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The influence of tropical cyclones on the thunderstorm activity on the Kamchatka peninsula for winter thunderstorms in the 2008–2018 period is studied. The temporal variations of the quasi-static electric field and meteorological values at the Paratunka Observatory of the Institute of Cosmophysical Research and Radio Wave Propagation (IKIR) of the Far Eastern Branch of the Russian Academy of Sciences (φ = 52.97° N; λ = 158.25° E) and data on the solar, seismic, and cyclonic activities available on the INTERNET are used as an indicator of thunderstorm activity. It is shown that, in addition to from solar flares, which are accompanied by increased radiation in the visible and infrared spectra, the infrared radiation from a series of closely located epicenters of average-intensity earthquakes, as well as distant tropical cyclones in the Pacific Ocean, can be a source of powerful flows of warm and humid air for the formation of thunderstorm activity.

The article presents the results of work on the study of synoptic conditions for the formation of a dangerous weather phenomenon for the territory of Western Siberia – a thundersnow. The material for the study was the archive of aerosynoptic material from the Khanty-Mansiysk Air Meteorological Center.

In 2015, the Gamma-Ray Observation of Winter Thunderstorms (GROWTH) collaboration launched a mapping observation campaign for high-energy atmospheric phenomena related to thunderstorms and lightning discharges. This campaign has developed a detection network of gamma rays with up to 10 radiation monitors installed in the cities of Kanazawa and Komatsu, Ishikawa Prefecture, Japan, where low-charge-center winter thunderstorms frequently occur. During four winter seasons from October 2016 to April 2020, a total of 70 gamma-ray glows, i.e., minute-lasting bursts of gamma rays originating from thunderclouds, were detected. Their average duration is 58.9 s. Among the detected events, 77% were observed at night. The gamma-ray glows can be classified into temporally symmetric, temporally asymmetric, and lightning-terminated types based on their count-rate histories. An averaged energy spectrum of the gamma-ray glows is well fitted with a power-law function with an exponential cutoff, whose photon index, cutoff energy, and flux are $0.613\ifmmode\pm\else\textpm\fi{}0.009$ MeV, $4.68\ifmmode\pm\else\textpm\fi{}0.04$ MeV, and $(1.013\ifmmode\pm\else\textpm\fi{}0.003)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ erg ${\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ (0.2--20.0 MeV), respectively. The present paper provides a catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas.

It has been theorized that thundersnow (TSSN) occurs in conjunction with heavy snowfall rates and in geographical regions where heavy-banded snow occurs more frequently. This study aims to objectively and quantitatively identify characteristics associated with TSSN to improve the situational awareness of heavy snowfall and associated hazards. The Geostationary Lightning Mapper (GLM), National Environmental Satellite Data and Information Services (NESDIS) merged Snowfall Rate (mSFR) product, and surface observations were utilized to characterize snowfall accumulation, snow-to-liquid ratio (SLR) values, and radar characteristics of heavy snowfall events from a GLM perspective. When at least 2 in. of snowfall accumulation occurred, areas with TSSN flashes identified by the thundersnow detection algorithm (TDA) were likely to receive, on average, a total of 24.5 cm (9.6 in.) of snowfall. TSSN was more likely to occur in snowfall rates less than 2.54 cm h−1 (1 in. h−1) and be associated with snow-to-liquid ratio (SLR) values between 8:1 and 10:1. It was determined that TSSN flashes observed by GLM occurred in isothermal reflectivity values less than 30 dBZ and average spatial offsets of 131 km between the lightning flash location and the heaviest snowfall rates were observed. GLM flashes in proximity of National Lightning Detection Network cloud-to-ground flashes and tall structures were found to be statistically different (p < 0.05) regarding snowfall rates, SLR values, and various Multi-Radar Multi-Sensor variables compared to other TSSN flashes. It was inferred that tower TSSN flashes, on median, were more likely to initiate within light-to-moderately rimed snowfall. Last, a heavy snowfall event was analyzed to demonstrate the capability of these products in identifying storm characteristics associated with TSSN.

