Displays

Characterisation of the vertical variation in the atmospheric electric field has been made for many decades, but normally in an ad-hoc manner, using instrumented weather balloons or manned aircraft, which are expensive to fly.  Such vertical measurements are required to measure the ionospheric potential and to characterise electric fields with clouds (both thunderstorm and non thunderstorm clouds) to understand the charging processes within them. 

Advances in electronics and battery technology has meant that small Unmanned Aerial Vehicles (UAVs) have now become available as a new science platform. These measurement platforms address many of the problems associated with manned aircraft while allowing in-situ measurements with an increased level of control and repeatability when compared to weather balloons. Despite their potential advantages, one of the main challenges to using UAVs for atmospheric electricity research is the lack of small, lightweight sensors which are commercially available. To overcome this barrier, this work describes the development of a lightweight, miniaturised electric field sensor to be integrated with a small UAV (<7kg, wingspan 2m). 

The sensor has been designed to allow measurements of the electric field intensities typical of non-thunderstorm low altitude (<6000 ft) clouds with a typical range of 0-2.5kV/m. It is based on the concept of an electric field mill, but with a translational shield rather than a rotating vane model. This allows the sensor to fit neatly within the wing of a small UAV, rather than the need to be mounted in the nose.  A custom designed 3D printed housing contains all elements of the sensor package, with the translational shield movement and data logging controlled by an onboard programmable microcontroller. This work will focus on the details regarding the experimental characterisation of the sensor package with a particular focus on the key influences of the integration with the airborne platform.

How to cite: Chindea, S. and Nicoll, K.: Electric Field Sensor for Small Unmanned Aerial Vehicles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7768, https://doi.org/10.5194/egusphere-egu2020-7768, 2020.

The study of the electrical properties of the atmospheric marine boundary layer is important as the effect of natural radioactivity in driving near surface ionisation is significantly reduced over the ocean, and the concentration of aerosols is also typically lower than over continental areas, allowing a clearer examination of space-atmosphere interactions. Furthermore, cloud cover over the ocean is dominated by low-level clouds and most of the atmospheric charge lies near the earth surface, at low altitude cloud tops.

The relevance of electric field observations in the marine boundary layer is enhanced by the the fact that the electrical conductivity of the ocean air is clearly linked to global atmospheric pollution and aerosol content. The increase in aerosol pollution since the original observations made in the early 20th century by the survey ship Carnegie is a pressing and timely motivation for modern measurements of the atmospheric electric field in the marine boundary layer. Project SAIL (Space-Atmosphere-Ocean Interactions in the marine boundary Layer) addresses this challenge by means of an unique monitoring campaign on board the ship-rigged sailing ship NRP Sagres during its 2020 circumnavigation expedition.

The Portuguese Navy ship NRP Sagres departed from Lisbon on January 5th in a journey around the globe that will take 371 days. Two identical field mill sensors (CS110, Campbell Scientific) are installed on the mizzen mast, one at a height of 22 m, and the other at a height of 5 meters. A visibility sensor (SWS050, Biral) was also set-up on the same mast in order to have measurements of the extinction coefficient of the atmosphere and assess fair-weather conditions. Further observations include gamma radiation measured with a NaI(Tl) scintillator from 475 keV to 3 MeV, cosmic radiation up to 17 MeV, and atmospheric ionisation from a cluster ion counter (Airel). The 1 Hz measurements of the atmospheric electric field and from all the other sensors are linked to the same rigorous temporal reference frame and precise positioning through kinematic GNSS observations.

Here the first results of the SAIL project will be presented, focusing on fair-weather electric field over the Atlantic. The observations obtained in the first three sections of the circumnavigation journey, including Lisbon (Portugal) - Tenerife (Spain), from 5 to 10 January, Tenerife - Praia (Cape Verde) from 13 to 19 January, and across the Atlantic from Cape Verde to Rio de Janeiro (Brasil), from January 22nd to February 14th, will be presented and discussed.

How to cite: Barbosa, S., Camilo, M., Almeida, C., Almeida, J., Amaral, G., Aplin, K., Dias, N., Ferreira, A., Harrison, G., Heilmann, A., Lima, L., Martins, A., Silva, I., Viegas, D., and Silva, E.: Atmospheric electric field in the Atlantic marine boundary layer: first results from the SAIL project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18767, https://doi.org/10.5194/egusphere-egu2020-18767, 2020.

Countries in arid and desert climates that have small amounts of rainfall each year use cloud seeding techniques to enhance the little rainfall that is present. Typically, this is achieved by seeding the cloud with hygroscopic nuclei to increase the rainfall. A possible alternative method is to inject the cloud with electric charge, which has been shown in models to alter the droplet size and distribution and influence rainfall properties.

Here, in-situ observations of the electrical and optical properties of clouds are described from a desert site. These are used to inform droplet growth models. For this, a yearlong campaign, during which 10 weather balloons carrying electric charge and optical sensors were launched through fog layers from Abu Dhabi airport, United Arab Emirates. Here we present 2 case studies. The first is a clear air ascent comparison between the desert site at Abu Dhabi and a temperate site in northern Finland. The second is a fog comparison between Abu Dhabi and a temperate site in the United Kingdom

The results show that the fogs in Abu Dhabi are highly charged with a charge density of 0.1-1 nC m^-3 as opposed to the charge densities of fogs in Northern Hemisphere temperate regions which have a typical charge density of 10 pC m^-3. The droplet concentration in the Abu Dhabi fog case study is significantly smaller, approximately 150 cm^-3 as opposed to droplet concentrations of 300-400 cm^-3 in fog over a temperate site.

The results suggest that dust contributes strongly to the atmospheric electrical conditions in the UAE region, due to charging of the dust tribo-electrically. This dust charge may also affect the droplet distribution within the fog. These new measurements of the vertical profile of charge through fog layers in desert climates will be used to improve understanding in droplet growth models.

How to cite: Marlton, G., Harrison, G., Nicoll, K., and Ambaum, M.: Measuring the electrical and optical properties of fog using balloon borne instrumentation in the UAE, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8037, https://doi.org/10.5194/egusphere-egu2020-8037, 2020.

A problem of the electric field dynamics in turbulent electro-active clouds (Cumulus, Stratus, Cumulonimbus) is one of the most relevant and complex problems of dynamical meteorology and atmospheric electricity. This problem is important for the study of intense large-scale electric field and its fluctuations that may lead to high-energy particle flows and lightning discharges, for electric current parameterization. Direct field measurements in convective clouds with a developed electrical structure are very difficult; so one of urgent tasks is the development of remote sensing methods for turbulence characteristics in electro-active clouds.

The growth of a large-scale electric field in a turbulent atmosphere is caused by the generation of an electric charge on colliding particles (hydrometeors and dust). Meanwhile, observations (including preliminary observations of the authors) and theoretical studies (Mareev and Dementyeva, 2017) show that intensification of thunderstorm activity can be associated with increased turbulence in the cloud. This paper presents new ideas and results of experimental and theoretical studies of the role of turbulence in electro-active clouds.

The main attention is paid to complex remote observations of different types of clouds with an experimental set-up including the microwave radiometers of 3 cm and 8 mm wavelengths (with a time resolution of order of one second), the network of electrostatic fluxmeters spaced by several kilometers each from another, and the meteorological radar. The data of recent several years were used for analysis. Note that recently space-borne passive microwave radiometry of intense convective clouds (see, for example, Peterson et al., 2017) attracted more attention compared to ground-based microwave observations. A principal idea of our approach is to use the wave-length channels allowing us to reveal both optically thick and optically transparent cloud events from the data on fluctuations in the brightness temperature of the atmosphere.

A special attention was paid to comparative analysis of the turbulence characteristics in thunderclouds and in clouds that do not have a developed electrical structure. The spectral characteristics of electric field and brightness fluctuations were found to be associated with atmospheric air turbulence and mostly are quantitatively described by Kolmogorov-type spectra. Compared with ordinary Cumulus and Stratus clouds, a limited band near a frequency of ~ 0.01 Hz with a higher level of fluctuations is distinguished in the spectral density of fluctuations in the brightness temperature of thunderclouds. The spectra of fluctuations of the electric field caused by thunderclouds, as well as turbulence interior thundercloud, are significantly different from the spectra caused by ordinary Cumulus and layered clouds.

The work was supported by the Russian Foundation for Basic Research (projects no. 19-05-00975 and 18-45-520010).

References

Mareev E.A., Dementyeva S.O. (2017), The role of turbulence in thunderstorm, snowstorm, and dust storm electrification. Journal of Geophysical Research: Atmospheres, V. 122, No. 13, P. 6976-6988. doi: 10.1002/2016JD026150.

Peterson M., Liu C., Mach D., Deierling W., Kalb C. (2015), A method of estimating electric fields above electrifi_ed clouds from passive microwave observations. J. Atmos. Ocean. Tech., V.32 (8), P.1429-1446. doi: 10.1175/ JTECH-D-14-00119.1.

How to cite: Mareev, E., Klimenko, V., Lubyako, L., Shatalina, M., Dementyeva, S., and Ilin, N.: Ground-based measurements of turbulence in electrified clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21073, https://doi.org/10.5194/egusphere-egu2020-21073, 2020.

This full-year study spanning portions of 2017-18 quantifies GOES-16 Geostationary Lightning Mapper (GLM) flash detection efficiency (DE) in central Florida using the Kennedy Space Center Lightning Mapping Array (LMA). Findings support the expectation that about 70% of all flashes are reported when averaged over all thunderstorms and times-of-day. When quantified as a function of LMA flash parameters, GLM exhibited an average of 40% DE for small (main channel length of 5-8 km), and even lower DE for shorter-length and/or short-duration (less than 200 milliseconds) flashes. Conversely, GLM exhibited more than 95% DE for long-duration flashes with main channel lengths of 50-100 km. DE was somewhat lower during daylight and higher at night.  Flash size and duration, on average are shown to be a critical parameter influencing GLM detection.  Given that this behavior occurred for severe and non-severe storms, it is likely an important contributing factor to the low flash detection efficiency for storms with high flash rates (and resulting small/short flashes) associated with severe weather, thereby modulating the effects of optical scattering and absorption within cloud volumes.

These findings can be explained by the time-evolution of cloud-top optical emissions derived from observations using the Lightning Imaging Sensor (LIS) onboard the Tropical Rainfall Measuring Mission (TRMM) Satellite. Specifically, LIS group area, energy density, and cloud-top energy in intra-cloud flashes, on average, reached a local maximum value in the very first few milliseconds of a flash and fell to their minimum values at around 10-20 milliseconds into the flash. After that, all parameters gradually increased over the next 80-100 milliseconds to reach the initial values, and then continued to increase for longer-duration flashes. In addition, statistical simulations based on long-term LIS group area observations indicate that about half of the above-threshold light sources are smaller than a LIS pixel (~ 4 km x 4 km) and are the smallest during initial breakdown in IC flashes.

These observations have implications for expectations about the performance of all satellite lightning observing instruments that are based on optical observations operating in the near-IR portion of the optical spectrum.  The specific values for optical source size and cloud-top energy provided by this study, as a function of time-in-flash, should help refine the expectations for the performance of the upcoming Lightning Imager on the Meteosat Third Generation geostationary satellite.

How to cite: Cummins, K. and Zhang, D.: Flash size and within-flash time evolution of cloud-top optical emissions: Implications for satellite-based lightning observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4225, https://doi.org/10.5194/egusphere-egu2020-4225, 2020.

Calculating the electric force between cloud drops is not straightforward. Since water drops are conducting, the electric force is not just simply the force between point charges, but instead the charge in each drop induces an infinite number of image charges in the other drop. The effect of these image charges can cause the electric force between two like charged cloud drops to become attractive on very short distances, when only applying Coulomb's law would result in a repulsive force. The attractive effect of image charges could potentially increase the collision rate of cloud drops. Within the United Arab Emirates Rain Enhancement Program (UAE REP) we are investigating the potential for rain enhancement by charging clouds.

Simulating the behaviour of cloud drops is numerically very expensive. A large number of drops needs to be simulated to obtain stable collision statistics. Additionally, the drops move in a complex turbulent environment with eddies spanning several orders of magnitude in size. Simulating the turbulent flow alone is an expensive task. Because of the typical sizes of cloud drops, their motion is predominantly influenced by the smallest turbulent scales in the flow. Therefore, Direct Numerical Simulation (DNS) is necessary and used to simulate the influence of turbulent flow on drop motion. In this work, instead of using DNS, we use an ABC flow to simulate the turbulent effect on cloud drops. This simple approximation for the turbulent flow allows to simulate the drop motion using much less computational resources then needed by DNS and therefore, allows to include the very expensive effect of electrical drop charge in our simulation of colliding drops in a turbulent environment.

To investigate the effect of electrical charge on drop collisions, a Lagrangian particle code for the interaction of cloud drops is used. It calculates the motion of individual drops based on the aerodynamical force due to the ABC flow and the gravitational force and registers drop collisions from which collision statistics can be calculated. In the cloud model all drops carry positive charges. The effect of the electric force is calculated by an approximation which uses Coulomb's law for the effect of the point charges and an additional term to approximate the effect of image charges which produce an attractive force on short distance.

