4,034 research outputs found
Reduced-rank spatio-temporal modeling of air pollution concentrations in the Multi-Ethnic Study of Atherosclerosis and Air Pollution
There is growing evidence in the epidemiologic literature of the relationship
between air pollution and adverse health outcomes. Prediction of individual air
pollution exposure in the Environmental Protection Agency (EPA) funded
Multi-Ethnic Study of Atheroscelerosis and Air Pollution (MESA Air) study
relies on a flexible spatio-temporal prediction model that integrates land-use
regression with kriging to account for spatial dependence in pollutant
concentrations. Temporal variability is captured using temporal trends
estimated via modified singular value decomposition and temporally varying
spatial residuals. This model utilizes monitoring data from existing regulatory
networks and supplementary MESA Air monitoring data to predict concentrations
for individual cohort members. In general, spatio-temporal models are limited
in their efficacy for large data sets due to computational intractability. We
develop reduced-rank versions of the MESA Air spatio-temporal model. To do so,
we apply low-rank kriging to account for spatial variation in the mean process
and discuss the limitations of this approach. As an alternative, we represent
spatial variation using thin plate regression splines. We compare the
performance of the outlined models using EPA and MESA Air monitoring data for
predicting concentrations of oxides of nitrogen (NO)-a pollutant of primary
interest in MESA Air-in the Los Angeles metropolitan area via cross-validated
. Our findings suggest that use of reduced-rank models can improve
computational efficiency in certain cases. Low-rank kriging and thin plate
regression splines were competitive across the formulations considered,
although TPRS appeared to be more robust in some settings.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS786 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Beyond Mixing-length Theory: a step toward 321D
We examine the physical basis for algorithms to replace mixing-length theory
(MLT) in stellar evolutionary computations. Our 321D procedure is based on
numerical solutions of the Navier-Stokes equations. These implicit large eddy
simulations (ILES) are three-dimensional (3D), time-dependent, and turbulent,
including the Kolmogorov cascade. We use the Reynolds-averaged Navier-Stokes
(RANS) formulation to make concise the 3D simulation data, and use the 3D
simulations to give closure for the RANS equations. We further analyze this
data set with a simple analytical model, which is non-local and time-dependent,
and which contains both MLT and the Lorenz convective roll as particular
subsets of solutions. A characteristic length (the damping length) again
emerges in the simulations; it is determined by an observed balance between (1)
the large-scale driving, and (2) small-scale damping.
The nature of mixing and convective boundaries is analyzed, including
dynamic, thermal and compositional effects, and compared to a simple model.
We find that
(1) braking regions (boundary layers in which mixing occurs) automatically
appear {\it beyond} the edges of convection as defined by the Schwarzschild
criterion,
(2) dynamic (non-local) terms imply a non-zero turbulent kinetic energy flux
(unlike MLT),
(3) the effects of composition gradients on flow can be comparable to thermal
effects, and
(4) convective boundaries in neutrino-cooled stages differ in nature from
those in photon-cooled stages (different P\'eclet numbers).
The algorithms are based upon ILES solutions to the Navier-Stokes equations,
so that, unlike MLT, they do not require any calibration to astronomical
systems in order to predict stellar properties. Implications for solar
abundances, helioseismology, asteroseismology, nucleosynthesis yields,
supernova progenitors and core collapse are indicated.Comment: 22 pages, 4 figures, 2 tables; significantly re-written, critique of
Pasetto, et al. model added, accepted for publication by Ap
Reduction of dynamical biochemical reaction networks in computational biology
Biochemical networks are used in computational biology, to model the static
and dynamical details of systems involved in cell signaling, metabolism, and
regulation of gene expression. Parametric and structural uncertainty, as well
as combinatorial explosion are strong obstacles against analyzing the dynamics
of large models of this type. Multi-scaleness is another property of these
networks, that can be used to get past some of these obstacles. Networks with
many well separated time scales, can be reduced to simpler networks, in a way
that depends only on the orders of magnitude and not on the exact values of the
kinetic parameters. The main idea used for such robust simplifications of
networks is the concept of dominance among model elements, allowing
hierarchical organization of these elements according to their effects on the
network dynamics. This concept finds a natural formulation in tropical
geometry. We revisit, in the light of these new ideas, the main approaches to
model reduction of reaction networks, such as quasi-steady state and
quasi-equilibrium approximations, and provide practical recipes for model
reduction of linear and nonlinear networks. We also discuss the application of
model reduction to backward pruning machine learning techniques
Phylogenetic analyses of trilobites from the Cambrian and Ordovician radiations
Trilobites are an ideal group used for phylogenetic studies due to their complex morphology and abundance in the Paleozoic fossil record. Because of this, trilobite phylogenetics has become an important component of understanding macroevolutionary patterns during key evolutionary radiations. Two radiations that this dissertation investigates are the Cambrian and Ordovician radiations. While both occur during the early Paleozoic, they present very different patterns. The Cambrian radiation was the sudden appearance of all major metazoan phyla, whereas in the Ordovician, those new beauplans were already in place and there was instead a great increase in diversity among those groups. Presented here are phylogenetic analyses of trilobites from those radiations to further our understanding of the evolutionary patterns occurring at those times
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