Winter thunderstorms in Kamchatka are a rare meteorological phenomenon. Temporal variations of the quasi-static electric field and meteorological values at the Paratunka observatory of the Institute of Cosmophysical Research and Radio Wave Propagation, Far East Branch, Russian Academy of Sciences (φ = 52.97° N; λ = 158.25° E), as well as data on solar, seismic and cyclonic activities available on INTERNET, are used to study the nature of this phenomenon. It is shown that powerful solar flares accompanied by increased radiation in the visible and infrared spectra, as well as the infrared radiation from the Earth that enters the atmosphere before powerful earthquakes with a magnitude of M > 8 may serve as an additional heat source in the surface atmosphere of Kamchatkan for the formation of thunderstorm activity. The contribution of tropical cyclones to these processes with weak seismic activity is not clearly defined and requires further detailed study.

Two types of high-energy events have been detected from thunderstorms. One is “terrestrial gamma-ray flashes” (TGFs), sub-millisecond emissions coinciding with lightning discharges. The other is minute-lasting “gamma-ray glows”. Although both phenomena are thought to originate from relativistic runaway electron avalanches in strong electric fields, the connection between them is not well understood. Here we report unequivocal simultaneous detection of a gamma-ray glow termination and a downward TGF, observed from the ground. During a winter thunderstorm in Japan on 9 January 2018, our detectors caught a gamma-ray glow, which moved for ~100 s with ambient wind, and then abruptly ceased with a lightning discharge. Simultaneously, the detectors observed photonuclear reactions triggered by a downward TGF, whose radio pulse was located within ~1 km from where the glow ceased. It is suggested that the highly-electrified region producing the glow was related to the initiation of the downward TGF., 金沢市の高校で捉えた放射線バーストで雷発生の瞬間に迫る. 京都大学プレスリリース. 2019-06-26.

We have been observing LF electromagnetic waves associated with lightning around Toyama Bay in winter thunderstorm seasons. From received LF signals at 7 stations, their sources are located in 3D using time of arrival method. Lightning flashes with the length of over 100km are detected in the observation results. Unlike general lighting discharges in winter, many return strokes are recorded along LF electromagnetic wave sources.

International audience; During three winter seasons from November 2016 to March 2019, 11 gamma ray glows were detected at a single observation site of our ground based gamma ray monitoring network in Kanazawa, Japan. These events are analyzed with observations of an X band radar network, a ceilometer, a disdrometer, and a weather monitor. All the detected glows were connected to convective high reflectivity regions of more than 35 dBZ, developed up to an altitude of >2 km. They were also accompanied by heavy precipitation of graupels. Therefore, graupels in the lower layer of thunderclouds that correspond to high reflectivity regions can form strong electric fields producing gamma ray glows. Also, these events are compared with a limited sample of nondetection cases, but no significant differences in meteorological conditions were found between detection and nondetection cases in the present study.

The distribution of cloud-to-ground lightning energies is well established, and its most extreme values appear only in extremely rare flashes (< 0.0001%), defined as lightning "super-bolts". There are varying definitions of the specific energy values of super-bolts, depending on the detector or mode of observation. When using optical energy as viewed from a satellite, one usually refers to the brightest flashes (103 times brighter than average), while when relating to the electromagnetic radiation received by lightning detection networks, the definition revolves around the strongest signals in the VLF or ELF range, or the largest peak-current or charge-moment-change (CMC) inferred from the signal. These are all different metrics for evaluating the lightning's intensity, and they are inter-related and exhibit mutual dependence (e.g. extreme values of peak current positively correlate with extreme VLF amplitudes). The global distribution of these extremely powerful lightning is remarkably different from that of normal lightning, which are concentrated in the 3 convective "chimneys" of tropical Africa, South-America and the maritime continent in South-east Asia. Superbolts are found mostly over the oceans and near coastlines, such as Sea of Japan, the North Sea and in the Andes mountains (Holzworth et al., 2019). They are also discovered in maritime winter storms over the Mediterranean Sea which is one of the most prolific regions, especially in the months November-January. We present the climatology of east-Mediterranean super-bolts (peak current > 200 kA), and compare data obtained by various lightning detection networks (ENTLN, WWLLN and ILDN). Some storms exhibit a larger percentage of superbolts compared with the global average, up to 0.65% of total flashes. While the physical mechanisms producing these powerful flashes remains unknown, we suggest that such flashes are more common when large amounts of desert dust aerosols, coming from the Sahara Desert, are ingested into maritime winter storms and contribute to convective invigoration, enhanced freezing and efficient charge separation. Initial modelling results will be discussed.