Results for the collision kernel with and without charge will be presented. The effect of the additional term to Coulomb's law will be shown for different drop sizes and drop charges. It will be discussed if the attractive force for like charged drops on short distances can lead to an enhancement in drop collisions and under which conditions the effect is the largest.

How to cite: Auerswald, T. and Ambaum, M.: Simulating collisions of charged cloud drops in an ABC flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12408, https://doi.org/10.5194/egusphere-egu2020-12408, 2020.

Many studies have shown that the characteristics of lightning such as size and peak current differ by geographical region as well as between ocean and continental thunderstorms. For example, several studies have shown that the lightning in oceanic thunderstorms are generally larger and have lightning with higher peak currents than in continental thunderstorms. In this study, as opposed to individual lightning flash characteristics, we focus on how thunderstorm characteristics change for various regions. We develop a lightning clustering algorithm that takes individual lightning strokes and creates thunderstorms based on their spatiotemporal proximity. We use lightning data from the Earth Networks Total Lightning Network and compare storms throughout regions of the U.S.A. and Europe. Once these thunderstorms are obtained, we can regionally categorize them and compare various characteristics (size, duration, flash rate, polarity, IC/CG ratio, etc.) to determine if any differences stand out. In this presentation, we will discuss the clustering algorithm used, analyze the results of the study, and discuss implications.

How to cite: Lapierre, J. and Stock, M.: Lightning Clustering to Study Regional Variations in Thunderstorm Characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20611, https://doi.org/10.5194/egusphere-egu2020-20611, 2020.

Most electrical activity in vertebrates and invertebrates occurs at extremely low frequencies (ELF), with characteristic maxima below 50Hz.  The origin of these frequency maxima is unknown and remains a mystery.  We propose that over billions of years during the evolutionary history of living organisms on Earth, the natural electromagnetic resonant frequencies in the atmosphere, continuously generated by global lightning activity, provided the background electric fields for the development of cellular electrical activity.  In some animals the electrical spectrum is difficult to differentiate from the natural background atmospheric electric field produced by lightning.  In this paper we present evidence for the link between the natural ELF fields and those found in many living organisms, including humans.

Price, C., E. Williams, G., Elhalel and D. Sentman, 2020:  Natural ELF Fields in the Atmosphere and in Living Organisms, Int. J. Biometeorology, in press.

How to cite: Price, C., Williams, E., Elhalel, G., and Sentman, D.: Lightning, Evolution and Biology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6988, https://doi.org/10.5194/egusphere-egu2020-6988, 2020.

Upward lightning triggered by elevated objects, such as wind turbines (WT), may increase significantly the number of lightning strikes to these objects. In the recently publishes 2^nd edition of the international standard IEC 61400-24 an environmental factor C_DWL for winter lightning conditions was introduced to account for this additional lightning risk in the lightning exposure assessment of a WT. Values for C_DWL should be 4 (in medium winter lightning activity areas) or 6 (high activity areas) or even higher in special cases. The main challenge is to get reliable data about the winter lightning activity for a given region and for first estimates maps of winter lightning activity for the continents are given in IEC 62400-24, Annex B.

A different approach is used in this contribution. As there is already a high number of WT installed in Europe, we have investigated the number (percentage) of existing WT that was at least struck one time in the winter periods of 2017/18 an 2018/19 based on data of the EUCLID lightning location system.

We have extracted the locations of 10.225 WT sites in Europe in the area from 45°N - 50°N and 10°W -30°E form OpenStreetMap database. Then we checked if there were any lightning strikes located by EUCLID within a 0.003° circular area (is about a 300 m radius) around each of these turbines during the cold season (October to April) in 2017/18 and 2018/2019, respectively. Out of the 10.225 WT 1.131 (11,1 %) and 913 (8,9 %) have been struck by lightning in cold season 2017/18 and 2018/19, respectively. It is worth noting, that only 101 WT (1%) were struck in both seasons, indicating that it is more a dependency on regional meteorological conditions changing from year to year, rather than on a specific group of WT. EUCLID detected flashes are likely to represent only about one half of the real occurring upward flashes from the WT. ICC_Only type upward lightning, which are discharges with current waveforms not followed by any return strokes are typically not detected by lightning location systems, and on instrumented towers this type of discharges makes up about 50% of all upward lightning. But there is a high chance, that a large fraction of this ICC_Only discharges were triggered by the same WT, where EUCLID detected some strokes.

In terms of dependency of the altitude of the WT site above sea level we observe a clear increase of probability of WT lightning with increasing altitude. About 10 % (29/315) of the 315 WT at altitudes up to 50 m ASL are struck by lightning increasing to almost 50 % (15/31) for WT at sites of 950 to 1000 m altitudes ASL. No clear trend is observed for higher altitudes, likely due to the low number of WT above 1000 m.

The obtained 10 % of the WTs triggering at least one upward lighting per cold season demonstrates the high probability of lightning to WT and emphasizes the need of proper protection of the WTs mechanical structure (rotor blades) as well as the entire electrical installation.

How to cite: Diendorfer, G.: Probability of lightning strikes to wind turbines in Europe during winter months, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3337, https://doi.org/10.5194/egusphere-egu2020-3337, 2020.

The variability of winter climate in the North Atlantic region is predominantly driven by a large scale alternation of atmospheric masses between the Icelandic Low and Azores High pressure systems called the North Atlantic Oscillation (NAO) and characterized by the NAO index. The calculation of the NAO index is based on the difference between sea-level pressure strengths of the Azores High and the Icelandic Low. Unusually high positive values of the NAO index were observed to manifest themselves by above-average precipitation and severe winter storms over British Isles and other parts of northwestern and northern Europe.

In the last two decades, the winter season 2014/2015 exhibited the highest positive monthly NAO indexes. During this winter, newspapers in the UK, Germany, Poland, and Scandinavia reported extremely strong storms which caused huge power outages, damages of buildings, and collapses of traffic which paralyzed the daily life. As winter thunderstorms are also characterized by a higher production of very energetic lightning, we use the World Wide Lightning Location Network (WWLLN) data and investigate properties of lightning which occurred in the north European region from October 2014 to March 2015.  The dataset consists of more than 90 thousand lightning detections. We focus on spatial and temporal distribution of lightning strokes, their energies and multiplicity.

We have found that the diurnal distribution of lightning was random from November till February, while the afternoon peak typical for summer storms was noticeable only in October and March. The median energy of lightning strokes observed in October, November and March reached only about 10-20% of the median energy of strokes detected in December, January and February. The most energetic strokes were concentrated above the ocean close to the western coastal areas and appeared exclusively at night and in the morning hours.

How to cite: Kolmašová, I., Rosická, K., and Santolík, O.: Characteristics of North European winter lightning related to a high positive North Atlantic Oscillation index, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4024, https://doi.org/10.5194/egusphere-egu2020-4024, 2020.

Lightning is formed in the atmosphere through the combination of complex dynamic and microphysical processes. Lightning can have a considerable influence on the environment and on the economy since it causes energy supply outages, forest fires, damages, injury and death of humans and livestock worldwide. Therefore, it is of great importance to be able to predict lightning incidence in order to protect people and installations. Despite numerous attempts to solve the important problem of lightning prediction (e.g., [1]–[3]), the complex processes and large number of parameters involved in the problem lend themselves to the potential application of a machine learning (ML) approach.

We recently proposed a ML-based lightning early-warning system with promising performance [4]. The proposed ML model is trained to nowcast lightning incidence during any one of  three consecutive 10-minute time intervals and within a circular area of 30 km radius around a meteorological station. The system uses the real-time measured values of four meteorological parameters that are relevant to the mechanisms of electric charge generation in thunderstorms, namely the air pressure at station level (QFE), the air temperature 2 m above ground, the relative humidity, and the wind speed. The proposed algorithm was implemented using the data from 12 meteorological stations in Switzerland between 2006-2017 with a granularity of ten minutes. The stations were selected to be well distributed among different ranges of altitude and terrain topographies.

The algorithm requires the filtering out of a portion of the data which are identified as outliers. However, the process of the automatic identification of outliers is a challenging task which could also affect the model’s performance. In this presentation, we discuss this problem and present approaches that can be used to optimize the process.

References

[1]      D. Aranguren, J. Montanya, G. Solá, V. March, D. Romero, and H. Torres, “On the lightning hazard warning using electrostatic field: Analysis of summer thunderstorms in Spain,” J. Electrostat., vol. 67, no. 2–3, pp. 507–512, May 2009.

[2]      G. N. Seroka, R. E. Orville, and C. Schumacher, “Radar Nowcasting of Total Lightning over the Kennedy Space Center,” Weather Forecast., vol. 27, no. 1, pp. 189–204, Feb. 2012.

[3]      Q. Meng, W. Yao, and L. Xu, “Development of Lightning Nowcasting and Warning Technique and Its Application,” Adv. Meteorol., vol. 2019, pp. 1–9, Jan. 2019.

[4]      A. Mostajabi, D. L. Finney, M. Rubinstein, and F. Rachidi, “Nowcasting lightning occurrence from commonly available meteorological parameters using machine learning techniques,” npj Clim. Atmos. Sci., vol. 2, no. 1, p. 41, 2019.

How to cite: Mostajabi, A., Finney, D., Rubinstein, M., and Rachidi, F.: Nowcasting Lightning Occurrence Using Machine Learning Techniques: The Challenge of Identifying Outliers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22302, https://doi.org/10.5194/egusphere-egu2020-22302, 2020.

We have been developing a ground-based lightning and AWS network system under the projects of a SATREPS “ULAT” and e-ASIA in order to realize precise real-time monitoring and issuing alert for torrential rainfall and typhoon extreme based on international cooperation among Japan, Philippines, Indonesia and other SE-Asian countries supported by JST, JICA, PHL-Microsat and other funding. The intensification of lightning activity is precursor of typhoon growth. In these projects, we are constructing ground-based lightning and AWS—automated weather station—network system with 12 sites for VLF radio wave measurement in nation-wide of Philippines and with 50 sites for electrostatic field measurement in Metro Manila together with infrasound sensor. We are going to complete the installation of the sensors at most of the planned ~60 sites by the end of this year. We already started with installed sensors and achieved preliminary results for typhoon and thunderstorm measurement. We are also doing practice in operating our micro-satellite which can make rapid target pointing at high accuracy. Using the photos captured from the satellite, now we can reproduce the detailed 3-D structure of the cloud at best quality even compared to the latest radar system.

How to cite: Takahashi, Y., Sato, M., Kubota, H., Ishida, T., Algodon, M., Castro, E., Estrebillo, L., Purwadi, P., Perez, G., Yamashita, K., Matsumoto, J., and Hamada, J.: Ground-based lightning and AWS network system for alert of torrential rainfall and typhoon combined with micro-satellite constellation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20990, https://doi.org/10.5194/egusphere-egu2020-20990, 2020.

EUMETSAT has provided the user community with more than three decades worth of satellite data, starting with the geostationary missions of the Meteosat First Generation, and since 2002 with the Meteosat Second Generation (MSG) series satellites.

The development of the next generation geostationary program, the Meteosat Third Generation (MTG), is now in its final stages. The MTG system will host a more advanced 16-channel VIS/IR Flexible Combined Imager (FCI) as well as a Lightning Imager (LI) on its geostationary imaging platform (MTG-I), whereas the sounding platform (MTG-S) will host the MTG InfraRed Sounder (IRS) and the Copernicus Sentinel-4 ultraviolet/near-infrared (UVN) sounding missions. The launch of the first two satellites MTG-I1 and MTG-S1 hosting the imaging and sounding instruments is foreseen in 2021 and 2023, respectively.

The new and improved capabilities will significantly enhance the potential for convective storm monitoring, from the earliest initial phases to full maturation and dissipation. In addition, as examples of dedicated applications where the improved capabilities will play a significant role, one can mention fog monitoring and especially the enhanced fire monitoring capability.

The presentation will give an overview of the MTG system, its observation missions, and the main improvements and novelties over Meteosat Second Generation (MSG) in terms of new missions and expected product performance. As a primarily Nowcasting mission, MTG will provide significant additions to the hazardous weather observations in the coming years. The emphasis of the presentation will be on the new observational capability provided by the Lightning Imager.

How to cite: Grandell, J., August, T., Coppens, D., Fowler, G., Lekouara, M., Munro, R., and Viticchie, B.: New and improved European satellite observation capabilities for hazardous weather to be available from 2022 onwards: Meteosat Third Generation (MTG), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2752, https://doi.org/10.5194/egusphere-egu2020-2752, 2020.

