Earle Williams

Lightning activity is strongly concentrated in tropical continental “chimneys”, among which the African lightning chimney stands out as the most intense globally. However, until recently, the incomplete and spatially uneven detection efficiency of ground‐based lightning networks, together with the limited spatio‐temporal coverage of satellite observations, have constrained the quantitative characterization of African lightning activity, particularly on the daily timescales. Since July 2024, the Lightning Imager (LI) onboard the Meteosat Third Generation (MTG) satellite has provided the first continuous, high‐temporal‐resolution observations of lightning activity over Africa and Europe from geostationary orbit. In this paper, we examine, based on the first year of MTG LI observations, how consistently daily peak activity in the African lightning chimney is represented in different observational data sets. For this purpose, we use MTG LI group and flash data, together with World Wide Lightning Location Network (WWLLN) stroke observations and Schumann resonance (SR) measurements, and focus on two key parameters: the daily peak activity and its timing. We find that daily peak activity derived from MTG flashes shows a strong positive correlation with SR intensity ( r = 0.749), while WWLLN stroke observations show much weaker correspondence. The timing of the daily peak is broadly consistent across the data sets, with the best agreement observed between MTG flashes and SR intensity. For all data sets, daily peak activity is strongly correlated with total daily activity. The strong correspondence between MTG flashes and SR intensity supports the use of both data sets in characterizing the daily activity of the African lightning chimney.

Past field experiments have demonstrated multiple cloud seeding techniques for modifying the electrical characteristics of developing thunderstorms with the purpose of reducing or eliminating lightning strikes. One motivation for these weather modification research programs was the prospect of preventing wildfires in inaccessible locations or particularly hazardous conditions by temporarily suppressing a prolific ignition source: lightning. It has been nearly 50 years since the last large-scale field effort in lightning suppression, and the ensuing five decades of technological innovation hold the promise of supporting this novel hazard mitigation approach. This study outlines observational and forecasting requirements, as well as an associated remote sensing architecture, that would support an operational wildfire prevention program based on lightning suppression by chaff seeding clouds. Two capabilities enabled by remote sensing observations are highlighted: (1) identifying potential regions of extreme wildfire behavior based on wildland fuels, topography, and weather, and (2) predicting regions where dry lightning strikes are probable. In combination, the proposed nowcasting system would predict the most likely areas of lightning-initiated wildfire danger. Dependent on land management strategies, surrounding infrastructure, firefighting capacity, and risk to human wellbeing, this information could be used to deploy lightning suppression technology to reduce or temporarily prevent wildfire ignitions in these high-risk situations.

We propose a plausible microscopic mechanism for charge transfer during rebounding collisions between ice crystals and graupel within thunderstorms. The key concept is that charge is transferred through the diffusional transport of H ⁺ ions across the ice‐ice contact junction during collisions. The transferred charges are stabilized by the trapping of H ⁺ ions at L ‐orientational defects in ice particles. The distinct formation pathways of ice crystals and graupel within a cloud lead to asymmetries in their respective H ⁺ ion and L ‐defect concentrations. Graupel, formed by the accretion of supercooled droplets containing dissolved atmospheric CO ₂ , has a higher H ⁺ concentration than ice crystals, which grow exclusively from vapor. Additionally, warming of graupel due to riming increases its L ‐defect concentration relative to that of crystals, according to microscopic analyses of ice growth processes. These differences in H ⁺ and L ‐defect concentrations between ice crystals and graupel drive charges in opposite directions, with the net charge transfer determined by the interplay of these two factors. A theoretical model based on this mechanism predicts that positive charge on graupel increases with rising liquid water content (LWC), while negative charge becomes stronger in low LWC regions at colder temperatures. These trends align with common features observed in most laboratory studies and with the formation of two polarities of storm dipoles under different LWC regimes. We show that the magnitude of charge separation during crystal‐graupel collisions, calculated based on this model, broadly agrees with the values found in laboratory studies.

The radar “first echo” appears in a fresh growing cloud when hydrometeors grow to large enough sizes to be detected by radar. Using 26‐year observations by the Ku band radars onboard the Global Precipitation Mission and the Tropical Rainfall Measurement Mission satellites, isolated pixels with detectable radar echoes at altitude and without precipitation at the surface are identified as candidate “first echoes.” In general, the appearance of the “first echoes” show three altitude‐related modes in the tropics. The shallow mode at 1.5–2 km is mainly from the growth of warm rain from rapid coalescence. The mid mode between 0°C and −7°C is found mainly over land and coastal regions, that can be explained by the enhanced radar reflectivity by ice particles through heterogenous ice nucleation at relative warmer temperatures, followed by active riming. The deep mode between −12°C and −25°C is frequently found over both land and ocean. Around 12% of isolated echoes, mainly over desert regions, could be the remnant of dissipating precipitation, like virga. Cloud base height and total column water vapor derived from ERA5 reanalysis are found positively related to the “first echo” height. The mid mode first echoes are more likely found in a moist (ice‐supersaturated) environment, a favorable condition for crystal growth following heterogeneous ice nucleation. While the shallow mode over ocean tends to have more sea salt aerosols, large smoke and sulfate aerosol concentrations are often associated with the mid mode, and large dust aerosol concentrations are often involved with the deep mode.