Since 2006, we have been performing the Gamma-Ray Observation of Winter Thunderclouds (GROWTH) experiment, an on-ground observation program for high-energy phenomena during winter thunderstorms in Japan. In 2015, we launched a mapping observation program; observation network with compact gamma-ray detectors has been constructed in coastal areas of the Japan Sea. This chapter summarizes the special characteristics of winter thunderstorms in Japan and the detection network of the GROWTH experiment.

A rare winter thunderstorm event near Hong Kong is documented in this paper. The thunderstorm occurrence was confirmed by the observation of one of the authors and lightning location information systems (both regional and global). It developed over the frontal cloud band associated with the winter monsoon. Observing the upper air measurements, however, the thermodynamic conditions did not seem favorable to the occurrence of the thunderstorm. The convective available potential energy could not be determined in the cool and dry lower troposphere. There was only a moister layer in the middle troposphere. The weather forecaster did not anticipate the occurrence of thunder and lightning. However, some features could have changed dynamically, which suggested the possibility that a thunderstorm would take place. For instance, waves could be identified in the middle troposphere and a broad troughing flow could be analyzed in the lower troposphere. The integrated wave vapor was also of a rather high level. It is hoped that this paper can serve as a useful reference for forecasters in assessing the possibility of thunderstorms occurring during winter over subtropical latitudes in association with frontal cloud bands.

Mediterranean thunderstorms climatology is analyzed for a 10-year period from 2005 to 2014. For this analysis, we applied a clustering method to the cloud-to-ground lightning stroke dataset, derived from the ZEUS detection system, operated by the National Observatory of Athens. Results showed that the annual days of thunderstorm occurrence are dependent on the diurnal cycle of insolation and on the underlying topographic features. Therefore, the majority of thunderstorms occur over land in spring and summer, while winter and autumn thunderstorms tend to occur over the sea. On average, winter thunderstorms are less frequent and of lower intensity than the ones of summer, but last longer, tend to attain larger sizes and are associated with significantly higher velocity. Regardless the season, all thunderstorms tend to propagate towards the east, while their speed is higher in winter and autumn. In particular, for thunderstorms occurring over land, we found that the higher the duration of the lifetime of a thunderstorm, the earlier in the day it is expected to occur. Mediterranean thunderstorms size and duration have been found to be correlated, while a linear relation was determined between thunderstorms intensity and rainfall over the Mediterranean region, where the rain yield was estimated to be about 1.8 108 kg of rainfall per CG lightning.

This study examined 34 lightning flashes within four separate thundersnow events derived from lightning mapping arrays (LMAs) in northern Alabama, central Oklahoma, and Washington DC. The goals were to characterize the in-cloud component of each lightning flash, as well as the correspondence between the LMA observations and lightning data taken from national lightning networks like the National Lightning Detection Network (NLDN). Individual flashes were examined in detail to highlight several observations within the dataset. The study results demonstrated that the structures of these flashes were primarily normal polarity. The mean area encompassed by this set of flashes is 375 km(2), with a maximum flash extent of 2300 km(2), a minimum of 3 km(2), and a median of 128 km(2). An average of 2.29 NLDN flashes were recorded per LMA-derived lightning flash. A maximum of 11 NLDN flashes were recorded in association with a single LMA-derived flash on 10 January 2011. Additionally, seven of the 34 flashes in the study contain zero NLDN identified flashes. Eleven of the 34 flashes initiated from tall human-made objects (e.g., communication towers). In at least six lightning flashes, the NLDN detected a return stroke from the cloud back to the tower and not the initial upward leader. This study also discusses lightning’s interaction with the human built environment and provides an example of lightning within heavy snowfall observed by GOES-16’s Geostationary Lightning Mapper.