Previous studies suggested that lightning activity could be an indicator of Tropical Cyclone (TC) intensity change but their relationships vary greatly and at times appear contradictory. The importance of total lightning for TC intensification study and forecasting applications has also been pinpointed by several studies. Recently, we revisited this problem using 16 years of TRMM Lightning Imaging Sensor (LIS) measurements and found that reduced (elevated) inner-core total lightning marked rapidly intensifying (weakening) TCs, whereas outer rainband total lightning had opposite trends. It is also shown that the reduced lightning frequency in the inner cores of rapidly intensifying storms was coincident with reduced volumes of 30-dBZ radar reflectivity in the mixed-phase cloud region (-5 to -40 oC), suggesting the lack of large ice particles (e.g., graupel) in the inner cores of rapidly intensifying TCs (which is considered to be important for cloud electrification). To better understand the physical process responsible for these results, we have examined the vertical profiles of radar reflectivity, distribution of precipitation/convection, overshooting radar echo tops (CloudSat), and microwave ice scattering signatures provided by GPM and CloudSat overpasses. This data fusion exercise uniquely provides a more complete understanding of storm electrification, convective intensity, ensemble precipitation microphysics, and storm dynamics in relation to TC intensity change. For example, we have distinguished the convective and microphysical structures between rapidly intensifying (RI) TCs with and without enhanced lightning activity, RI and steady-state TCs, and RI and rapidly weakening TCs.

How to cite: Xu, W.: Changes of Lightning Activity and Vertical Structure in the Inner Core Preceding Tropical Cyclone Rapid Intensification , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4085, https://doi.org/10.5194/egusphere-egu2020-4085, 2020.

The Atmosphere-Space Interaction (ASIM) mission was launched on April 2, 2018 and installed on an external platform of the Columbus Module of the International Space Station the 13th.

The main objectives of the mission are to observe and study thunderstorms and their interaction with the atmosphere. ASIM embarks two main instruments pointing at Nadir, the Modular Multispectral Imaging Array (MMIA) observing in the visible and the Modular X- and Gamma- ray Sensor (MXGS) observing in the X- and Gamma-ray bands.

In this presentation we focus on observations made by the MMIA which includes two cameras operating in the bands 337/5 nm and 777.4/3 nm and three photometers operating in the bands 180-230 nm, 337/5 nm and 777.4/5 nm. Specifically, we analyze the short duration pulses recorded in the 180-230 nm band.

After about 2 years of operations, more than 2500 of such events were identified in the data. They are likely to be recordings of ELVEs (Emissions of Light and Very low frequency perturbation due to Electromagnetic pulse sources), occurring in the ionosphere in response to lightning currents.

We show the amplitude, spatial and temporal distributions of the events and compare the results with those of previous studies. We present an analysis of the temporal characteristics of the pulses themselves and of their delays regarding the parent lightning observed in the other ASIM photometers or in the GLD360 ground lightning detection network recordings. Finally, we compare some typical events with modeling.

How to cite: Chanrion, O., Neubert, T., Zuccoti, C., Heumesser, M., Dimitriadou, K., Gordillo, F. J., Perez-Invernon, F. J., Østgaard, N., Mezentsev, A., and Reglero, V.: Analysis of Elves observations from about 2 years of ASIM operation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17913, https://doi.org/10.5194/egusphere-egu2020-17913, 2020.

The search for the physical mechanisms of lightning, transient luminous events and terrestrial gamma-ray flashes is receiving an extraordinary support by new space observations that have recently become available. Next to lightning detectors on geostationary satellites, new low orbit experiments are giving an unprecedented insight in the very source of these processes. Looking at the physics behind these new observations requires however to have a variety of different instruments covering the same event, and this is proving extremely challenging. Here, we present observations of UV emissions of elves and lightning taken for the first time simultaneously from the two instruments Mini-EUSO and ASIM operating on the international space station. Mini-EUSO was designed to perform observations of the UV-light night emission from Earth. It is a wide field of view telescope (44°x44° square FOV) installed for the first time on October 2019 inside the Zvezda Module of the ISS, looking nadir through a UV transparent window. Its optical system consists of two Fresnel lenses for light collection. The light is focused onto an array of 36 multi-anode photomultiplier tubes (MAPMT), for a total of 2304 pixels. Each pixel has a footprint on ground of ~5.5 km. The instrument is capable of single-photon counting on three different timescales: a 2.5 microsecond (D1) and a 320 microsecond (D2) timescale with a dedicated trigger system, and a 40.96ms timescale (D3) used to produce a continuous monitoring of the UV emission from the Earth. ASIM is an experiment dedicated to lightning and atmospheric processes. Its Modular Multispectral Imaging Array (MMIA) is made of an array of 3 high speed photometers probing different wavelength sampling at rates up to 100 kHz, and 2 Electron Multiplication Charge Coupled Devices (EM-CCDs) with a sub-km spatial resolution with an 80° FOV and recording up to 12 frames per second. Mini-EUSO detected several bright atmospheric events like lightning and elves, with a few km spatial resolution and different time resolutions, probing therefore different stages of the electromagnetic phenomena. Observations from Mini-EUSO were simultaneously captured by ASIM instruments, allowing for the first time to compare and complement the capabilities of the two instruments with a time inter-calibration based on unambiguous series of lightning detections.

How to cite: Battisti, M., Arnone, E., Bertaina, M., Casolino, M., Chanrion, O., Fuglesang, C., and Neubert, T. and the Mini-EUSO team for the JEM-EUSO collaboration: High-speed UV imaging of elves and lightning from space: first simultaneous detections from the Mini-EUSO and ASIM instruments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15025, https://doi.org/10.5194/egusphere-egu2020-15025, 2020.

The Modular Multispectral Imaging Array (MMIA) of the Atmosphere-Space Interactions Monitor (ASIM) contains 3 photometers and 2 cameras, that monitors electrical discharges in and above thunderstorms. The 3 photometers sample in the bands:  337/4 nm, the VUV band 180-230 nm and 777.4/5 nm at 100 kHz; and the 2 cameras record in the bands 337/5 nm and 777.4/3 nm, with a temporal resolution of 12 frames per second. The 337 nm band corresponds to the strongest line of N₂2P, the VUV band include part of the N2 LBH and the 777.4 nm band corresponds to the OI line which is the strongest emission line of lightning leader channel. Here, we analyse observations of flashes that are predominantly blue. We will discuss the leader/streamer nature of these flashes. The analysis incorporates satellite cloud observations and weather radar measurements for the characterization of the thunderstorm clouds and their phase of development. In our optical analysis we incorporate also comparisons with data from NASA’s Lightning Imaging Sensor on the ISS (ISS-LIS) and VAISALA’s lightning location network GLD360.

How to cite: Dimitriadou, K., Chanrion, O., Neubert, T., Heumesser, M., Protat, A., Louf, V., Christian, H., Blakeslee, R., Köhn, C., Østgaard, N., and Reglero, V.: Analysis of Blue Discharges in Thunderclouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18845, https://doi.org/10.5194/egusphere-egu2020-18845, 2020.

Sprites are transient illuminations of the middle atmosphere above thunderclouds which often occur after intense lightning discharges. Here we report optical recordings of sprites and lightning taken with a video camera and photometers in northern Colombia during October 2019.

Optical observations of sprites are often superimposed on the scattered light produced by the parent lightning discharge. This superposition of two optical sources can result in a misinterpretation of the photometer recordings, for example the determination of the rise time of an optical waveform.

Here we propose to use the green light emissions from ~495-505 nm to discriminate between sprite and lightning. This experimental discrimination has become possible because recent modeling studies suggest that lightning emits green light whilst sprite do not emit green light (Gordillo Vazquez et al., 2011; Xue et al., 2015).

The optical signals are detected by a white light video camera and a photometer which is fitted with a ~495-505 nm band pass filter to detect green light. The observed lightning discharges are characterized by significant green emissions in the ~495-505 nm wavelength band. These green emissions are part of the diffuse glow detected by the video camera which is caused by the scattered light from the lightning discharge. This light is scattered during its propagation through the atmosphere which is most likely caused by aerosols, for example related to the ambient humidity and dust. The majority of sprite observations are contaminated by such a diffuse glow with significant ~495-505 nm emissions. The observation of one particular sprite does not exhibit any significant ~495-505 nm emissions and it is therefore attributed to a ‘pure sprite’. The rise time of these optical emissions and the characteristics of other wavelengths recorded by several photometers will be reported for this particularly pure sprite event.

The knowledge gained from these ground-based observations may assist the interpretation of measurements with photometers onboard the ASIM payload on the International Space Station and the forthcoming TARANIS satellite.   

Gordillo-Vazquez, F.J., Luque, A. and Simek, M.(2011). Spectrum of sprite halos. Journal of Geophysical research, 116, A09319. doi: 10.1029/2011JA016652.

Xue, S., Yuan, P., Cen, J., Li, Y. and Wang, X.(2015). Spectral observations of a natural bipolar cloud-to-ground lightning. Geophysical Research Letters, 120, 1972–1979. doi:10.1002/2014JD022598

How to cite: Ghilain, S., Fullekrug, M., Gordillo Vazquez, F. J., and Sergejevs, A.: Optical discrimination of sprite and lightning by use of green light from ~495-505 nm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10093, https://doi.org/10.5194/egusphere-egu2020-10093, 2020.

The Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station is providing important observations of terrestrial gamma-ray flashes (TGFs), including new measurements of optical emissions associated with TGFs and new measurements of multi-pulsed TGFs.  TGFs are thought to be produced by bremsstrahlung emissions from relativistic runaway electrons accelerated inside thunderstorms.  However, the exact mechanisms for generating the large number of runaway electrons required to account for the observed TGF luminosities remains an active area of debate.  Two mechanisms being considered are cold-runaway electron production by streamer heads or leader tips in the high-field regions near lightning, and the self-sustained production of runaway electrons by relativistic feedback involving backward propagating runaway positrons and backscattered x-rays.  Because both mechanisms may require the presence of lightning leaders inside thunderstorms -- for the cold-runaway mechanism to emit the runaway electrons and for the relativistic feedback mechanism to drive the electric field above the feedback threshold -- it has been challenging to test which TGF production mechanisms are occurring.  The new ASIM TGF observations should help constrain TGF models and possibly identify which mechanisms are primarily responsible for the runaway electron production.  In this talk, I will present new TGF modeling results and compare them with available ASIM observations.   

How to cite: Dwyer, J.: Modeling terrestrial gamma-ray flashes observed by ASIM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19320, https://doi.org/10.5194/egusphere-egu2020-19320, 2020.

Terrestrial Gamma-Ray Flashes (TGFs) observed from space appear to be generated after a few milliseconds of optical activity and before the onset of a main optical pulse. The pre-activity is thought to be from a propagating leader and the main optical pulse the emissions from the ensuing stroke. Scattering of photons in the cloud increases the rise time and durations of the pulses and thus allows for estimates of their optical path from their sources.

In this presentation we estimate the depth inside thunderclouds of pulses associated with more than 100 TGFs observed by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS). The observations are in narrow bands at 337 nm, to include the strongest line of N₂2P and 777.4 nm of OI, considered a strong lightning emission line. With the assumption that the sources are instantaneous and at single points within a cloud, we find optical paths for the events by using typical cloud properties. Combined with cloud top heights from a recent study on TGF producing thunderstorms, this gives an estimate at which altitude the optical detections are produced.

Data from VAISALA’s lightning location network GLD360 and NASA’s Lightning Imaging Sensor on the ISS (ISS-LIS) will be used to assess the results from the optical analysis. This includes investigations of the correlations between TGF durations, detected peak lightning current and optical path in the cloud.

How to cite: Heumesser, M., Chanrion, O., Neubert, T., Dimitriadou, K., Köhn, C., Gordillo-Vazquez, F. J., Luque, A., Pérez-Invernón, F. J., Christian, H., Blakeslee, R. J., Østgaard, N., Mezentsev, A., and Marisaldi, M.: Source Altitudes of Optical Emissions Associated with TGFs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2467, https://doi.org/10.5194/egusphere-egu2020-2467, 2020.

On February 8, 2019 the Atmosphere-Space Interaction Monitor (ASIM) passed a thunderstorm system north east of Puerto Rico and observed a TGF and an Elve from the same lightning stroke at the very beginning of a lightning flash. A second Elve was observed 456 ms later but without any signature of a TGF about 300 km south-east of the first Elve.
The strokes associated with the two Elve events were detected by WWLLN and Vaisala, which allows for an absolute timing accuracy of the ASIM measurements of at least 100 us. Images of the lighting strokes support the source locations for the Elves and TGF.  
Both the rise time of the UV pulse by ASIM MMIA photometer and radio measurements from Puerto Rico indicate that the first stroke was an intracloud positive while the latter was a cloud-to-ground stroke.
The UV emissions from the Elves preceded the optical emissions in 777 nm by
50 us and 90 us, respectively. This can partly be explained by the scattering of 777 nm within the cloud.
Current moments derived from radio measurements at Puerto Rico and Duke University  indicate a fast (30 us) and large (200 kA) current pulse emitting an electromagnetic wave that produces an Elve and a slow (1-2 ms) current producing the optical signals.

How to cite: Ostgaard, N., Cummer, S., Mezentsev, A., Neubert, T., Reglero, V., Olivier, O. A., Marisaldi, M., Kochkin, P., Lehtinen, N., Sarria, D., Maiorana, C., Skeie, C. A., Lindanger, A., Pu, Y., Christiansen, F., Ullaland, K., and Genov, G.: One TGF and two elves produced by the same thunderstorm system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5556, https://doi.org/10.5194/egusphere-egu2020-5556, 2020.

