Hunt H.G.P., K.J. Nixona, I.R. Jandrella, W. Schulz:
Can we model the statistical distribution of lightning location system errors better?

Electric Power Systems Research (EPSR), Volume 178, 106042, 2020

Lightning location systems geolocate lightning strokes. Given assumptions made in the geolocation models, errors in the reported locations can occur. Modelling these errors as a bivariate Gaussian distribution of historic stroke detections has found success in the form of confidence ellipses. However, the presence of outliers - strokes with large location errors - indicate that there is a better model for these errors. The Students’ t-distribution is a “heavier” tailed distribution. This paper investigates whether the bivariate Students’ t-distribution is a better model for such errors. A methodology for modelling and evaluating the distribution of location errors using maximum likelihood estimation, expectation-maximization and a Mahalanobis distance quality-of-fit test is described. This method is applied to stroke reports from the South African Lightning Detection Network and the Austrian Lightning Detection and Information System time-correlated with photographed lightning events to the Brixton Tower, South Africa and current measurements to the Gaisberg Tower, Austria respectively. In both cases, we find outliers in the distribution of location errors - even as the performance of the networks increase. Using the Mahalanobis test, we find the bivariate Students’ t-distribution to be a better statistical model for both the South African and the Austrian events.

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Schwalt L., St. Pack; W. Schulz:
Ground Truth Data of Atmospheric Discharges in Correlation with LLS Detections

Electric Power Systems Research (EPSR), Manuscript Number: EPSR-D-19-00476, 2019

This paper presents recent ground truth data analyses in the Austrian Alps run by Graz University of Technology in cooperation with Austrian Lightning Detection and Information System (ALDIS). The project "Lightning Observation in the Alps - LiOn" was established in 2017 at the Institute of High Voltage Engineering and System Performance at Graz University of Technology. Atmospheric discharges are observed at different measurement locations in Austria by using a Video and Field Recording System (VFRS). This system consists of a high speed video camera (2000 frames per second) and a flat plate antenna to measure the electric field. The recorded data can be used to better understand the atmospheric discharges, especially in the alpine area. For the present analysis a data set of the measurement periods 2015, 2017 and 2018 was used for validation of the Lightning Location System (LLS) data of ALDIS. In total 463 negative cloud-to-ground (CG) flashes and 1527 negative CG strokes were recorded in Austria during 51 days. The LLS performance parameter, location accuracy and detection efficiency and further the flash multiplicity and peak currents have been analyzed for these three years. Values for location accuracy are in the range of 90 m to 130 m. Flash detection efficiencies in the range from 96.08 % to 98.62 % and for the stroke detection efficiencies in the range from 76.36 %to 85.60 % have been determined. Mean multiplicity values determined with the VFRS data are comparable to the results of previous analyses in the Austrian Alps. The median values of negative stroke peak currents are around 10 % to 30 % lower for 2015, 2017 and 2018, than for detections of older VFRS measurements in the Alps.

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Paul C., F. H. Heidler , W. Schulz:
Performance of the European Lightning Detection Network EUCLID in Case of Various Types of Current Pulses From Upward Lightning Measured at the Peissenberg Tower

IEEE Transactions on Electromagnetic Compatibility, DOI: 10.1109/TEMC.2019.2891898, 2019

In this paper, we present current pulses from upward lightning, which have been measured since 2011 at the new top structure of the Peissenberg Tower, Germany. The study comprises 38 negative and two positive flashes, which contained 199 current pulses. About 49 of them were return stroke current pulses, 133 of them were current pulses, which superimposed the initial continuous current (ICC-pulses), and 17 of them were M-component current pulses, which superimposed the continuing current of a preceding return stroke. The current pulses are used to evaluate the performance of the European lightning location system EUCLID. Fifty one (25.6%) out of the 199 current pulses were detected by EUCLID, 40 (81.6%) return stroke current pulses, and 11 (8.5%) ICC-pulses or M-component current pulses. The peak currents ranged from 0.1 to 40.8 kA. Two groups of current pulses could be identified. The first group is related to branches of nearby downward lightning which got in contact with the tower. Therefore, EUCLID reported much higher peak currents (more than 100%) compared to the peak currents measured at the Peissenberg Tower. The first group comprises the total of nine current pulses (six ICC-pulses, three return stroke current pulses). The second group of current pulses is related to upward lightning initiated from the top of the tower. The peak current inferred from EUCLID deviates much less from peak current measured at the Peissenberg Tower. The peak current was overestimated by about 20% by EUCLID. The second group comprises the total of 42 current pulses (4 ICC-pulses, 1 M-component current pulses, and 37 return stroke current pulses). The peak currents ranged from 3.1 to 40.8 kA, the geometric mean (GM) was 9.4 kA. About 30% of these events were misclassified as intra-cloud pulses by EUCLID. The GM of the location error was 161 m for all events, and 132 m considering only the return stroke current pulses.

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Poelman D. R., W. Schulz:
Comparing lightning observations of the ground-based European lightning location system EUCLID and the space-based Lightning Imaging Sensor (LIS) on the International Space Station (ISS)

Atmospheric Measurement Techniques (AMS), 13, 2965–2977, 2020

The Lightning Imaging Sensor (LIS) on the International Space Station (ISS), hereafter referred to as ISS-LIS, detects lightning from space by capturing the optical scattered light emitted from the top of the clouds. The ground-based European Cooperation for Lightning Detection (EUCLID) makes use of the low-frequency electromagnetic signals generated by lightning discharges to locate them accordingly. The objective of this work is to quantify the similarities and contrasts between these two distinct lightning detection technologies by comparing the EUCLID cloud-to-ground strokes and intracloud pulses to the ISS-LIS groups in addition to the correlation at the flash level. The analysis is based on the observations made between 1 March 2017 and 31 March 2019 within the EUCLID network and limited to 54∘ north. A Bayesian approach is adopted to determine the relative and absolute detection efficiencies (DEs) of each system. It is found that the EUCLID relative and absolute flash DE improves by approximately 10 % towards the center of the EUCLID network up to a value of 56.3 % and 69.0 %, respectively, compared to the averaged value over the full domain, inherent to the network geometry and sensor technology. In contrast, the relative and absolute ISS-LIS flash DE over the full domain is 48.4 % and 71.3 %, respectively, and is somewhat higher than the values obtained in the center of the EUCLID network. The behavior of the relative DE of each system in terms of the flash characteristics of the other reveals that the greater the value, the more likely the other system will detect the flash. For instance, when the ISS-LIS flash duration is smaller than or equal to 200 ms, the EUCLID relative flash DE drops below 50 %, whereas it increases up to 80 % for ISS-LIS flashes with a duration longer than 750 ms. Finally, the distribution of the diurnal DE indicates a higher DE for the ISS-LIS and a lower DE for EUCLID at night.

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Watanabe N., A. Nag, G. Diendorfer, H. Pichler, W. Schulz, H. Rassoul:
Characteristics of Currents in Upward Flashes Transferring Negative Charge to Ground

AGU Fall Meeting, 2020

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