173 research outputs found
Sim2Real for Environmental Neural Processes
Machine learning (ML)-based weather models have recently undergone rapid
improvements. These models are typically trained on gridded reanalysis data
from numerical data assimilation systems. However, reanalysis data comes with
limitations, such as assumptions about physical laws and low spatiotemporal
resolution. The gap between reanalysis and reality has sparked growing interest
in training ML models directly on observations such as weather stations.
Modelling scattered and sparse environmental observations requires scalable and
flexible ML architectures, one of which is the convolutional conditional neural
process (ConvCNP). ConvCNPs can learn to condition on both gridded and
off-the-grid context data to make uncertainty-aware predictions at target
locations. However, the sparsity of real observations presents a challenge for
data-hungry deep learning models like the ConvCNP. One potential solution is
'Sim2Real': pre-training on reanalysis and fine-tuning on observational data.
We analyse Sim2Real with a ConvCNP trained to interpolate surface air
temperature over Germany, using varying numbers of weather stations for
fine-tuning. On held-out weather stations, Sim2Real training substantially
outperforms the same model architecture trained only with reanalysis data or
only with station data, showing that reanalysis data can serve as a stepping
stone for learning from real observations. Sim2Real could thus enable more
accurate models for weather prediction and climate monitoring.Comment: 4 pages, 3 figures, To be published in Tackling Climate Change with
Machine Learning workshop at NeurIP
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
A weighted configuration model and inhomogeneous epidemics
A random graph model with prescribed degree distribution and degree dependent
edge weights is introduced. Each vertex is independently equipped with a random
number of half-edges and each half-edge is assigned an integer valued weight
according to a distribution that is allowed to depend on the degree of its
vertex. Half-edges with the same weight are then paired randomly to create
edges. An expression for the threshold for the appearance of a giant component
in the resulting graph is derived using results on multi-type branching
processes. The same technique also gives an expression for the basic
reproduction number for an epidemic on the graph where the probability that a
certain edge is used for transmission is a function of the edge weight. It is
demonstrated that, if vertices with large degree tend to have large (small)
weights on their edges and if the transmission probability increases with the
edge weight, then it is easier (harder) for the epidemic to take off compared
to a randomized epidemic with the same degree and weight distribution. A recipe
for calculating the probability of a large outbreak in the epidemic and the
size of such an outbreak is also given. Finally, the model is fitted to three
empirical weighted networks of importance for the spread of contagious diseases
and it is shown that can be substantially over- or underestimated if the
correlation between degree and weight is not taken into account
Rainbow scattering in the gravitational field of a compact object
We study the elastic scattering of a planar wave in the curved spacetime of a compact object such as a
neutron star, via a heuristic model: a scalar field impinging upon a spherically symmetric uniform density
star of radius R and mass M. For R<rc, there is a divergence in the deflection function at the light-ring
radius rc ¼ 3GM=c2, which leads to spiral scattering (orbiting) and a backward glory; whereas for R>rc,
there instead arises a stationary point in the deflection function which creates a caustic and rainbow
scattering. As in nuclear rainbow scattering, there is an Airy-type oscillation on a Rutherford-like cross
section, followed by a shadow zone. We show that, for R ∼ 3.5GM=c2, the rainbow angle lies close to 180°,
and thus there arises enhanced backscattering and glory. We explore possible implications for gravitational
wave astronomy and dark matter models
Environmental Sensor Placement with Convolutional Gaussian Neural Processes
Environmental sensors are crucial for monitoring weather conditions and the
impacts of climate change. However, it is challenging to maximise measurement
informativeness and place sensors efficiently, particularly in remote regions
like Antarctica. Probabilistic machine learning models can evaluate placement
informativeness by predicting the uncertainty reduction provided by a new
sensor. Gaussian process (GP) models are widely used for this purpose, but they
struggle with capturing complex non-stationary behaviour and scaling to large
datasets. This paper proposes using a convolutional Gaussian neural process
(ConvGNP) to address these issues. A ConvGNP uses neural networks to
parameterise a joint Gaussian distribution at arbitrary target locations,
enabling flexibility and scalability. Using simulated surface air temperature
anomaly over Antarctica as ground truth, the ConvGNP learns spatial and
seasonal non-stationarities, outperforming a non-stationary GP baseline. In a
simulated sensor placement experiment, the ConvGNP better predicts the
performance boost obtained from new observations than GP baselines, leading to
more informative sensor placements. We contrast our approach with physics-based
sensor placement methods and propose future work towards an operational sensor
placement recommendation system. This system could help to realise
environmental digital twins that actively direct measurement sampling to
improve the digital representation of reality.Comment: In review for the Climate Informatics 2023 special issue of
Environmental Data Scienc
Friction and wear phenomena of vegetable oil based lubricants with additives at severe sliding wear conditions
The tribological responses of palm oil and soybean oil, combined with two commercial antiwear
additives (zinc dialkyl dithiophosphate and boron compound), were investigated at a lubricant
temperature of 100 °C and under severe contact conditions in a reciprocating sliding contact. The
friction coefficient of palm oil with zinc dialkyl dithiophosphate was closest to the commercial
mineral engine oil, with a 2% difference. The soybean oil with zinc dialkyl dithiophosphate
produced a 57% improvement in wear resistance compared to its pure oil state. The existence of
boron nitride in vegetable oils was only responsive in reduction of wear rather than friction. The
response of commercial antiwear additives with vegetable oils showed a potential for the future
improvement in the performance of vegetable oils
Seasonal Arctic sea ice forecasting with probabilistic deep learning.