One year after the starting of ASIM operational phase, we have succeeded to perform accurate Imaging of 54 TGF.  Among them, some have been analysed at extreme imaging conditions in terms of TGF position at the MXGS partially coded field of view.  20 TGF events have angular distances larger than 40º respect to the MXGS FOV centre. Extreme cases at angular distances larger than 50º are presented. Validation of TGF position by WLN data is included in the discussion.

The canonical value of 32 LED cnts as the minimum fluency for TGF imaging defined during MXGS development was checked using low luminosity TGF.  At the present, we have succeeded to obtain imaging solution for 7 TGF with less than 20 cnts. A sample is presented with indication of position accuracy and S/N ratios.  

Last part of the presentation is the discussion of a TGF with a very large and asymmetric probability distribution at the MXGS FOV that suggest the TGF as an extended source. Imaging data projected to the Earth surface is compared with GOES data, showing that the TGF is at the edge of a large convective cell, close to the TGF imaging data map.  Therefore, we can conclude that for some bright TGF it is possible to estimate the TGF fireball dimensions generated by the iteration of TGF photons with local atmospheric asymmetric matter distributions. The presence of a large CZT tail is coherent with the size of the convective cell.

How to cite: Reglero, V., Connell, P., Navarro, J., Eyles, C., Ostgaard, N., Neubert, T., Fabro, F., Montanya, J., Mezentsev, A., Chanrion, O., Christiansen, F., Kochkin, P., and Marisaldi, M.: Extreme TGF Imaging by ASIM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8515, https://doi.org/10.5194/egusphere-egu2020-8515, 2020.

Terrestrial Gamma-ray Flashes (TGFs) are short bursts of gamma radiation originating from thunderclouds; they propagate upwards and are then detected by satellites such as AGILE, Fermi and ASIM. ASIM is the first mission specifically designed for the study of thunderstorm-related phenomena (Neubert et al., 2019); being placed on the ISS, it can for the first time detect TGF events up to more than 51 degrees in latitude.

Among the previous missions, RHESSI was the one reaching the highest latitude: 38 degrees. We then consider “high-latitude” for ASIM the band between 35 and 51 degrees of latitude. 9 events have already been observed in this band, inside four distinct geographical regions. At such latitudes, TGFs are expected to experience greater absorption in the troposphere, which makes them more difficult to detect. Moreover, we expect an intrinsically lower production rate due to the lower lightning activity (Smith et al., 2010, Williams et al., 2006).

In this work we present the characteristics of those events, in the context of the global ASIM sample collected so far. We also examine whether the observed number of events is statistically compatible with the atmospheric absorption, taking into account the local flash activity and ASIM’s exposure at high latitude.

How to cite: Maiorana, C., Marisaldi, M., Mezentsev, A., Fullekrug, M., Soula, S., Lindanger, A., Skeie, C. A., Sarria, D., Kochkin, P., Lehtinen, N., Bjørge-Engeland, I., Østgaard, N., Ullaland, K., Genov, G., Neubert, T., Christiansen, F., and Reglero, V.: Observation of TGFs at High Latitude, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9152, https://doi.org/10.5194/egusphere-egu2020-9152, 2020.

In designing the MXGS coded mask imager on the ASIM mission to the ISS many simulations of its performance were made using a model of TGF origin as a RREA in a vertical electric ffield at about 15 km altitude. One consequence was the prediction that imaging scatter background from high energy photons would be 15-20% of CZT detector counts, decreasing with TGF off-axis observation angle.

Analysis of the linear image reconstruction model shows the maximum scatter background to be 40% in some cases, with sources at the same off-axis angle having both small and large scatter background. The obvious reason to explain this asymmetry is that a TGF beam is not primarily vertical, but at large angles, and provokes an inference about TGF origin.

The phenomenon can be explained by TGF origin at the tips of lightning leader channels, resulting in a wide range of random beam angles, or in the macro electric ffield of the induced negative shfielding charge above a stormcloud. This charge might begin as concentrated near the top-centre of a stormcloud but should slowly spread out to form a torus-like charge with the greatest electric field on the circular boundary of the torus, over a range of angles from vertical to horizontal to downward - with many TGFs absorbed or expanding spherically as a low energy Compton Scatter Remant.

In this scenario the TGF would originate near the upper radial edge of the cloud, but not within it, either by lightning leader electron injection or electron positron injection from a cosmic ray shower, posing the question if this location of origin can be observed.

We made a study of over 6000 TGF locations from FERMI-WWLLN observations where the centroid centrepoint of the nearest lightning cluster to the TGF was located, allowing for wind drift, its RMSQ cluster radius determined, and its distance vector from the cluster centrepoint. If the cluster would represent stormcloud location and area, then the macro E-field scenario of TGF origin should result in an annular distribution of the TGF-WWLLN vector location, but convolved with the lightning location error distribution. We present the results here, showing there is indeed a significant increase in TGF origin at the outer boundary of stormcloud lightning clusters.

How to cite: Connell, P.: Using FERMI TGF observation data to show an enhanced likelihood of TGF origin at the edges of stormcloud lightning clusters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10077, https://doi.org/10.5194/egusphere-egu2020-10077, 2020.

Recent measurements by the Atmosphere-Space Interactions Monitor (ASIM) indicate that the production of energetic electrons and of subsequent terrestrial gamma-ray flashes (TGFs) occurs immediately prior to intracloud lightning breakdown. Inspired by this finding, we relate the production of high-energy particles to the occurrence of streamer coronas initiated during the final leader step when the leader is in the vicinity of the upper cloud charge layer. Therefore, we model the acceleration of electrons and the subsequent production of energetic photons in the electric fields of the two encountering streamer coronas which are initiated in the vicinity of the leader tip and of the charge layer. Applying a particle Monte Carlo code, we first initiate thermal electrons in the electric field of the leader tip and subsequently turn on the streamer coronas and simulate the acceleration of electrons from thermal energies to energies of several tens of MeV. We present the temporal evolution of the electron and photon energies and spectra, and discuss the role of the electric fields of the encountering streamer coronas. Finally, we relate our results to ASIM measurements and discuss the duration and the relative timing of TGF bursts.

How to cite: Köhn, C., Chanrion, O., Matthias, H., Dimitriadou, K., and Neubert, T.: Modelling the production of terrestrial gamma-ray flashes during the final leader step, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4645, https://doi.org/10.5194/egusphere-egu2020-4645, 2020.

We will present maps of negative leaders imaged in the 30-80 MHz band by the LOFAR radio telescope, which is a distributed radio telescope in the Northern Netherlands that can map lightning with meter length and nanosecond timing accuracy. These VHF images show that negative leaders emit bursts of VHF that are about 1-3 µs in duration, most likely in relation to leader stepping. The median time between bursts is around 40 μs, and the median distance is about 7.5 m. Each of these bursts contains around 3-10 discrete VHF pulses. 2/3 of these pulses are consistent with coming from the same location (with 1 meter location accuracy), and the other 1/3 come from up to 3 m away. These data are consistent with the hypothesis that these VHF bursts are due to corona flashes during leader stepping, that the discrete pulses we locate are due to the few very strongest streamers in the corona flash, and the majority of streamers in a corona flash are too weak to be observed as discrete VHF pulses. From these data, we estimate that the strongest streamers in a natural corona flash emit about 4x10^-6 J in our 30-80 MHz band.

How to cite: Hare, B., Scholten, O., Dwyer, J., Ebert, U., and Nijdam, S. and the LOFAR CR KSP: Meter-scale Measurements of VHF structure of natural leader streamers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7957, https://doi.org/10.5194/egusphere-egu2020-7957, 2020.

While the spectrum of lightning electromagnetic radiation is known to peak around 5-10 kHz in the very low frequency (VLF) range, intense high frequency/very high frequency (HF/VHF) radiation can be produced by various lightning related processes. In fact, thunderstorm narrow bipolar events (NBEs), which are capable of initiating lightning, are the most powerful HF/VHF sources in nature on Earth. But even for NBEs, the spectral intensity in HF/VHF is still many orders of magnitude weaker than that of lower frequencies (Liu et al., JGR, 124, https://doi.org/10.1029/2019JD030439, 2019). HF/VHF bursts with weak VLF signals, however, can also be produced by thunderstorms. These bursts may be related to the thunderstorm precursor events noted by Rison et al. (Nat. Commun., 7, 10721, 2016) and are also found to precede a large fraction of lightning initiation (Lyu et al., JGR, 124, 2994, 2019). They are also known as continual radio frequency (CRF) radiation associated with volcanic lightning (Behnke et. al., JGR, 123, 4157, 2018).

In this talk, we report a theoretical and modeling study to investigate a physical mechanism for production of those HF/VHF bursts. The study is built on the theory developed recently concerning the radio emissions from an ensemble of streamers (Liu et al., 2019). We find an ensemble of streamer discharges that develop in random directions can produce HF/VHF radiation with intensity comparable to those all developing in a single direction, but the VLF intensity is many orders of magnitude weaker. The results of our study support the conclusions of Behnke et. al (2018) that CRF is produced in the absence of large-scale electric field, it results in insignificant charge transfer, and it is caused by streamers. In the context of the HF/VHF bursts preceding lightning initiation (Lyu et. al, 2019), our results imply that highly localized strong field regions exist in thunderstorms and streamers take place in those regions, which somehow precondition the medium for lightning initiation.

How to cite: Liu, N. and Dwyer, J.: Investigating Thunderstorm HF/VHF Radio Bursts with Weak Lower Frequency Radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20497, https://doi.org/10.5194/egusphere-egu2020-20497, 2020.

It’s a common knowledge for the spark discharge researches that there are space leaders inside the negative leader streamer zone. They arise from plasma formations of the volume of about 1 cm³ which are called space stems. But there is no any established idea about how space stems form in conditions when the background electric field magnitude inside a negative leader corona is about three times less than the dielectric strength of air. In this study, we propose a new mechanism of space stem precursors (ionization centers, which are capable to generate positive streamers) formation which is based on the joint action of ionization and drifting processes. The most possible location of proposed mechanism realization is the external boundary of the negative corona streamer burst, where electric field strength reaches a maximum value. The process takes place in the presence of strongly inhomogeneous stochastic electric field relief, which is formed by chaotically positioned clusters of negative charge transported to the negative corona streamer burst periphery by the negative streamer heads. The last are emanated from the leader tip during the negative corona streamer burst finishing each step-formation process. The only thing needed for the space stem precursor formation is the increased level of streamer heads spatiotemporal appearance frequency inside the very small area of space, which scale is of the order of a few millimeters. One important conclusion derived from this study is that the relatively strong electric field strength, overabundance of negative charge, and increased level of both reduced electric field and detachment frequency, which accompany ionization center formation, facilitate survival and growth of positive streamers initiated from a space stem precursor. The model is applied to specify the range of conditions, under which space stem precursor genesis is possible, and to analyze times of its formation at the range of altitudes of 0-12 km.

This work was supported by the Russian Science Foundation (project 19-17-00183).

How to cite: Syssoev, A. and Iudin, D.: A possible mechanism of space stem precursors formation at the negative lightning leader corona streamer burst periphery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11022, https://doi.org/10.5194/egusphere-egu2020-11022, 2020.

Thunderstorm ground enhancements (TGEs) are events of energetic particle flux increases, discovered and observed at the Aragats Research Station (Armenia). Energetic particles are accelerated and multiplied in the electric field of clouds, and may be registered by ground-based detectors. Analysis of the structure of thunderclouds producing TGEs is crucial for clarifying the mechanism of particle acceleration.

In the present study the hydrometeor dynamics are analysed on the basis of the state of the atmosphere modeling by means of Weather Research and Forecasting Model. Meteorological characteristics typical of TGE occurrence in the mountainous region of Aragats are discovered. A technique has been developed for estimation of the charge distribution in a cloud on the basis of comparison of the simulations and experimental data. The retrieved cloud electrical structure is used to estimate the dependence of the electrification process on the temperature and liquid water content.

An unusually low concentration of ice particles leads to the great importance of snow particles in the process of charge separation. A typical charge distribution in a TGE-producing cloud is found to be well approximated by a two-layered charge structure with a lower positive charge region formed by graupel particles and an upper negative region formed by snow particles. Characteristic charge density is 0.01 C/km^3 for graupel cluster and 0.02 C/km^3 for snow cluster. A vertical distance of about 1-2 km between the lower positive and upper negative layers is sufficient for the development of an energetic particle avalanche.

The obtained estimation of the hydrometeor content and the electrical structure of a TGE-producing cloud provides new evidence on particle acceleration mechanisms in the atmosphere and processes of charge distribution in mountainous conditions.

How to cite: Svechnikova, E., Ilin, N., and Mareev, E.: Meteorological Parameters of Thunderstorm Ground Enhancements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-699, https://doi.org/10.5194/egusphere-egu2020-699, 2020.