Lightning data as observed by the European Cooperation for Lightning Detection network EUCLID are used in combination with radar data to retrieve the temporal and spatial behaviour of lightning outliers, i.e. discharges located on a wrong place, over a 5-year period from 2011 to 2016. Cloud-to-ground stroke and intracloud pulse data are superimposed on corresponding 5-min radar precipitation fields in two topographically different areas, being Belgium and Austria, in order to extract lightning outliers based on the distance between each lightning event and the nearest precipitation. It is shown that the percentage of outliers is sensitive to changes in the network and to the location algorithm itself. The total percentage of outliers for both regions varies over the years between 0.8 % and 1.7 % for a distance to the nearest precipitation of 2 km, with an average of approximately 1.2 % in Belgium and Austria. Outside the European summer thunderstorm season the percentage of outliers tends to increase somewhat and could result from the fact that more sensor upgrades occur during winter or that precipitation of winter thunderstorms is underestimated compared to the vertically extended summer storms. The majority of all the outliers are low peak current events with absolute values falling between 0 to 10 kA. More specifically, positive cloud-to-ground strokes are more likely to be classified as outliers compared to all other type of discharges. Furthermore, it turns out that the number of sensors participating in locating a lightning discharge is different for outliers versus correctly located events, with outliers having the least amount of sensors participating. In addition, it is shown that in most cases the semi-major axis assigned to a lightning discharge as a confidence indicator in the location accuracy is smaller for correctly located events compared to the semi-major axis of outliers.

The diurnal variations in electrical (quasistatic electric field and electrical conductivity) and meteorological (temperature, pressure, relative humidity of the atmosphere, and wind speed) parameters, measured simultaneously before strong earthquakes in Kamchatka region (November 15, 2006, М = 8.3; January 13, 2007, М = 8.1; January 30, 2016, М = 7.2), are studied for the first time in detail. It is found that a successively anomalous increase in temperature, despite the negative regular trend in these winter months, was observed in the period of six–seven days before the occurrences of earthquakes. An anomalous temperature increase led to the formation of “winter thunderstorm” conditions in the near-surface atmosphere of Kamchatka region, which was manifested in the appearance of an anomalous, type 2 electrical signal, the amplification of and intensive variations in electrical conductivity, heavy precipitation (snow showers), high relative humidity of air, storm winds, and pressure changes. With the weak flow of natural heat radiation in this season, the observed dynamics of electric and meteorological processes can likely be explained by the appearance of an additional heat source of seismic nature.

This paper presents lightning mapping array (LMA) observations of corona discharges from a windmill and its lightning protection tower in winter thunderstorms in Japan. Corona discharges from the windmill, called windmill coronas, and those from the tower, called tower coronas, are distinctly different. Windmill coronas occur with periodic bursts, generally radiate larger power, and possibly develop to higher altitudes than tower coronas do. A strong negative electric field is necessary for the frequent production of tower coronas but is not apparently related with windmill coronas. These differences are due to the periodic rotation of the windmill and the moving blades which can escape space charges produced by corona discharges and sustain a large local electric field. The production period of windmill coronas is related with the rotation period of the windmill. Surprisingly, for one rotation of the windmill, only two out of the three blades produce detectable discharges and source powers of discharges from these two blades are different. The reason for this phenomenon is still unclear. For tower coronas, the source rate can get very high only when there is a strong negative electric field, and the source power can get very high only when the source rate is very low. The relationship between corona discharges and lightning flashes is investigated. There is no direct evidence that corona discharges can increase the chance of upward leader initiation, but nearby lightning flashes can increase the source rate of corona discharges right after the flashes. The peak of the source height distribution of corona discharges is about 100 m higher than the top of the windmill and the top of the tower. Possible reasons for this result are discussed.

Superbolts are defined as lightning flashes that are a thousand times stronger than normal ones, and their occurrence is estimated to be less than 0.001% of total number of lightning on earth. The global distribution of these extremely powerful lightning flashes is remarkably different than that of regular lightning, which are concentrated in the well-known convective "chimneys" in tropical Africa, South-America and the maritime continent in South-East Asia. The physical mechanisms producing these powerful flashes remain unknown, and the puzzle is exacerbated by the fact that they are discovered mostly over oceans, in maritime winter storms.The Mediterranean Sea is one of the most prolific regions where super-bolts occur, especially in the months November-January (Holzworth et al., 2019). We analyzed 8 years of lightning data obtained from the Israeli Lightning Detection Network (ILDN), defining a 200kA peak current threshold for superbolts. We mapped the spatial and temporal distribution of superbolts and their monthly frequency in winter season thunderstorms (DJF) in the eastern Mediterranean, and identified the meteorological and microphysical circumstances in such storms.Our working hypothesis is that large amounts of desert dust aerosols, coming from the Sahara Desert, are ingested into maritime winter storms over the eastern Mediterranean. The large dust contributes to convective invigoration, enhanced freezing and efficient charge separation, implying that superbolts are more likely to occur in the presence of large dust. We will present the results of simulation conducted using the WRF-ELEC numerical model, and WRF with spectral bin microphysics coupled with Lynn et al.'s (2012) Dynamic Lightning Scheme (DLS) and the Lightning Potential Index (Yair et al., 2010; LPI), for selected case studies when an enhanced fraction of superbolts was observed. Holzworth, R. H., McCarthy, M. P., Brundell, J. B., Jacobson, A. R., and Rodger, C. J. (2019). Global distribution of superbolts. J. Geophys. Res. Atmos., 124., doi:10.1029/2019JD030975.Lynn, B., Y. Yair, C. Price, G. Kelman and A. J. Clark (2012). Predicting cloud-to-ground and intracloud lightning in weather forecast models. Weather and Forecasting, 27, 1470-1488, doi:10.1175/WAF-D-11-00144.1.Yair, Y., B. Lynn, C. Price, V. Kotroni, K. Lagouvardos, E. Morin, A. Mugnai, and M. d. C. Llasat (2010). Predicting the potential for lightning activity in Mediterranean storms based on the Weather Research and Forecasting (WRF) model dynamic and microphysical fields, J. Geophys. Res., 115, D04205, doi:10.1029/2008JD010868