Anthropogenic warming has led to an unprecedented year-round reduction in Arctic sea ice extent. This has far-reaching consequences for indigenous and local communities, polar ecosystems, and global climate, motivating the need for accurate seasonal sea ice forecasts. While physics-based dynamical models can successfully forecast sea ice concentration several weeks ahead, they struggle to outperform simple statistical benchmarks at longer lead times. We present a probabilistic, deep learning sea ice forecasting system, IceNet. The system has been trained on climate simulations and observational data to forecast the next 6 months of monthly-averaged sea ice concentration maps. We show that IceNet advances the range of accurate sea ice forecasts, outperforming a state-of-the-art dynamical model in seasonal forecasts of summer sea ice, particularly for extreme sea ice events. This step-change in sea ice forecasting ability brings us closer to conservation tools that mitigate risks associated with rapid sea ice loss
Three-dimensional general relativistic hydrodynamics II: long-term dynamics of single relativistic stars
This is the second in a series of papers on the construction and validation
of a three-dimensional code for the solution of the coupled system of the
Einstein equations and of the general relativistic hydrodynamic equations, and
on the application of this code to problems in general relativistic
astrophysics. In particular, we report on the accuracy of our code in the
long-term dynamical evolution of relativistic stars and on some new physics
results obtained in the process of code testing. The tests involve single
non-rotating stars in stable equilibrium, non-rotating stars undergoing radial
and quadrupolar oscillations, non-rotating stars on the unstable branch of the
equilibrium configurations migrating to the stable branch, non-rotating stars
undergoing gravitational collapse to a black hole, and rapidly rotating stars
in stable equilibrium and undergoing quasi-radial oscillations. The numerical
evolutions have been carried out in full general relativity using different
types of polytropic equations of state using either the rest-mass density only,
or the rest-mass density and the internal energy as independent variables. New
variants of the spacetime evolution and new high resolution shock capturing
(HRSC) treatments based on Riemann solvers and slope limiters have been
implemented and the results compared with those obtained from previous methods.
Finally, we have obtained the first eigenfrequencies of rotating stars in full
general relativity and rapid rotation. A long standing problem, such
frequencies have not been obtained by other methods. Overall, and to the best
of our knowledge, the results presented in this paper represent the most
accurate long-term three-dimensional evolutions of relativistic stars available
to date.Comment: 19 pages, 17 figure
Environmental sensor placement with convolutional Gaussian neural processes
Environmental sensors are crucial for monitoring weather conditions and the impacts of climate change. However, it is challenging to place sensors in a way that maximises the informativeness of their measurements, particularly in remote regions like Antarctica. Probabilistic machine learning models can suggest informative sensor placements by finding sites that maximally reduce prediction uncertainty. Gaussian process (GP) models are widely used for this purpose, but they struggle with capturing complex non-stationary behaviour and scaling to large datasets. This paper proposes using a convolutional Gaussian neural process (ConvGNP) to address these issues. A ConvGNP uses neural networks to parameterise a joint Gaussian distribution at arbitrary target locations, enabling flexibility and scalability. Using simulated surface air temperature anomaly over Antarctica as training data, the ConvGNP learns spatial and seasonal non-stationarities, outperforming a non-stationary GP baseline. In a simulated sensor placement experiment, the ConvGNP better predicts the performance boost obtained from new observations than GP baselines, leading to more informative sensor placements. We contrast our approach with physics-based sensor placement methods and propose future steps towards an operational sensor placement recommendation system. Our work could help to realise environmental digital twins that actively direct measurement sampling to improve the digital representation of reality
Architectures for Cognitive Radio Testbeds and Demonstrators – An Overview
Wireless communication standards are developed at an ever-increasing rate of pace, and significant amounts of effort is put into research for new communication methods and concepts. On the physical layer, such topics include MIMO, cooperative communication, and error control coding, whereas
research on the medium access layer includes link control, network topology, and cognitive radio. At the same time, implementations are moving from traditional fixed hardware architectures towards software, allowing more efficient development. Today, field-programmable gate arrays (FPGAs) and regular
desktop computers are fast enough to handle complete baseband processing chains, and there are several platforms, both open-source and commercial, providing such solutions. The aims of this paper is to give an overview of five of the available platforms and their characteristics, and compare the features and performance measures of the different systems
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