Many phenomena of atmospheric electricity are still not well understood, as most of the processes involved can only be observed in real nature. For this purpose, reliable and stable measurements of the electric field strength are mandatory. While for high-frequency fields, there exists a large variety of equipment, in the quasi-static and especially static regime, such systems are scarce. The „standard“ device for the application is the electrostatic field mill which uses a rotating, electrically grounded shutter electrode to alternatingly expose and shield measurement electrodes to/from the electric field. While they achieve good-enough resolution, there are many inherent problems associated with the measurement principle, such as mechanical wear, massive field distortions, size and weight. As a consequence, they are typically installed at a fixed points and cannot be easily moved or mounted. Miniaturised field mills have minimised some of these issues, the shutter priniciple leads to very fragile structures.

We present an alternative way of measuring low-frequency and static electric fields (E-field), which does not suffer from the hindering drawbacks of field mills. The underlying mechanism converts the E-field to a mechanical oscillation of a microelectromechanical system (MEMS). This is achieved by applying an AC voltage to a compliant mechanical structure. As a result of the AC voltage, alternating charges accumulate at the surface of the MEMS. When exposed to the E-field, this leads to a force deflecting the structure at a known frequency. For this kind of active mechanism, the power consumption is minimal, since the current flow is practically zero. Therefore, the system can be used in a floating way without grounded connections and therefore minimum field distortions. The mechanical motion can then be read out optically, also to avoid field distortions and backaction. If the system is driven at the mechanical resonance, the quality factor can be exploited to boost the sensitivity. In this case the bandwidth of the system ranges from 0 Hz to twice the resonance frequency.

Several MEMS sensors with different resonance frequencies (ranging from ~100 Hz to ~1 kHz) have been fabricated and tested in the laboratory. The sensors have been mounted between two parallel field plates supplied with a DC voltage, which provides the static electric field. A tiny hole in one of the field plates allowed for optical readout of the sensor movement with a laser-Doppler vibrometer (Polytec MSA-400). The sensors have been tested for different field strengths (10 V/m – 30 kV/m) and different AC voltages (0.02 V – 20 V) confirming linearity in both quantities. In terms of field strength, a resolution as good as ~25 V/m was achieved for a sensor with a resonance frequency of 167 Hz. These promising results substantiate that this sensor is a potentially low-weight, low-cost alternative for classical field mills. The next steps will be to investigate long-term stability and environmental effects on the sensor (temperature, humidity) and, finally, installation and test in the open area during fair weather and thunderstorm activity.

How to cite: Kainz, A., Hortschitz, W., Kahr, M., Keplinger, F., and Diendorfer, G.: Microsensor for Atmospheric Electric Fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8562, https://doi.org/10.5194/egusphere-egu2020-8562, 2020.

Cloud processes leading to rainfall generation are suspected to be influenced by droplet charge. Droplet charging is abundant, and even in layer clouds, charging of droplets readily occurs at the horizontal cloud-air boundary. Droplet charging in such circumstances is proportional to the vertical current driven through the cloud by the global electric circuit. Small global circuit variations from natural influences, such as solar modulation of cosmic rays can be used to investigate this, but an alternative is presented by artificial introduction of ionisation. The atmospheric nuclear weapons test period, which reached its peak 1962-1964, caused exceptional anthropogenic disturbance to the global circuit, through the increased ionisation from steady sedimentation of stratospheric radioactive debris.

Measurements of the vertical current J_z made at Kew Observatory near London (51°28′N, 0°19′W) were several times greater than normal during 1962-1964, as a result of the widespread extra ionisation in the lower atmosphere. At Lerwick, Shetland (60°09′N, 1°08′W) where deposition of radioactive material occurred, the atmospheric electrical parameters were strongly affected by the enhanced ionisation. To investigate tropospheric ionisation effects on local cloud processes, rainfall days at Lerwick in 1962-64 have been analysed by considering reduced and enhanced ionisation periods. During the enhanced ionisation, the Lerwick rainfall distribution shifted towards heavier rainfall and is significantly different from the rainfall distribution for reduced ionisation days; the Lerwick cloud was also significantly optically thicker during the enhanced ionisation. This contrasts with other years of the Kew record, when J_z was relatively undisturbed. Whilst the ionisation conditions of 1962-64 were exceptional, controlled methods of enhancing tropospheric ionisation by non-radioactive means - such as corona emission - may nevertheless be promising for local rainfall modification, or even geoengineering of cloud properties.

How to cite: Harrison, R. G., Nicoll, K., Ambaum, M., Marlton, G., Aplin, K., and Lockwood, M.: Ionisation effects on precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10160, https://doi.org/10.5194/egusphere-egu2020-10160, 2020.

We present here the study of six hail events and five snow events in Xanthi, N. Greece, on Potential Gradient (PG). All hail events occurred in the spring-summer season of the years 2011-2018. A decrease in PG has been observed which has been around 2000-3000 V/m during the three hail events which occurred concurrently with rain. In three events with no rain, the decrease has been varying between 60 and 6000 V/m. In the case of only 60 V/m drop, no concurrent drop in temperature has been observed, while for the other cases it appears that for each degree drop in temperature the drop in PG is 1000 V/m, hence it appears that the intensity of the hail event regulates the drop in PG, although we do not have hail amount measurements to validate this. Regarding snow events,  the situation is more complicated, with PG fluctuating rapidly between high positive and high negative values. We present also a preliminary study of the impact of PM1.0 and PM2.5 on PG from measurements performed during 2019. We acknowledge support of this work by the project “PANhellenic infrastructure for Atmospheric Composition and climatE change” (MIS 5021516) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

How to cite: Kourtidis, K., Karagioras, A., Papadopoulou, E., Mihalopoulos, N., and Stavroulas, I.: A study of the effects of hail, snow and PM on Potential Gradient, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1298, https://doi.org/10.5194/egusphere-egu2020-1298, 2020.

At about 22:43:30 BJT (Beijing Time = UTC + 8) on 13 August 2016, two amateur astronomers in Shikengkong, Guangdong province, and Jiahe County, Hunan province, respectively, fortunately captured a gigantic jet (GJ) event simultaneously and the GJ exact location could be triangulated. The parent thunderstorm was in a very humid environment [Precipitable Water (PWAT) in excess of 60 mm], featuring high convective available potential energy (CAPE) and weak 0-6 km vertical wind shear. The GJ occurred in the region with the coldest cloud top brightness temperature of −64 °C, suggesting the GJ was associated with strong vertical development of the thunderstorm. Vertical cross sections of radar reflectivity also show that the GJ occurred near the thunderstorm strong convection region as indicated by the results that a region of 25 dBZ (and 35 dBZ) in excess of the local tropopause (overshooting top in the parent thunderstorm) during a time window containing the GJ. The negative cloud-to-ground flashes dominated during the thunderstorm evolution. Three positive narrow bipolar events (NBEs) were detected within 30s before and after the GJ. It indicates that the NBEs were distributed between 11 and 13 km and occurred in the upper and middle layers of thunderstorm with radar reflectivity of 30-35 dBZ.

How to cite: Yang, J.: Analysis of a gigantic jet in southern China: morphology, meteorology, storm evolution, lightning and narrow bipolar events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1812, https://doi.org/10.5194/egusphere-egu2020-1812, 2020.

Lightning is a common high-energy phenomenon. In particular, cloud-to-ground lightning (CG lightning) generates shock wave and electrical discharge on the ground and forms the associated geological evidence including melting and shock lamella on rocks, termed fulgurites. Because lightning strikes on different protolith (cohesive or non-cohesive rocks), Pasek et al. (2012) divided the fulgurites into four types: (I) sand fulgurites, (ii) soil/clay fulgurites, (iii) calcic-soil fulgurites, and (iv) rock fulgurites. Compared with the reported fulgurites derived from non-cohesive rocks, the recognition of rock fulgurites was rare and remains unclear. Here we report the detailed characterization of rock fulgurites formed in a very recent CG lightning event with microanalytical methods including optical microscope, Field-Emission Scanning Electron Microscope (FESEM), Transmission Electron Microscope (TEM), regular and synchrotron X-ray Powder Diffraction (XRD), and Raman spectroscope. We also provide a CG lightning energy dissipation model constrained by the observed current values. The CG lightning event (the current value is ~ 162 kA) took place on granitic gneiss in Kimen county, Taiwan, on May. 7th, 2018. Our results show that the rock fulgurites were characterized with a black-to-brown thin (~10 μm in thickness) glassy crust with some vesicles covering on the host rock. Hydrous sulfates, including jarosites and gypsums, were recognized to locally deposit on fulgurites, likely suggesting the presence of hydrothermal condition in near-surface exposures after the cessation of the CG lightning. Planer deformation features derived from high pressures (up to several GPa) were found in k-feldspar located beneath the glassy crust, suggesting the presence of shock waves also on the surface. In addition, the estimated melting energy for the observed fulgurite (~20 m² in area with the thickness of 100 μm) is much less than one one-hundredth of the observed CG lightning. It supports the previous studies that documented most of the electrical discharge was dissipated into ground. Our study establishes a reference rock fulgurites data originated from CG lighting on granitic rocks set for future on-site drilling and presents an application of these data for studies of ancient rock fulgurite relicts.

How to cite: Ting-Ju, M., Li-Wei, K., Chien-Chih, C., Wen-Jeng, H., and Tze-Yuan, C.: The mineralogical, microstructural, chemical characteristics of Recently Formed Fulgurite in Kinmen, Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3264, https://doi.org/10.5194/egusphere-egu2020-3264, 2020.

Natural and artificial lakes are able to change the climate of their surroundings. These modifications are collectively known as lake effects and range from microscale to synoptic scale. The presence of the lake can cause negative effect on the local thunderstorm activity in summertime decreasing the convection and precipitation over lakes due to the greater stability created by the lower atmosphere and the colder surfaces of the lake [1, 2]. However, it also can have a positive impact on thundercloud generation when the temperature difference between air in 850 mb height and near earth's surface is more than 13 C causing instability in the atmosphere [3].

The main objective of the present study is to investigate the impact of Lake Fertő (Neusiedler See, located in Hungary and Austria) on local thunderstorm activity by applying statistical analysis on meteorological and lightning data and event studies. Data of the Blitzortung lightning location network, local meteorological data (temperature, precipitation) measured at stations around the lake, water temperature measured at Fertőrákos and temperature measured at 850 mb in Vienna station were used for the analysis. The local thunderstorm activity was investigated during summertime (May - September) in 2015, 2016 and 2017. Lightning distribution maps above and around the lake for the investigated period have been determined based on the Blitzortung data.

According to the lightning distribution maps we can not observe any positive impact of the lake on the lightning activity when water temperature was higher than the air temperature around the lake. Furthermore, we can not conclude that there is a clear negative effect of the lake on the lightning activity based on the lightning distribution maps when the air temperature is higher than the water temperature. Nevertheless, there are some months when it seems a clear border between the lightning activity measured above the lake and at the coast (e. g. in June and July 2015, June 2016). The negative effect also seems to appear in some cases of the investigated local individual thunderstorms, namely the thunderstorm activity is larger above the surrounding surface than directly above the lake. This seems to strengthen the hypothesis that "Deep convection is not often formed in summer above the lakes, and existing storms dissipate significantly when moving above the lakes due to the greater stability created by the lower atmosphere and the colder surfaces of the lake" [1].

[1] Lyons, W. A., Some effects of Lake Michigan upon sqall lines and summertime convention. Proc. 9th Conf. Great Lakes Research, Great Lakes Res. Div. Publ. No. 15, University of Michigan, 259–273, 1966

[2] Scott, R. W., & Huff, F. A. . Impacts of the Great Lakes on Regional Climate Conditions. Journal of Great Lakes Research, 22(4), 845–863., 1996

[3] Wilson, J. W. : Effect of Lake Ontario on precipitation. Mon. Wea. Rev. 105, 207–214., 1977

How to cite: Karapetyan, G. and Barta, V.: Investigation of the lake-effect on the local thunderstorm activity around the Lake Fertő, Hungary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10387, https://doi.org/10.5194/egusphere-egu2020-10387, 2020.

The electrification of mineral sand/dust particles during aeolian processes is a well-documented phenomenon both in natural settings and in laboratory experiments. When in motion, small airborne dust particles collide with other suspended particles or impact the surface through the kinetic energy they acquire from the ambient wind. Field experiments will be conducted in conjunction with the AMEDEE-2020 Analog Mars Mission, planned for November 2020 in the Ramon Crater in southern Israel and led by the Austrian Space Forum. During SANDEE, we will deploy a portable wind-tunnel (Katra et al., 2016) at the site, and record particle movements in conditions that simulate sand storms of varying speeds. We will use local Negev desert, as well as Mars-simulant, soil samples that will be placed inside the wind-tunnel. We will measure particles' dynamic, mineralogical and electrical characteristics as they are blown by wind inside the tunnel.  A JCI 114 portable electric field detector will be used to to measure the amplification of the ambient electric field during sand movement. A vertical array of traps oriented along the wind direction will be used for sampling particles, in order to calculate the related sand fluxes and to analyze particle characteristics. The experiment will be repeated at night under dark conditions, in order to observe if light is emitted from electrified dust, due to corona discharges.