Lightning occurrence throughout Europe is at a minimum in winter and mostly confined around the coastlines of the Mediterranean. Limited extent winter thunderstorms at higher latitudes are nevertheless found to produce intense CG strokes that may result in short-lived optical phenomena above thunderstorms in the region between the stratosphere and the lower ionosphere that are collectively referred to as transient luminous events (TLEs). Recent examples of sprite observations have been reported in northern Europe, at latitudes larger than about 49N, during very low flash-rate and small-scale winter storms. This study focuses on the characteristics of the sprite-producing strokes and the context in which they occurred. The sprite parent strokes are identified through the Météorage lightning detection network, providing additional information on the polarity and the peak current. A further characterization of the electromagnetic signal associated with these events is performed combining data from a series of quasi-electrostatic lightning sensors deployed in UK, a wideband ELF-VLF-LF radio receiver at the University of Bath (UK) and an ELF station in Poland, used for the calculations of the related current moment waveform (CMW) and charge moment change (CMC). The characteristics of the thunderstorm, as the cloud top temperature (CTT), the size and the meteorological context, are considered in order to better understand the conditions leading to the observed events.

During a winter thunderstorm on 2017 November 24, a strong burst of gamma rays with energies up to $\sim$10~MeV was detected coincident with a lightning discharge, by scintillation detectors installed at Kashiwazaki-Kariwa Nuclear Power Station at sea level in Japan. The burst had a sub-second duration, which is suggestive of photoneutron productions. The leading part of the burst was resolved into four intense gamma-ray bunches, each coincident with a low-frequency radio pulse. These bunches were separated by 0.7--1.5~ms, with a duration of $\ll$1~ms each. Thus, the present burst may be considered as a ``downward" terrestrial gamma-ray flash (TGF), which is analogous to up-going TGFs observed from space. Although the scintillation detectors were heavily saturated by these bunches, the total dose associated with them was successfully measured by ionization chambers, employed by nine monitoring posts surrounding the power plant. From this information and Monte Carlo simulations, the present downward TGF is suggested to have taken place at an altitude of 2500 $\pm$ 500~m, involving $8^{+8}_{-4} \times 10^{18}$ avalanche electrons with energies above 1~MeV. This number is comparable to those in up-going TGFs., 6 pages, 4 figures, accepted for publication in Physical Review Letters

Since 2006, The Gamma Ray Observation of Winter Thunderclouds (GROWTH) collaboration has operated radiation measurement networks at the coastal area of Japan sea. The area is famous for its frequent occurrence of winter thunderstorms. We aims at elucidating how particles in lightning and thunderclouds are accelerated to relativistic energies. More than 10-years observations reveal that there are two types of radiation bursts associated with winter thunderstorms. One is a "long burst" or a "gamma-ray glow" lasting for a few tens of seconds to a few minutes. The other is a "short burst" in association with lightning. In order to expand the observational network, we have developed a small type of a radiation detector. In this paper, we focus on recent several findings obtained by the new detectors.One is a combination of a short burst and a long one, showing simultaneous detection of prompt gamma rays extending to $\sim$10 MeV and the 511-keV annihilation ones. These gamma-ray signals demonstrate the occurrence of photonuclear reactions in lightning. Another is an event that a gamma-ray glow suddenly ceased just prior to lightning, suggesting a relation between lightning and the two types of radiation bursts. On the basis of these results, we discuss the production mechanism of gamma rays related to thunderstorms and the lightning initiation problem.

