11,076 research outputs found
Deducing effective light transport parameters in optically thin systems
We present an extensive Monte Carlo study on light transport in optically
thin slabs, addressing both axial and transverse propagation. We completely
characterize the so-called ballistic-to-diffusive transition, notably in terms
of the spatial variance of the transmitted/reflected profile. We test the
validity of the prediction cast by diffusion theory, that the spatial variance
should grow independently of absorption and, to a first approximation, of the
sample thickness and refractive index contrast. Based on a large set of
simulated data, we build a freely available look-up table routine allowing
reliable and precise determination of the microscopic transport parameters
starting from robust observables which are independent of absolute intensity
measurements. We also present the Monte Carlo software package that was
developed for the purpose of this study
Spatial interpolation of high-frequency monitoring data
Climate modelers generally require meteorological information on regular
grids, but monitoring stations are, in practice, sited irregularly. Thus, there
is a need to produce public data records that interpolate available data to a
high density grid, which can then be used to generate meteorological maps at a
broad range of spatial and temporal scales. In addition to point predictions,
quantifications of uncertainty are also needed. One way to accomplish this is
to provide multiple simulations of the relevant meteorological quantities
conditional on the observed data taking into account the various uncertainties
in predicting a space-time process at locations with no monitoring data. Using
a high-quality dataset of minute-by-minute measurements of atmospheric pressure
in north-central Oklahoma, this work describes a statistical approach to
carrying out these conditional simulations. Based on observations at 11
stations, conditional simulations were produced at two other sites with
monitoring stations. The resulting point predictions are very accurate and the
multiple simulations produce well-calibrated prediction uncertainties for
temporal changes in atmospheric pressure but are substantially overconservative
for the uncertainties in the predictions of (undifferenced) pressure.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS208 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models
Measurements of atmospheric O2/N2 ratios and CO2 concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O2/N2 + CO2) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996-2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low-latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O2 and CO2 flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time-independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air-sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere. Copyright 2006 by the American Geophysical Union
Spatio-temporal Variability in Surface Ocean pCO2 Inferred from Observations
The variability of surface ocean pCO2 is examined on multiple spatial and temporal
scales. Temporal autocorrelation analysis is used to examine pCO2 variability over multiple
years. Spatial autocorrelation analysis describes pCO2 variability over multiple spatial
scales. Spatial autocorrelation lengths range between <50 km in coastal regions and other
areas of physical turbulence up to 3,000 km along major currents. Analysis of the drivers
of pCO2 shows that ocean currents are the primary driver of spatial variability. Autocorrelation
lengths of air-sea CO2 fluxes are approximately half as long as for pCO2 due to
the effects of highly variable wind speeds.
The influence of modes of climate variability on ocean pCO2 and related air-sea CO2
fluxes is examined through correlations of climate indices with interannual pCO2 anomalies
separated from the long-term trend and mean seasonal cycle. Changes in the El NiËœno
Southern Oscillation alter pCO2 levels by -6.6 � 1.0 �atm per index unit (�atm i
Renormalization of radiobiological response functions by energy loss fluctuations and complexities in chromosome aberration induction: deactivation theory for proton therapy from cells to tumor control
We employ a multi-scale mechanistic approach to investigate radiation induced
cell toxicities and deactivation mechanisms as a function of linear energy
transfer in hadron therapy. Our theoretical model consists of a system of
Markov chains in microscopic and macroscopic spatio-temporal landscapes, i.e.,
stochastic birth-death processes of cells in millimeter-scale colonies that
incorporates a coarse-grained driving force to account for microscopic
radiation induced damage. The coupling, hence the driving force in this
process, stems from a nano-meter scale radiation induced DNA damage that
incorporates the enzymatic end-joining repair and mis-repair mechanisms. We use
this model for global fitting of the high-throughput and high accuracy
clonogenic cell-survival data acquired under exposure of the therapeutic
scanned proton beams, the experimental design that considers -H2AX as
the biological endpoint and exhibits maximum observed achievable dose and LET,
beyond which the majority of the cells undergo collective biological
deactivation processes. An estimate to optimal dose and LET calculated from
tumor control probability by extension to cells per -size voxels
is presented. We attribute the increase in degree of complexity in chromosome
aberration to variabilities in the observed biological responses as the beam
linear energy transfer (LET) increases, and verify consistency of the predicted
cell death probability with the in-vitro cell survival assay of approximately
100 non-small cell lung cancer (NSCLC) cells
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