We expect that SANDEE will help decipher wind-speed/aerosol/electrical charge relationships. These have practical implications for future Mars landers, because airborne sand particles are likely to interfere with communications and also to impede the energy output of solar panels due to the electrical adhesion of charged aerosol.

How to cite: Katra, I. and Yair, Y.: The SANDEE campaign: Electrical effects during sand transport by aeolian processes in the Negev desert and implications for Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20343, https://doi.org/10.5194/egusphere-egu2020-20343, 2020.

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

How to cite: Yair, Y., Lynn, B., Ziv, B., and Yaffe, M.: Lightning super-bolts in Eastern Mediterranean winter thunderstorms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1788, https://doi.org/10.5194/egusphere-egu2020-1788, 2020.

About 5% of the wildfires in the Mediterranean basin are produced by lightning [1]. Lightning-ignited fires tend to occur in remote areas and can spread significantly before suppression. The occurrence of lightning-caused fires is closely related with intense drought periods and high temperatures [2]. Therefore, drier conditions and higher temperatures in a changing climate are expected to lead to a future increase in lightning-ignited fires occurrence. The development of a lightning-ignited fire parameterization for Earth system models arises as a necessary tool to predict the future occurrence of these extreme events and to study their impact on atmospheric chemistry.

Long Continuing Current lightning (LCC-lightning), preferable taking place in dry thunderstorms, is believed to be the main precursor of lightning-ignited fires. This was originally proposed by McEachron and Itagenguth in 1942 [3] working with laboratory sparks, which suggested that ignition by natural lightning is usually caused by a discharge having an unusual long-continuing current phase. Later in 1967 this hypothesis was confirmed by Fuquay et al. [4].

In this work, we analyse three fire databases of lightning-ignited fires in Spain, Portugal and Southern France between 2009 and 2015. Furthermore lightning measurements from the World Wide Lightning Location Network (WWLLN) and the Earth Networks Total Lightning Network (ENTLN), and land and atmospheric variables from the new ERA-5 reanalysis are combined to investigate the electrical characteristics and environmental conditions of the fires. This preliminary data analysis will be useful to set new relationships between the characteristics of thunderstorms and the initiation of wildfires. It is the first step towards the development of a detailed lightning-ignited fire parameterization for the atmospheric chemistry-climate model EMAC.

[1] Vázquez, A., and Moreno, J. M. (1998). Patterns of lightning-, and people-caused fires in peninsular Spain. International Journal of Wildland Fire, 8(2), 103-115.

[2] Pineda, N., and Rigo, T. (2017). The rainfall factor in lightning-ignited wildfires in Catalonia. Agricultural and Forest Meteorology, 239, 249-263.

[3] McEachron, K. B., and Itagenguth, J. It (1942), Effect of lightning on thin metal surfaces, AIEE Trans., 61, 559-564, 1942.

[4] Fuquay, D. M., Baughman R. G, Taylor, A. R. and Hawe, R. G. (1967). Characteristics of seven lightning discharges that caused forest fires. Journal of Geophysical Research, 72 (24).

How to cite: Pérez-Invernón, F. J., Huntrieser, H., Soler Lopez, S., Gordillo-Vázquez, F. J., Navarro-Gonzalez, J., Reglero, V., Montanyà, J., and van der Velde, O. A.: Electrical characteristics and environmental conditions of lightning-ignited fires in the Iberian Peninsula and Mediterranean France between 2009 and 2015, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13729, https://doi.org/10.5194/egusphere-egu2020-13729, 2020.

Under future climate uncertainties, a better understanding of wildfires is necessary both from physical and operational points of view, which are the goals of the CILIFO (Centro Ibérico para la Investigacion y Lucha contra Incendios Forestales) Interreg POCTEP project. Among several sources of fire ignition, lightnings are the main natural source of wildfires and an important contributor to burned areas in many regions. In 2017, devastating forest fires were reported in Portugal. The fires near Pedrógão Grande created a huge wall of flames, killing at least 60 people. The goal of this study is to discuss the atmospheric conditions that were supportive of lightning flashes to cause a fire during this event, as well as to check the possibility to correctly diagnose cloud-to-ground flashes using high resolution simulations with the non-hydrostatic atmospheric Meso-NH model. A set of meteorological data was used to validate the model results and to describe the prevailing atmospheric environment during the afternoon of 17th June 2017 over central Portugal. The Portuguese Institute for Sea and Atmosphere (IPMA) provided the data for this study. The Meso-NH model was configured in order to provide an explicit representation of the clouds and their electrical activity, through the activation of the CELLS electrical scheme. The ICE3 microphysical scheme predicts the mixing ratio of six atmospheric water categories. The Meso-NH system also includes a grid point radar diagnostic given by the total equivalent radar reflectivity, as well as a Plan Position Indicator (PPI) that is a representation mode in which sweeping cones are projected on a horizontal plane determined by scanning the atmosphere at constant elevation. The description of the electrical state of a thunderstorm is based on the monitoring of the electrical charge densities, the computation of the electric field and the production of lightning flashes. The cloud charging involves mostly the non-inductive mechanism, and both Intra-Cloud (IC) and Cloud-to-Ground (CG) flashes are considered. The CELLS scheme provides a realistic representation of the electrical properties of precipitating cloud systems. The simulation was carried out with two nested domains of 4 km and 1 km horizontal resolution. Concerning the atmospheric conditions, the dry thunderstorm environment configured a perfect scenario for the natural ignition and evolution of some fires, since lightning activity came from high-base thunderstorms with relatively dry air at lower levels favouring the evaporation of rain before it reaches the ground, as well as intense outflows. Therefore, the fires on 17th June 2017 occurred in an exceptional hot day, with fire ignitions in places with complex terrain and a favourable vegetation state producing uncontrolled wildfires. The spatial distribution of the simulated CG lightnings showed a good agreement with the lightning strokes obtained from the national lightning detection network. Besides the identification of favourable conditions for the occurrence of wildfires, this study introduces a possible application of the Meso-NH electrical scheme, namely the study of forest fire ignition by lightning strokes.

How to cite: Couto, F. T., Iakunin, M., Salgado, R., Pinto, P., Viegas, T., and Pinty, J.-P.: Modelling dry thunderstorm environment during a wildfire episode in Portugal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10796, https://doi.org/10.5194/egusphere-egu2020-10796, 2020.

Schupy is an open-source python package aimed at modeling and analyzing Schumann resonances (SRs), the global electromagnetic resonances of the Earth-ionosphere cavity resonator in the lowest part of the extremely low frequency band (<100 Hz). Its very-first function forward_tdte applies the solution of the 2-D telegraph-equation introduced recently by Prácser et al. (2019) for a uniform cavity and is able to determine theoretical SR spectra for arbitrary source-observer configurations. It can be applied for modeling both the amplitude and phase of extraordinarily large SR-transients and the power spectral density of SRs excited by incoherently superimposed lightning strokes within an extended source region.

In this contribution, test results of planned new functionalities of the package are presented. A new function aims at removing sections of the measured data, e.g. Q-bursts, which bias spectral characteristics of  natural “background” electromagnetic noise. This way, PSD will be calculated from a sanitized time series. Other new functions are introduced for determining the spectral parameters (amplitude/intensity, frequency, Q-factor) of SR modes using different approaches, i.e., symmetrical and asymmetrical Lorentzian fitting, complex demodulation, and the weighted average method. We would like to encourage the community to join our project in developing open-source modeling and signal analyzing capacities for SR research as part of the schupy package.

How to cite: Bozoki, T., Satori, G., Pracser, E., Bor, J., Szabone Andre, K., Rodríguez-Camacho, J., Dalya, G., and Neska, M.: Schupy: a python package for modeling and analyzing Schumann resonances, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4872, https://doi.org/10.5194/egusphere-egu2020-4872, 2020.

Measured time series of the extremely low frequency (ELF, 3 Hz-3 kHz) band electromagnetic field can be considered as a superposition of background and transient signals. Transient signals produced by exceptionally powerful lightning strokes far from the recording station are named Q-bursts. The direction of the source lightning stroke at the recording station can be calculated using the horizontal components of the Poynting vector. The source lightning stroke can be identified in the lightning database of the World Wide Lightning Location Network (WWLLN, wwlln.net) by the matching detection time and direction calculated from ELF measurements.

Schumann resonance (SR) peaks appear at ~8Hz, ~14Hz, ~20 Hz, etc., in the spectra computed from the background ELF timeseries. SRs are natural electromagnetic resonances with wavelengths comparable to the circumference of the Earth-ionosphere waveguide. Peak amplitudes and frequencies in the resonance spectrum detected in the ELF band at any given location on the Earth depend on the distribution and intensity of the global lightning activity which excites SR.

ELF measurements are routinely performed in the Széchenyi István Geophysical Observatory (NCK, 47°38' N, 16°43' E) near Nagycenk, Hungary. Vertical electric and the horizontal magnetic components of the atmospheric electromagnetic field are monitored by the Schumann resonance recording system. In this work, we study the variation of the number of lightning strokes with high charge moment change (CMC; indicated by the number of large amplitude Q-bursts recorded at NCK) and the variation of the number of lightning strokes with large peak current (indicated by the number of WWLLN-detected energetic lightning strokes). In addition to considering the total number of WWLLN-detected lightning strokes and Q-bursts, we analyze lightning strokes occurring  only in west, south, east, and north directions from NCK, corresponding predominantly to the three main lightning producing regions of the tropical lands in America, Africa, and Indonesia as well as to the Pacific Ocean. Time variations of the number of high CMC and large peak current lightning strokes during November, 2014 are compared with time variation of the cumulative SR intensity detected at NCK station in the vertical electric field component in the same month. Similarities and differences in the time variations of the considered quantities are discussed in order to show how these indicators mirror the changing distributions of the global lightning activity.

How to cite: Szabóné André, K., Bór, J., Sátori, G., Bozóki, T., and Steinbach, P.: Comparison of global lightning activity variations inferred from Q-bursts, Schumann resonances, and WWLLN-detected lightning strokes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13501, https://doi.org/10.5194/egusphere-egu2020-13501, 2020.

Since April 2018, the Atmosphere-Space Interactions Monitor (ASIM) has been in operation on board the International Space Station (ISS). ASIM is composed of the Modular X-and Gamma Ray Sensor (MXGS) as well as a multispectral and high resolution array of photometers and cameras, called the Modular Multispectral Imaging Array (MMIA). These instruments allow us to investigate Terrestrial Gamma-Flashes, Transient Luminous Events and their interactions with thunderstorms and lightning flashes.

The Colombia Lightning Mapping Array (COL-LMA), operational since 2017, is the first VHF range network installed and working in a tropical region, and can contribute to the electrical understanding of thunderstorms and lightning leader processes associated with high energy phenomena in the upper atmosphere.

This work employs data from the MMIA array to investigate optical emission patterns at different bands (337 nm, 180-230 nm and 777.4 nm) caused by lightning leader development and cloud-to-ground flashes, derived from the COL-LMA and LINET network respectively. All cases are also correlated with optical observation from the Lightning Imaging Sensor (LIS) on board the ISS, and the Geostationary Lightning Mapper sensor on the GOES-R satellite.

The region of study is defined by the high detection-efficiency area of the COL-LMA around the Magdalena river valley. MMIA-ASIM information since July 2019 corresponding to passes over this tropical region has been analysed.

How to cite: López, J. A., Montanyà, J., van der Velde, O., Fabró, F., Navarro, J., Reglero, V., Chanrion, O., Neubert, T., Dimitriadou, K., and Østgaard, N.: Simultaneous detection of lightning flashes by MMIA-ASIM and Colombia Lightning Mapping Array, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21858, https://doi.org/10.5194/egusphere-egu2020-21858, 2020.

A Mesoscale Convective System (MCS), consisting of three Super Cells
formed over South-east Indian, is assessed in detail with satellite and ground based
data-sets. The MCS under investigation generated a total of Ten (10) upward
electrical discharges (9 Sprites and 1 Gigantic Jet) commonly named as Transient
Luminous Events (TLEs). The TLEs were recorded from TLE observation station
located at Allahabad, India. The event occurred in the Post-Monsoon period of 2013
on October 7, during 15-23 UT hours. The MCS was spread over a region of 25000 sq.
Kilometers. A lowest cloud top temperature value of -84.7 0 C was observed in the
mature stage of the MCS, during 2130 UT hours, and the cloud top altitude was
reaching 17.6 km. The coldest cloud top region was covering an average area of
13000 sq. Km. The measured Convective Available Potential Energy (CAPE) value was
606.9 J/kg at 00 UT on 7 th October which dropped to 211 J/kg at 00 UT on 8 th
October. The mean lightning flash rate during the formation and maturity stages of
the MCS was around 46.03 min -1 . During the entire lifespan of the thunderstorm,
peak currents were found to be reaching ±400 kA. Such high electric currents,
extreme cold temperature and towering altitudes of the convective complexes show
how much a MCS is dynamically active and the TLEs which it produced are known to
electrically connect the lower atmosphere to the upper space environment.

How to cite: Moharana, S. S. and Singh, R.: Probing a post monsoon Mesoscale Convective System (MCS) and the generated Transient Luminous Events (TLEs) over Indian Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19407, https://doi.org/10.5194/egusphere-egu2020-19407, 2020.

