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James W. C. White

Publications and source records attributed to James W. C. White.

4 recordsLinked to original sources
arXiv2020

Detection of Local Mixing in Time-Series Data Using Permutation Entropy

While it is tempting in experimental practice to seek as high a data rate as possible, oversampling can become an issue if one takes measurements too densely. These effects can take many forms, some of which are easy to detect: e.g., when the data sequence contains multiple copies of the same measured value. In other situations, as when there is mixing$-$in the measurement apparatus and/or the system itself$-$oversampling effects can be harder to detect. We propose a novel, model-free technique to detect local mixing in time series using an information-theoretic technique called permutation entropy. By varying the temporal resolution of the calculation and analyzing the patterns in the results, we can determine whether the data are mixed locally, and on what scale. This can be used by practitioners to choose appropriate lower bounds on scales at which to measure or report data. After validating this technique on several synthetic examples, we demonstrate its effectiveness on data from a chemistry experiment, methane records from Mauna Loa, and an Antarctic ice core.

cs.IT↗
arXiv2018

Anomaly Detection in Paleoclimate Records using Permutation Entropy

Permutation entropy techniques can be useful in identifying anomalies in paleoclimate data records, including noise, outliers, and post-processing issues. We demonstrate this using weighted and unweighted permutation entropy of water-isotope records in a deep polar ice core. In one region of these isotope records, our previous calculations revealed an abrupt change in the complexity of the traces: specifically, in the amount of new information that appeared at every time step. We conjectured that this effect was due to noise introduced by an older laboratory instrument. In this paper, we validate that conjecture by re-analyzing a section of the ice core using a more-advanced version of the laboratory instrument. The anomalous noise levels are absent from the permutation entropy traces of the new data. In other sections of the core, we show that permutation entropy techniques can be used to identify anomalies in the raw data that are not associated with climatic or glaciological processes, but rather effects occurring during field work, laboratory analysis, or data post-processing. These examples make it clear that permutation entropy is a useful forensic tool for identifying sections of data that require targeted re-analysis---and can even be useful in guiding that analysis.

physics.data-an↗
arXiv2018

Climate entropy production recorded in a deep Antarctic ice core

Paleoclimate records are extremely rich sources of information about the past history of the Earth system. Information theory, the branch of mathematics capable of quantifying the degree to which the present is informed by the past, provides a new means for studying these records. Here, we demonstrate that estimates of the Shannon entropy rate of the water-isotope data from the West Antarctica Ice Sheet (WAIS) Divide ice core, calculated using weighted permutation entropy (WPE), can bring out valuable new information from this record. We find that WPE correlates with accumulation, reveals possible signatures of geothermal heating at the base of the core, and clearly brings out laboratory and data-processing effects that are difficult to see in the raw data. For example, the signatures of Dansgaard-Oeschger events in the information record are small, suggesting that these abrupt warming events may not represent significant changes in the climate system dynamics. While the potential power of information theory in paleoclimatology problems is significant, the associated methods require careful handling and well-dated, high-resolution data. The WAIS Divide ice core is the first such record that can support this kind of analysis. As more high-resolution records become available, information theory will likely become a common forensic tool in climate science.

physics.geo-ph↗