department of Agriculture (USDA) Forest Service and a National Science Foundation (NSF) grant to the University of Wisconsin-Madison to support the North Temperate Lakes Long-Term Ecological Research (NTLLTER) Site (DEB-#1440297)."https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL07721
On the stability of tidal streams in action space
In the Gaia era it is increasingly apparent that traditional static,
parameterized models are insufficient to describe the mass distribution of our
complex, dynamically evolving Milky Way (MW). In this work, we compare
different time-evolving and time-independent representations of the
gravitational potentials of simulated MW-mass galaxies from the FIRE-2 suite of
cosmological baryonic simulations. Using these potentials, we calculate actions
for star particles in tidal streams around three galaxies with varying merger
histories at each snapshot from 7 Gyr ago to the present day. We determine the
action-space coherence preserved by each model using the Kullback-Leibler
Divergence to gauge the degree of clustering in actions and the relative
stability of the clusters over time. We find that all models produce a
clustered action space for simulations with no significant mergers. However, a
massive (mass ratio prior to infall more similar than 1:8) interacting galaxy
not present in the model will result in mischaracterized orbits for stars most
affected by the interaction. The locations of the action space clusters (i.e.
the orbits of the stream stars) are only preserved by the time-evolving model,
while the time-independent models can lose significant amounts of information
as soon as 0.5--1 Gyr ago, even if the system does not undergo a significant
merger. Our results imply that reverse-integration of stream orbits in the MW
using a fixed potential is likely to give incorrect results if integrated
longer than 0.5 Gyr into the past
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