Using 5 years of operational Doppler radar, cloud-to-ground lightning observations and NECP reanalysis data, this study, for the first time for such a purpose, examines the spatial and temporal characteristics of and correlations between summer storm and lightning over the Yangtze-Huaihe River Basin (YHRB), with a special emphasize on their diurnal cycles. The sub-seasonal variability of the lifetime, storm top, max reflectivity and cell-based vertical integrated liquid (VIL) water of storms are also investigated using the Storm Cell Identification and Tracking algorithm. Results show that storms over YHRB occur most frequently during the Meiyu period. Storms are largely associated with Meiyu fronts during the period and show a fast-moving speed and moderate intensity (proxies including storms top, max reflectivity and VIL). The diurnal variations of storms embedded in Meiyu front are weak. The storm intensity becomes much stronger in the post-Meiyu period due to the increased atmospheric instability. Higher occurrence frequency of CG lighting can also be found during the post-Meiyu period. The diurnal cycles of storm and CG lightning in the post-Meiyu period show a unimodal pattern with an afternoon peak corresponding to solar heating effect. An inverse correlation between the lightning numbers and the mean value of peak current (MPC) for the negative CG lightning is found during the pre-Meiyu and Meiyu periods. The diurnal variation of MPC for the negative CG lightning agrees well with the storm intensity to some extent.

How to cite: Yang, J.: The diurnal cycle of lightning and storms during the pre-Meiyu, Meiyu and post-Meiyu period over Yangtze-Huaihe River Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12263, https://doi.org/10.5194/egusphere-egu2020-12263, 2020.

The vertical atmospheric potential gradient is particularly affected by high aerosol loading in cities as the air’s conductivity is reduced through aerosol attachment of free ions. The reduction of ion concentrations decreases the conductivity and, as the air-earth current remains constant, the potential difference increases. Aerosol size distributions can be affected by the relative humidity dependent on the aerosol hygroscopicity, if an aerosol is sufficiently hygroscopic, it will grow as humidity increases. As larger aerosols are, in principle, more prone to effectively scavenge ions, an increase in relative humidity may increase the size of hygroscopic aerosols, decrease ion concentrations and hence increased measured potential gradient. Measurements of atmospheric potential gradient in Lisbon, Portugal, demonstrated an increase in potential gradient associated with increasing relative humidity (in the range 60-90%), mainly for wind directions corresponding to marine air.

A JCI 131 field mill (Chilworth) and Maximet 500 (Gill) weather station were positioned on the roof of the University of Bristol School of Chemistry between May and September 2016. Particle number concentration was determined using a condensation particle counter (TSI 3010) with an upper limit of 10,000 particles cm^-3. A dilution system was put in place to increase this range to 14,000 cm^-3. The same field mill and weather station were used in Thailand. Measurements at 1 Hz (averaged to 1-minute samples) were taken on the roof of a 6-floor building, approximately 100 m from a busy toll road in Lak Si, northern Bangkok. Aerosol concentrations were taken with a Condensation Particle Counter (Grimm Aerosol Technik) at the same height. The measurement period began on March 8th 2018 after which there were 8 weeks of particle number count data.

In the Bristol measurement between 50% and 80% relative humidity, the median potential gradient increased, but above this it sharply decreases, which may be due to disturbed weather at the highest humidities. Initial analysis of the relationship between relative humidity and potential gradient in Bangkok shows a decrease in median potential gradient as relative humidity increases. This may be due to a large proportion of traffic related aerosol which could be less hygroscopic, but the potential for effects of disturbed weather and traffic to mask hygroscopic effects will be considered.

How to cite: Wright, M., Matthews, J., Silva, H., Navasumrit, P., Ruchirawat, M., and Shallcross, D.: Relationship between aerosol concentration, relative humidity and atmospheric electric potential gradient in cities , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21927, https://doi.org/10.5194/egusphere-egu2020-21927, 2020.

Using the Modular X- and Gamma-ray Sensor (MXGS) and the Modular Multi-spectral Imaging Array (MMIA) of the Atmosphere-Space Interactions Monitor (ASIM), we investigate the time sequence of the Terrestrial gamma-ray flashes and the optical emissions from the associated lighting. A common observation in the ASIM data is that the TGFs are observed before or during a weak increase in the optical signals in 337 nm and 777.4 nm, and prior to- or at the onset of the main optical pulse. Using data from the MXGS and MMIA instruments for the period from April 2019, we assess the time sequence and the relationship between the observed TGF duration and the time between the onset of the TGF and the onset of the main optical pulse, with a relative timeing uncertainty of +/- 5 µs. The data prior to April 2019 is presented in Bjørge-Engeland et al.

How to cite: Skeie, C. A., Østgaard, N., Bjørge-Engeland, I., Mezentsev, A., Neubert, T., Reglero, V., Marisaldi, M., Kochkin, P., Lehtinen, N., Sarria, D., Maiorana, C., Lindanger, A., Ullaland, K., Genov, G., Heumesser, M., Christiansen, F., and Chanrion, O.: Assessing the relationship between the TGF durations and the onset times of the TGFs and the main optical pulses as detected by ASIM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6967, https://doi.org/10.5194/egusphere-egu2020-6967, 2020.

Launched and installed at the International Space Station in April 2018, the Atmosphere-Space Interactions Monitor (ASIM) provides science data since June 2018. Suite of onboard instruments contains optical and high energy detectors payloads. Modular Multi-spectral Imaging Array (MMIA) includes three photometers (180-240 nm, 337 nm and 777.4 nm) sampling at 100 kHz, and two cameras (337 nm and 777.4 nm) sampling at 12 Hz. It allows for lightning and transient luminous events (TLEs) observations during the orbital eclipses. The Modular X- and Gamma-ray Sensor (MXGS) detects X- and Gamma-ray photons, and is dedicated to detection of Terrestrial Gamma-ray Flashes (TGFs). The mutual relative timing accuracy between MXGS and MMIA is as good as +/- 5 µs.

TGFs are known to be associated with the +IC lightning discharges. ASIM provides a unique possibility for simultaneous observations of TGFs together with the underlying optical activity inside the thundercloud. In this contribution we summarize the almost two years of ASIM observations to make an overview of the various optical contexts accompanying the TGF production.

How to cite: Mezentsev, A., Østgaard, N., Marisaldi, M., Kochkin, P., Neubert, T., Chanrion, O., Heumesser, M., Reglero, V., Christiansen, F., Genov, G., and Ullaland, K.: Lightning optical context associated with ASIM TGFs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8157, https://doi.org/10.5194/egusphere-egu2020-8157, 2020.

In 2018, the Atmospheric Space Interactions Monitor (ASIM) was launched and mounted onboard the Columbus module of the International Space Station (ISS). Using data from the Modular X- and Gamma-Ray Sensor (MXGS) and the Modular Multispectral Imaging Array (MMIA), we investigate the time sequence of the TGFs detected by MXGS and the optical pulses detected by the MMIA. The optical pulses are observed in the 337 nm and 777.4 nm, and the X- and gamma-rays are detected by the High Energy Detector of MXGS, which is sensitive to energies from 300 keV to more than 30 MeV. We will also look into the TGF duration and any correlation with the time between the TGFs and the main optical signals. The data used is from June 2018 (shortly after mounting on the Columbus module) until the end of March 2019, when the relative timing uncertainty between the two instruments was +/- 80 us. The data after this is presented in Skeie et al.

How to cite: Bjørge-Engeland, I., Østgaard, N., Skeie, C. A., Mezentsev, A., Neubert, T., Reglero, V., Marisaldi, M., Kochkin, P., Lehtinen, N., Sarria, D., Maiorana, C., Lindanger, A., Ullaland, K., Genov, G., Christiansen, F., Chanrion, O., and Heumesser, M.: Time sequence of TGFs and optical pulses detected by ASIM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7113, https://doi.org/10.5194/egusphere-egu2020-7113, 2020.

The anthropogenic increase in atmospheric CO₂ is not only considered to drive global warming, but also ocean acidification. Previous studies have shown that acidification will affect many aspects of carbon uptake and release in the surface water of the ocean through increased primary productivity and decreased biogenic calcification and CaCO₃ dissolution. In this report we present a potential novel impact of acidification on the flash intensity of lightning discharged into the oceans. Our experimental results show that a decrease in ocean pH corresponding to the predicted increase in atmospheric CO₂ according to the IPCC RCP 8.5 worst case emission scenario will increase the Lightning Flash Intensity (LFI) by ca. 30% by the end of the 21^st century relative to 2000. This increase in LFI may have broader implications for the atmospheric NO_x production and precipitation as well as the atmospheric ozone budget (O₃ and N₂O production). In turn, these feedback processes may impact both marine and terrestrial biological uptake of carbon that should be considered in global carbon and climate models.

How to cite: Asfur, M., Silverman, J., and Price, C.: Ocean acidification may be increasing the intensity of lightning over the oceans, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5363, https://doi.org/10.5194/egusphere-egu2020-5363, 2020.

We report on the improvements of our lightning imaging technique over what was reported in Hare2019, where we map lightning in 3D using timing obtained from the cross-correlation of the signals from antenna pairs in broadband VHF (30 — 80 MHZ). We use the infrastructure offered by LOFAR (LOw Frequency Array), a software radio telescope.

The infrastructure of LOFAR allows us to use a large number of simple dual-polarized dipole antennas arranged in stations of 48 antennas with a diameter of about 60m. We limit ourselves to the use of the Dutch stations only, which gives us baselines of up to 100 km. The data are sampled at 200 MHz giving 5 nanoseconds time between samples. We use LOFAR in the mode where it saves the full time-series spectra for five seconds for every antenna in the array. Upon a trigger, the data for all Dutch stations is stored for later off-line processing.

In imaging a flash our bottleneck is to handle the confusion limit. Because of the high density of sources, pulses that are detected in one time-order in the first antenna may have changed order in a second that is at an appreciable distance from the first. The pulse density where this problem surfaces depends on the imaging technique. In our new imaging method we use an approach inspired by the Kalman-filter technique. In the presentation the new technique will be explained. This allows us to obtain a larger number of located sources as compared to the approach used in Hare2019 (sometimes as much as three times as many) which allows for a more detailed analysis of small structures seen in lightning.

To show the strength of the new technique we show some images of positive and negative leader development as well as a return stroke.

Hare2019:  B. Hare et al., Nature 568, 360–363 (2019).

How to cite: Scholten, O., Hare, B., Pel, A., Bonardi, A., Buitink, S., Corstanje, A., Falcke, H., Huege, T., Hoerandel, J., Krampah, G., Mitra, P., Mulrey, K., Nelles, A., Pandya, H., Rachen, J., Rossetto, L., Trinh, G., Veen, ter, S., and Winchen, T.: Precision Lightning Imaging with LOFAR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9975, https://doi.org/10.5194/egusphere-egu2020-9975, 2020.

Cosmic ray particles have extreme energies, 10¹⁶ eV/nucleon and up. Upon arrival at the higher atmosphere and collisions with the gas molecules there, the cosmic ray particles convert into an cascade of different secondary particles that finally arrive at soil level in the form of an extensive air shower (EAS): high-energy gamma’s, electrons and muons. In the HIgh School Project on Astrophysics Research with Cosmics (Hisparc, www.hisparc.nl) about 100 EAS detector stations are distributed over the Netherlands and several neighboring countries. These stations are mostly placed on the roof of secondary schools, where they have been built by pupils to attract them towards STEM studies.

Each station consists of two or four detectors with 0.5 m² plastic scintillator plates to record the passage of the EAS. At coincidence, the scintillator signals are individually recorded, accurately timed with GPS. All data are sent to and collected at the NIKHEF institute (www.nikhef.nl) and made available (open-access) for further analysis by pupils and scientists.

The sensitivity of the detectors is commonly adjusted such that each detector records a few hundred hits per second. The number of coincidences within 1.5 μs is then about 1 in 3 seconds, in part due to an actual EAS, in part due to random local radioactive processes.

During intense rainfall of a particular summer storm several two-detector systems recorded an increase in the coincidence frequency of up to a factor of 7. When comparing different stations we could follow the associated storm front moving northwards over NL. Within the coincidence interval of 1.5 μs the increased individual signals of both detectors were evenly distributed. Actual EAS signals tend to be synchronous to within 100 ns. We therefor attribute the increase to random signals. As possible source we suggest gamma radiation due to radon daughters in the atmosphere that are washed out by the rain and accumulate on the roof close to the detectors. The delay between rain and signal increase is noted and in accordance with the washing process time.

How to cite: van Deursen, A. P. J., Fokkema, D., van Dam, K., and van Eijk, B.: Hisparc cosmic ray detector’s response to heavy rain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10888, https://doi.org/10.5194/egusphere-egu2020-10888, 2020.