In lightning research, there is a growing interest in measuring the extremely low frequency (ELF, 3 Hz–3 kHz) electromagnetic (EM) radiation of lightning, as this frequency band can be used to infer various characteristics of lightning discharges that are currently not available from state‐of‐the‐art lightning detection networks. One of these characteristics is the presence of a continuing current (CC), which can last for hundreds of milliseconds and therefore poses an increased risk of physical lightning damage. In this paper, we investigate the modeling capability of the global EM resonance field excited by lightning with a CC using a modified version of a well‐known analytical model describing Schumann resonances (SRs) and a full FDTD model. Since analytical models are much faster and require significantly less memory than full numerical models, they are widely used to interpret ELF data. On the other hand, the flexibility of a full numerical model allows the simulation of model configurations that cannot be described by analytical models. To use the two models confidently, it is important to check their consistency for similar configurations. Here, we demonstrate that, for a uniform Earth‐ionosphere cavity, the theoretical ELF spectra provided by the analytical and full numerical models show good agreement ∼7(±5) % for both the impulse‐like (describing SRs) and exponentially decaying (describing the presence of a CC) current sources. Our results confirm that the analytical model is well suited to interpret ELF measurements for the purpose of studying global lightning activity or individual lightning discharges.

B.F.J. Schonland, advised and encouraged by C.T.R. Wilson, made two unsuccessful searches for runaway electrons from thunderstorms in the 1930s. These findings stand in marked contrast with research results over the last decade and ironically set this field of research back many decades. Schonland's lack of success is traced to gamma ray attenuation in the atmosphere above Johannesburg (1,780 m MSL) and to his restriction to nine thunderstorms.

The effect of anthropogenic aerosols on lightning is one of the least understood aspects of human‐induced climate change. Global aerosol clearly diminished during the COVID pandemic by 7.6%. A pronounced decrease in global lightning activity in the range 3.0%–5.8% is identified from various detection systems during this natural experiment. The Maritime Continent lightning chimney shows the largest reduction of 7.0% in aerosol accompanied by a lightning drop of 15%. The COVID period in 2020 also experiences a transition from pre‐COVID El Niño to a strong and sustained La Niña. Compensation for ENSO forcing of lightning activity is implemented to disclose the distinct responses of three global lightning chimneys to competing thermodynamic and aerosol effects. Our observational findings indicate a marked influence of aerosol on a global scale by virtue of the extraordinary COVID‐induced aerosol alteration.

Both simulations and measurements indicate the existence of stable annual behavior of the global electric circuit (GEC) intensity, but the exact pattern of variation is very difficult to reliably determine. Here we present further analysis of this problem using the results of ionospheric potential (IP) simulations with two different models of atmospheric dynamics. From a theoretical perspective, the annual variation of the GEC is eventually determined by seasonal changes in the distribution of convection, associated with the annual cycle of insolation. Simulations clearly demonstrate that the contribution to the IP from the vicinity of the equator has two maxima following the equinoxes, whereas the contribution of nonequatorial latitudes in each hemisphere has one maximum during the local summer. The resulting seasonal variation of the GEC, being the sum of three clear patterns offsetting each other, is rather subtle, and its prediction in simulations may vary depending on the model and IP parameterization. It is likely that the actual seasonal variation of the diurnal mean GEC intensity has one pronounced maximum during the Northern Hemisphere summer and one pronounced minimum during the Northern Hemisphere winter, in agreement with potential gradient measurements in Antarctica and in agreement with the larger portion of the Northern Hemisphere occupied by land. Physical parameters characterizing convection do not provide a very good measure of contributions to the IP, notably overestimating the role of equatorial latitudes; by including surface air temperature in the IP parameterization, one can achieve a better agreement between simulations and observations.

In lightning research, there is a growing interest in measuring the extremely low frequency (ELF, 3 Hz - 3 kHz) electromagnetic (EM) radiation of lightning, as this frequency band can be used to infer various characteristics of lightning discharges that are currently not available from state-of-the-art lightning detection networks. One of these characteristics is the presence of a continuing current (CC), which can last for hundreds of milliseconds and therefore poses an increased risk of physical lightning damage. In this contribution, we investigate the modelling capability of the global EM resonance field excited by lightning with a CC using a modified version of a well-known analytical model describing Schumann resonances (SRs) and a full FDTD model. Since analytical models are much faster and require significantly less memory than full numerical models, they are widely used to interpret ELF data. However, analytical models need to be validated as they rely on considerable simplifications. Here we demonstrate that the theoretical ELF spectra provided by the analytical model show good agreement with the FDTD spectra for both the impulse-like (describing SRs) and exponentially decaying (describing the presence of a CC) current sources. Our results confirm that the analytical model is well suited to interpret ELF measurements for the purpose of studying global lightning activity or individual lightning discharges.