An unusual negative lightning flash was recorded at the Santis Tower on June 15, 2012. The flash did not contain an initial continuous current typical of upward negative lightning, which is the most common type of event at the Santis Tower. The flash contained four strokes, the last three of which were normal while the current associated with the first stroke resembled a Gaussian pulse with an unusually high peak value of 102.3 kA, a long risetime of 28.4 μs, and a pulsewidth of 53.8 μs, which was followed by an opposite polarity overshoot with a peak value of 8.5 kA. Our current records suggest the involvement of a long upward connecting positive leader in response to the approaching downward negative leader in the formation of this flash. Lightning location system (LLS) data indicate that a positive cloud-to-ground stroke occurred 1 ms prior to the first stroke of the flash. In this paper, we present a detailed description of the data associated with this event. Moreover, both a return stroke model and an M-component model are used to reproduce the far-field waveform of this bipolar stroke. The simulations result in a radiated electric field waveform that is similar to those of large bipolar events (LBEs) observed in winter thunderstorms in Japan. A sensitivity analysis of the used simulation models reveals that, by proper selection of the input parameters, all field waveform characteristics, except for the positive half-cycle width, can be made to fall in the range of LBE field characteristics reported in Japan.

The analysis of high-energy background radiation (0.1 – 2 MeV) enhancements during eight winter thunderstorms and ?ve summer storms in the Ebro delta region in the northeast of Spain is presented. For the ?rst time, high-energy radiation counts, precipitation, radar re?ectivity, and very high frequency lightning detections to infer charge regions altitude have been analyzed in order to ?nd out what produces the measured background radiation increments associated with storms. The good agreement between radar re?ectivity and precipitation with increases in background radiation counts coupled with the spectrum analysis comparing rain/no rain periods suggests that radon-ion daughters play a major role in the radiation increments reported. No evidence has been found supporting that measured background radiation enhancements can be produced by storm electric ?elds. Finally, a single case of a high-energy radiation increase was prior to a cloud-to-ground lightning stroke, which reinforces the theory that a lower positive charge layer’s existence is important for the production of Terrestrial Ground Enhancements.

We present a simple, efficient, and flexible three-dimensional electrostatic model for calculating the magnitude and direction of the electric field from the ground to the base of the ionosphere for a given thunderstorm charge configuration, with the aim of evaluating the possibility for sprite inception. The model is based on a method-of-images solution to Poisson's equation, assuming near vacuum conditions and with perfectly conducting upper and lower boundaries. A dipole electrical structure within each thundercloud is assumed, with a screening (shielding) charge above the cloud. The charge centers (main positive, main negative, and screening charge) are modeled with average structural characteristics of summer and winter thunderstorms. To simulate a positive or negative cloud-to-ground lightning discharge, the main positive or main negative charge center, respectively, is removed from the domain. The computed electric potential at each grid point is converted to the electric field and is compared against the value of the conventional breakdown field to obtain an indication of the possibility of electrical breakdown and hence the possibility of sprite inception. This simple model is particularly useful for performing a sensitivity study with respect to variation in thunderstorm cell charge configuration, with no assumption of symmetry in the horizontal or vertical directions. Implications of the presence of neighboring clouds at different relative stages of development on the possibility of sprite inception and on the displacement of sprites from the location of the parent thunderstorm are also examined, as well as clouds with inverted dipole charge configuration.

Sprites are transient luminous events (TLEs) that occur at mesospheric altitudes between 50 and 90 km. They last up to several milliseconds, and are caused mostly by positive lightning discharges from the thunderstorm cells below. Infrasound from sprites was first observed in detail a decade ago by a team from the French Atomic Energy Commission (CEA), as part of a renewed international interest in infrasound measurements brought about by the Comprehensive Nuclear Test Ban Treaty (CTBT) ( Farges et al., 2005 ). They used optical images of sprites obtained during the EuroSprite observation campaign in order to form the temporal and azimuthal basis necessary for searching for the infrasound signatures from these events, which appear as unique “chirp” or “inverted chirp” signatures depending on the sprite's distance from the infrasound array. In this paper we follow this methodology to see if the nascent Israeli infrasound arrays can detect these signals from sprites in the Eastern Mediterranean, for which there are nearly 8 years of winter-time optical observations. We calculated the expected arrival time of the infrasound from optically observed sprites, and then used a basic ray-tracing method in order to confirm that we were in fact able to observe several of these sprites, at various distances, exhibiting both “chirp” and “inverted chirp” signals. We then compared these observations with observations of lightning activity made by the World Wide Lightning Location Network (WWLLN).

