Atmospheric electricity ‘CTR Wilson’ meeting 2023

On 16 November 2023, UK and international researchers gathered at the University of Bath for the 11th CTR Wilson Meeting on atmospheric electricity. The CTR Wilson Institute for Atmospheric Electricity honours the Nobel Prize winner who also pioneered the global circuit concept and published extensively on atmospheric electrical topics. It is integrated with the Special Interest Group of the Royal Meteorological Society, to advance understanding of atmospheric electricity. (Atmospheric electricity concerns diverse and varied electrical phenomenon in the atmosphere, including lightning, atmospheric ionisation and the global electric circuit.) There were 39 registrants for this meeting, which was organised by Martin Fullekrug, Karen Aplin, Alec Bennett, Keri Nicoll and Giles Harrison. A photograph of meeting attendees is shown in Figure 1.

Figure 1

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Attendees of the 11th CTR Wilson Meeting on atmospheric electricity, held in the Department of Electronic and Electrical Engineering at the University of Bath. Photograph courtesy of Mike Protts.

The first session was chaired by Martin Fullekrug and opened by Tamás Bozóki (Institute of Earth Physics and Space Science, Sopron, Hungary), who presented on transient signals from Schumann Resonances (SRs, resonant radio waves in the earth-ionosphere cavity produced by lightning) and their applications in lightning location. He introduced the topic of SR transients, which exceed the ELF (extremely low frequency) radio noise generated by global lightning strikes. They indicate a large change in vertical electric charge moment in large lightning strikes, and there are several methods to geolocate their source strikes. Using a network of 30 sites across the globe, he presented several case studies, including for the Tonga volcano eruption of 2022.

An online presentation was given by Yoav Yair (Reichman University, Israel), who discussed lightning geolocation data showing that lightning superbolt incidence rates were increased over ship tracks in the Mediterranean. This behaviour was argued to result from aerosol pollution from ship exhausts, increasing storm activity and lightning. He was able to find evidence that lightning superbolts (defined by the highest percentiles in the energy distribution of detected strikes) were enhanced in the eastern Mediterranean, in areas with known heavy shipping traffic.

Next, Graeme Marlton (Met Office), presented material on the use of radio telescope VHF (very high frequency) lightning imaging data (at the LOFAR radio telescope, which is able to create images of lightning formation from radio waves). These data were used to study lightning events also detected by the new Met Office lightning detection system, LEELA. They analysed a particular flash in the latest version of the LEELA software and captured by LOFAR, allowing examination of different portions of the lightning event: the dart leader, an inter-cloud strike and a cloud-to-ground strike.

After a short break, the next session (chaired by Jon Wilkinson) was opened by a presentation from Abdullah Kahraman (Newcastle University), on a study of the expected changes in lightning incidence following climate predictions from the RCP8.5 greenhouse gas emission scenario. His study covered all of Europe and was based on a convection-permitting numerical scheme from the Met Office. The general trends suggested more convective instability but less cloud ice, which resulted in a complicated picture across different regions of Europe. He noted that the United Kingdom would see a summertime increase in lightning in RCP8.5, based on his results.

Masashi Kamogawa (University of Shizuoka, Japan) discussed the variation in a long-term set of thunder day data from Japan. He found a very weak correlation between the number of thunder days and the number of cloud-to-ground strikes. The number of thunder days, which had been reconstructed from old records, including diaries, showed similar values from about 1810 to 1950, before an increase in sea surface temperatures and other factors seemed to lead to an increase in thunder.

Next, Isabel Smith (University of Reading) presented her work on whether lightning intensity (i.e. discharge current) will be affected by climate change. In a storm system from 2012, she had found that lightning increased where increased CAPE occurred, although there was a non-linear relationship. She also reported that an increase in CAPE had occurred between 1940 and 2010, noting that it was therefore likely that thresholds for increased lightning currents would be achieved increasingly often.

After lunch, in the session chaired by Alec Bennett, Giles Harrison (University of Reading) spoke on datasets of historical PG (potential gradient) records which are newly available. PG measurements were made for most of the twentieth century at several UK Met Office observatories: Eskdalemuir, Lerwick and Kew. Techniques included a Kelvin water dropper equaliser, which uses a spray of water droplets to continuously measure the PG. Such data can be useful for understanding local pollution and global circuit effects; monthly datasets are now available online.1

Blair McGinness (University of Reading) shared his study on a point-discharge sensor for measurements of atmospheric electricity. He noted that, despite a long history of usage, their operation is not fully understood. He described his methods of calibrating measurements of point discharge current with simultaneous electric field measurements. In his data, there was a phase shift between the current and electric field, which he was able to align by using both static and dynamic components in the calibration. Although retrieving electric fields from point discharge sensor currents is non-trivial, the sensors are still useful for monitoring variability.

Next, Caleb Miller (University of Reading) shared a presentation on his work concerning the behaviour of PG in fog, especially as a method for fog prediction. He noted that there was general inconclusiveness in the previous work, which could be resolved by using a larger dataset of many years' data, such as the measurements from the Reading University Atmospheric Observatory. Using this data, he identified significant PG increases which occurred during fog events, remarking that, in many cases, this increase happened sufficiently early to provide useful information for fog prediction beyond that possible from visibility measurements alone.

The final session (chaired by Keri Nicoll) was opened by David Reid (University of Bristol), who talked on modelling the electric and magnetic fields likely to be produced by dust devils on Mars. With simplifications such as assuming circular motion of the dust particles, he was able to produce a simulation which used the radius profile, height profile, charge profile, rotation frequency and particle loading. His results were approximately consistent with (extremely rare) real-world dust devil measurements made in Arizona, USA.

This session included a presentation from Justin Tabbett (University of Bristol) on a balloon-borne radioactivity detector. He shared that this CsI(Tl) scintillator-based particle detector, which is a small instrument which could be deployed on a weather balloon, could be used to measure ionising particles which are important to the study of atmospheric electricity. He presented results from tests in different environments, including a passenger airline flight and a thermal environment chamber. A microcontroller system is used for formatting the data transmitted to the launch site.

The final presentation was online, from Ronald Holle and Daile Zhang, from Holle Meteorology & Photography and the University of Maryland, respectively, about a recently published book entitled Flashes of Brilliance: The Science and Wonder of Arizona Lightning. They described the chapters of this book, which includes information about the science of lightning, the history of lightning research in Arizona (including interviews with Philip Krider, Kenneth Cummins and Richard Orville), and beautiful photographs of lightning in Arizona. Books describing the progress of lightning research for a single state or country are rare.

The meeting concluded with a lively poster discussion. Posters were presented by Gayane Karapetyan (University of Magdeburg) on lightning climatology, Hripsime Mkrtchyan (University of Reading) on fair weather PG classification methods, Keri Nicoll (University of Reading) on an aircraft charge emitter system and Gregory Marsden (University of Bristol) on volcanic ash charging.