Thundersnow (TSSN) is a mesoscale event that is typically associated with large amounts of precipitation and both in-cloud and cloud-to-ground lighting. TSSN is most often seen in the northwest and northeast sectors of dynamic mid-latitude cyclones (Market et al. 2002). Starting in 2002, the Research on Convective Snows (ROCS) group began issuing TSSN outlooks each day during the cold season for the area of the United States pictured in Figure 1. The outlooks were issued at 1800 UTC and expired at 1800 UTC the following day. The purpose of the outlooks was to inform users on whether TSSN should be expected in the central U.S. during the ensuing 24-hour period and for what location, if applicable. The issuance of daily outlooks continued for five seasons from 2003 through 2008. Although these TSSN outlooks had been issued for some time, there has never been any significant verification performed on these forecasts. This paper will look at three different TSSN events and determine the reasons for either a successful or unsuccessful forecast.

We compare two observations of gamma-rays before, during, and after lightning flashes initiated by upward leaders from a tower during low-altitude winter thunderstorms on the western coast of Honshu, Japan. While the two leaders appear similar, one produced a terrestrial gamma-ray flash (TGF) so bright that it paralyzed the gamma-ray detectors while it was occurring, and could be observed only via the weaker flux of neutrons created in its wake, while the other produced no detectable TGF gamma-rays at all. The ratio between the indirectly derived gamma-ray fluence for the TGF and the 95% confidence gamma-ray upper limit for the gamma-ray quiet flash is a factor of $1\times10^7$. With the only two observations of this type providing such dramatically different results -- a TGF probably as bright as those seen from space and a powerful upper limit -- we recognize that weak, sub-luminous TGFs in this situation are probably not common, and we quantify this conclusion. While the gamma-ray quiet flash appeared to have a faster leader and more powerful initial continuous current pulse than the flash that produced a TGF, the TGF-producing flash occurred during a weak gamma-ray "glow", while the gamma-ray quiet flash did not, implying a higher electric field aloft when the TGF was produced. We suggest that the field in the high-field region approached by a leader may be more important for whether a TGF is produced than the characteristics of the leader itself., 21 pages, 6 figures, accepted for publication by the Journal of Geophysical Research - Atmospheres

A continuous recording system for digital signals associated with lightning discharge is designed to enhance lightning discharge processes understanding. The original VHF interferometer occasionally misses small amplitude impulses, and this disadvantage causes imperfect imaging of lightning progressions, especially positive breakdowns. To overcome this difficulty, the method of phase difference standard deviation of frequency components is applied. Numerical simulations and winter thunderstorm observations are presented to show and evaluate the capability of the new system.

In this paper we have simulated the far-field waveform characteristic of large bipolar events (LBEs) occurred in winter thunderstorms in Japan and compared the field-to-current conversion factors (FCCFs) of LBEs with that of the lightning cloud-to-ground (CG) return stroke (RS) in summer thunderstorm. As for the physical process of LBEs, Wu et al. (2014) considered that LBEs may be very similar to the typical lightning RS (RS-like process) or caused by an initial continuous current pulse (ICC-like process) in upward lightning flashes. We assume that the lightning channel length of LBEs ranges from 500 m to 1000 m, and the height of tall object struck by LBEs is from 100 m to 300 m. By using the bouncing wave model, we found that only when the injected current waveform of LBEs is characterized with a symmetric Gaussian pulse, the simulated far-field waveform of LBEs both for RS-like process and ICC-like process is similar to that observed by Wu et al. (2014). For striking tall objects with heights from 100 m and 300 m, the FCCFs of LBEs are positively correlated with its channel length and derivatives of injected current waveform, and the FCCF for RS-like process is about similar to that for ICC-like process. However, the FCCFs of LBEs are very different from lightning RS in summer thunderstorm; that is to say, the FCCFs developed for the well-known lightning RS in summer thunderstorm are not suitable for LBEs.

Extratropical cyclones give rise to much of the precipitation over California. Observed California winter precipitation is highly correlated to a metric of extratropical cyclone activity over the Eastern Pacific. The lack of precipitation over the recent winters is coincident with consecutive winters of much below average cyclone activity. Analysis of variability in cyclone activity and California precipitation simulated by models participating in Coupled Model Intercomparison Project Phase 5 indicates that most models can simulate the relationship between cyclone activity and precipitation well. Examination of projected change suggests (1) no evidence of a long-term downward trend in California region cyclone activity within the examined scenarios and (2) that the intermodel spread in California precipitation projection can be largely explained by the spread in the projection of extratropical cyclone activity. This highlights the need to further understand physical mechanisms for the variation in projection of cyclone activity in this region.

The development and verification of a probabilistic forecast system for winter thunderstorms around Amsterdam Airport Schiphol is described. We have used Model Output Statistics (MOS) to develop the probabilistic forecast equations. The MOS system consists of 32 logistic regression equations, i.e. for two forecast periods (0–6 h and 6–12 h), four 90×80 km2 regions around Amsterdam Airport Schiphol, and four 6-h time periods. For the predictand quality-controlled Surveillance et Alerte Foudre par Interférométrie Radioélectrique (SAFIR) total lightning data were used. The potential predictors were calculated from postprocessed output of two numerical weather prediction (NWP) models – i.e. the High-Resolution Limited-Area Model (HIRLAM) and the European Centre for Medium-Range Weather Forecasts (ECMWF) model – and from an ensemble of advected lightning and radar data (0–6 h projections only). The predictors that are selected most often are the HIRLAM Boyden index, the square root of the ECMWF 3-h and 6-h convective precipitation sum, the HIRLAM convective available potential energy (CAPE) and two radar advection predictors. An objective verification was done, from which it can be concluded that the MOS system is skilful. The forecast system runs at the Royal Netherlands Meteorological Institute (KNMI) on an experimental basis, with the primary objective to warn aircraft pilots for potential aircraft induced lightning (AIL) risk during winter.

Meteorological conditions and thunderstorm characteristics related to lightning threats to wind turbines are discussed in this paper. Due to the rotating blades, wind turbines may be regarded peculiar tall objects, more susceptible to lightning strikes than other tall man-made structures. In the present study, Lightning Mapping Array and weather radar observations allowed to draw a clear picture of the thunderstorm characteristics leading to lightning strokes to wind turbines, in a coastal area of the Mediterranean basin. Results showed that lightning threats to wind turbines tend to occur during transitional periods (spring and autumn), although the main thunderstorm activity concentrates in the warm summer months. Thunderstorms with downward strokes to wind turbines presented particular features, like a limited vertical development and a dominant lower positive charge layer. Downward cloud-to-ground strokes hitting wind turbines were mainly of negative polarity and with peak currents above the average. On the other hand, conditions for self-initiated upwards from wind turbines resemble those reported in Japan and the U.S winter thunderstorms, with low-cloud based large electrified stratiform regions. These particular conditions, leading to lightning threats to wind turbines, should be properly included in lightning protection standards.

All lightning strokes generate electromagnetic pulses –atmospherics– which can travel over distances of thousands of kilometers. Night-side atmospherics show typical frequency dispersion signatures caused by sub-ionospheric propagation. Their analysis can be used to determine the distance to the source lightning, and therefore it represents a safe tool for investigation of distant thunderstorms, as well as for indirect observations of the lower ionosphere. However, such analysis has never been done on the dayside. Here we present the first results which show unusual daytime atmospherics with dispersion signatures originating from strong thunderstorms which occurred during winter months 2015 in the North Atlantic region. Using newly developed analysis techniques for 3-component electromagnetic measurements we are able to determine the source azimuth and to attribute these rare atmospherics to both positive and negative lightning strokes in northern Europe. We consistently find unusually large heights of the reflective ionospheric layer which are probably linked to low fluxes of solar X rays and which make the dayside subionospheric propagation possible. Although the atmospherics are linearly polarized, their dispersed parts exhibit left handed polarization, consistent with the anticipated continuous escape of the right-hand polarized power to the outer space in the form of whistlers.