The Atmosphere Space Interaction Monitor (ASIM) mission onboard the International Space Station is the first mission specifically dedicated to the observation of Terrestrial Gamma-ray Flashes (TGF) and Transient Luminous Events (TLE). ASIM, together with the Fermi and AGILE satellites, are the only three currently operating missions capable to detect TGFs from space. Depending on orbital parameters, pairs of these missions periodically get closer than few hundreds kilometers, observing the same region on the Earth surface for up to several tens of seconds. This offers the unique chance to observe the same TGF from two different viewing angles. Such observations allow to probe the TGF production geometry and possibly put constraints on production models and electric field geometry at the source.

Here we present four TGFs detected by ASIM and simultaneously detected by Fermi (three events) or AGILE (one event) in the period June 2018 - November 2019. We present location data, light curves, and possible constraints to emission geometry based on coupled observations and Monte Carlo simulations. 

How to cite: Marisaldi, M., Mezentsev, A., Sarria, D., Lindanger, A., Østgaard, N., Neubert, T., Reglero, V., Kochkin, P., Lehtinen, N., Maiorana, C., Skeie, C. A., Bjørge-Engeland, I., Ullaland, K., Genov, G., Christiansen, F., Christian, H., Al Nussirat, S., Briggs, M., Ursi, A., and Tavani, M.: ASIM - Fermi - AGILE simultaneous observation of Terrestrial Gamma-ray Flashes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12804, https://doi.org/10.5194/egusphere-egu2020-12804, 2020.

Terrestrial Gamma-ray Flashes (TGFs) are sub milliseconds bursts of high energy photons associated with lightning flashes in thunderstorms. The Atmosphere-Space Interactions Monitor (ASIM), launched in April 2018, is the first space mission specifically designed to detect TGFs. We will mainly focus on data from the High Energy Detector (HED) which is sensitive to photons with energies from 300 keV to > 30 MeV, and include data from the Low Energy Detector (LED) sensitive in 50 keV to 370 keV energy range. Both HED and LED are part of the Modular X- and Gamma-ray Sensor (MXGS) of ASIM.

The energy spectrum of TGFs, together with Monte Carlo simulations, can provide information on the production altitude and beaming geometry of TGFs. Constraints have already been set on the production altitude and beaming geometry using other spacecraft and radio measurements. Some of these studies are based on cumulative spectra of a large number of TGFs (e.g. [1]), which smooth out individual variability. The spectral analysis of individual TGFs has been carried out up to now for Fermi TGFs only, showing spectral diversity [2]. Crucial key factors for individual TGF spectral analysis are a large number of counts, an energy range extended to several tens of MeV, a good energy calibration as well as knowledge and control of any instrumental effects affecting the measurements.

We strive to put stricter constraints on the production altitude and beaming geometry, by comparing Monte Carlo simulations to energy spectra from single ASIM TGFs. We will present the dataset and method, including the correction for instrumental effects, and preliminary results on individual TGFs.

Thanks to ASIM’s large effective area and low orbital altitude, single TGFs detected by ASIM have much more count statistics than observations from other spacecrafts capable of detecting TGFs. ASIM has detected over 550 TGFs up to date (January 2020), and ~115 have more than 100 counts. This allows for a large sample for individual spectral analysis.

References:

How to cite: Lindanger, A., Marisaldi, M., Østgaard, N., Mezentsev, A., Neubert, T., Reglero, V., Kochkin, P., Lehtinen, N., Sarria, D., Carlson, B. E., Maiorana, C., Skeie, C. A., Bjørge-Engeland, I., Ullaland, K., Genov, G., Christiansen, F., and Köhn, C.: Energy spectrum from single TGFs detected by ASIM , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16206, https://doi.org/10.5194/egusphere-egu2020-16206, 2020.

In the last two decades, it has been discovered that lightning strikes can emit high-energy radiation.
In particular, a phenomenon has been observed from space called "Terrestrial Gamma-ray Flash'' (TGF), which consists of an intense burst of gamma radiation that can be produced during thunderstorms. This phenomenon has met with considerable interest in the scientific community and its mechanism is still not fully understood. Nowadays several satellites for astrophysics like AGILE and FERMI are able to detect and map TGFs and specific instruments like the ASIM detector on the ISS are studying this phenomenon from space.
In the atmosphere, the high-energy radiation undergoes a strong absorption exponentially proportional to the air density which makes it more difficult to detect TGFs on the ground. Nonetheless, ground measurements were conducted and observed that even in cloud-to-ground lightning high-energy radiation were produced. In particular, the works of Moore et al. [2001] and Dwyer et al. [2005] highlight two lightning processes in which the X-ray emission could be produced: downward negative stepped leader and dart leader. Currently, it is not clear if the emissions revealed on the ground and the TGFs observed in space are essentially the same phenomenon or how these phenomena are related. For these reasons, it is particularly interesting to study high-energy emissions also from ground instruments because, despite the strong absorption of the high-energy radiation, ground observations can reach a better accuracy in time and space and provide crucial information to investigate the origin and conditions under which these emissions occur.
A privileged instrument for this research is the VHF Lightning Interferometer, a system of antennas that allows you to map lightning through the very high frequency (VHF) emission. Due to the high resolution of this instrument, should be possible to locate the origin of the high-energy emissions and hopefully provide a better understanding of the radiation mechanism.
The aim of this research is, therefore, to develop a 3D interferometry system to identify as accurately as possible the origin and the conditions in which the X-ray emission occurs in cloud-to-ground lightning and investigate the relation of the VHF emissions with the TGFs.
Recently an observation campaign was conducted in Colombia with two VHF Lightning Interferometers and two X-rays detectors. This interferometry system was installed in the coverage area of a Lightning Mapping Array (LMA) and LINET to take advantage of the complementary information that these lightning location networks could provide. At the moment, about 15 lightning events with X-ray emissions were observed, including five X-ray bursts from downward negative leaders and two emissions from dart leaders. Further studies and analysis of the collected data are still ongoing.

How to cite: Urbani, M., Montanyà, J., Van der Velde, O., and López, J. A.: High-energy radiation from natural lightning observed in coincidence with a VHF Lightning Interferometer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16383, https://doi.org/10.5194/egusphere-egu2020-16383, 2020.

TARANIS (Tool for the Analysis of RAdiations from lightNIngs and Sprites) is a French CNES microsatellite dedicated to the study of the impulsive energy transfer between the Earth’s atmosphere and the space environment as widely observed above the active thunderstorm regions. After years of development and testing, the satellite is approaching to its launch (expected in June 2020). The comprehensive satellite scientific payload incorporates optical, field, and particle sensors including the energetic electron instrument (IDEE) with very high sensitivity and time resolution. Its main scientific tasks are: to measure high resolution energetic electron spectra (70 keV to 4MeV) and pitch angle distributions, to separate upward accelerated electrons and downward precipitated electrons, to detect burst of electrons associated with Terrestrial Gamma ray Flashes, to identify Lightning-induced Electron Precipitation (LEP), and to provide alert signals about high-energy electron bursts to other TARANIS experiments.  The aim of this contribution is to describe the final design and expected performance of the IDEE experiment, including the data products. We also want to show how we are going to enhance the today’s scientific knowledge of the thunderstorm related phenomena in synergy with other ground-based and space-born experiments.

How to cite: Prech, L., Blelly, P.-L., Devoto, P., Sauvaud, J.-A., Wong, K., Orttner, G., Baby, N., and Cermak, I.: The energetic electron instrument (IDEE) onboard the TARANIS spacecraft to search lightning-connected energetic electron populations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17075, https://doi.org/10.5194/egusphere-egu2020-17075, 2020.

We present first observational evidences of an HO₂ production in the mesosphere above sprite‐producing thunderstorms derived from low‐noise SMILES (Submillimeter-Wave Limb-Emission Sounder) observation spectra in relation with sprite detections by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument. Three events were identified with enhanced HO₂ levels of approximately 10²⁵ molecules at altitudes of 75-80 km a few hours after sprite occurrence. These first direct observations of chemical sprite effects are compared to results of plasma chemistry model simulations of electrical discharges in the mesosphere, and processes which can lead to an increase of mesospheric HO₂ on timescale of a few hours after a sprite event are analysed.

How to cite: Winkler, H., Yamada, T., Kasai, Y., and Notholt, J.: HO2 enhancements due to sprite discharges - observations and model simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8313, https://doi.org/10.5194/egusphere-egu2020-8313, 2020.

Long spark discharges of about one meter and natural lightning show a polarity asymmetry.  While positive discharges propagate continuously, negative discharges propagate in a stepped manner. This stepped propagation is mediated by the so-called space stem, an isolated region in the streamer corona of depleted electron density and enhanced electric field. Kostinskiy et al. 2018 [1] reported the stepping of positive leaders under high humidity conditions and Malagón-Romero et al. 2019 [2] showed that positive leader steps, if they exist, would be shorter and thus harder to observe in experiments. 

In this work we present the results of our simulations for the evolution of a space stem precursor [2] under dry and humid air conditions. These results show that the presence of water molecules enhances the electric field and the heating rate of the space stem, reaching 2000 K considerably faster than in dry air. This could make feasible the stepping of positive leader discharges under high humidity conditions as observed by Kostinskiy et al. 2018 [1].

[1] Kostinskiy, A. Y., Syssoev, V. S., Bogatov, N. A., Mareev, E. A., Andreev, M. G., Bulatov, M. U., & Rakov, V. A. (2018). Abrupt elongation (stepping) of negative and positive leaders culminating in an intense corona streamer burst: Observations in long sparks and implications for lightning. Journal of Geophysical Research: Atmospheres, 123, 5360–5375.

[2] Malagón-Romero, A., & Luque, A. (2019). Spontaneous emergence of space stems ahead of negative leaders in lightning and long sparks. Geophysical Research Letters, 46, 4029–4038. https://doi.org/10.1029/ 2019GL082063

How to cite: Malagón-Romero, A. and Luque, A.: Leader discharge stepping in dry and humid air, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19238, https://doi.org/10.5194/egusphere-egu2020-19238, 2020.

Lightning flashes emit intense optical radiation that can be detected from space. Several space missions work by observing this light in order to investigate lightning, thunderstorms, and other phenomena closely associated to them such as Transient Luminous Events (TLEs) and Terrestrial Gamma-ray Flashes (TGFs).

In its path towards a satellite-borne observing device, the optical radiation emitted by a flash is scattered many times by the droplets and ice crystals in the cloud. The detected signal is thus shaped by and contains information about the cloud geometry and composition. This is particularly relevant for instruments with a high spatial resolution such as the cameras in the Modular Multispectral Imaging Array (MMIA), which is part of the Atmosphere-Space Interactions Monitor (ASIM) currently onboard the International Space Station. These cameras provide images of lightning-illuminated cloud tops with a resolution of about 400 m.

We present a numerical code that can simulate light scattering in clouds with complex geometries and location-dependent droplet density and effective radius. The cloud geometry is specified by a number of elementary shapes (e.g. spheres and cylinders) that can be linearly deformed as well as combined by set operations such as unions, intersections and subtractions. The cloud composition can be specified by arbitrary functions. Designed to aid in the interpretation of satellite images, the code simulates spatially resolved observations from an arbitrary viewpoint. Some examples and applications of this tool will be discussed.

How to cite: Luque Estepa, A., Gordillo-Vázquez, F. J., Li, D., Malagón-Romero, A., Soler, S., Pérez-Invernón, F. J., Chanrion, O., Heumesser, M., and Neubert, T.: Scattering of lightning optical radiation by complex, inhomogeneous clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19506, https://doi.org/10.5194/egusphere-egu2020-19506, 2020.

Multi-band observation of transient luminous events (TLEs) is one of the useful methodologies to be employed in sprite campaigns. Here, we show a method to estimate the Boltzmann vibrational temperature of N₂ (B³Π_g) by analyzing the 630nm-filtered, N₂ 1P-filtered and 762 nm-filtered images of TLEs. Our advanced method is validated in compassion with derived relative vibrational distributions by sprite spectrum (Kanmae et al., 2007). The imager recorded N₂ 1P-filtered emission (I_1P,  623 – 754 nm) of TLEs indicates the intensity of N₂ 1P Δv=3 and partial with Δv=2 where dominated emissions with upper state vibrational number v=4, 5 and 6, i.e., N₂ 1P (4, 2), (4, 1), (5, 2) and (6, 3). The imager recorded 630 nm-filtered emissions (I₆₃₀) were contributed primarily from N₂ 1P (10, 7) with v=10 while N₂ 1P (3, 1) for 762 nm-filtered emissions (I₇₆₂) with v=3. Hence, we calculated the emission ratios of I₆₃₀ to I_1P, I₆₃₀ to I₇₆₂ and I₇₆₂ to I_1P. The emission ratios of I₆₃₀ to I_1P, I₆₃₀ to I₇₆₂ and I₇₆₂ to I_1P  also reflect the relative vibrational distributions of vibrational levels with LOW v=3 (I₇₆₂), MIDDLE v=4, 5, 6 (I_1P,  623 – 754 nm), and HIGH v=10 (I₆₃₀). Therefore, we use the Boltzmann temperature for indicating the relative vibrational distributions of the specified group (LOW/MIDDLE/HIGH) of N₂ (B³Π_g) vibrational levels. For ISUAL recorded sprites, the average brightness of N₂ 1P (I_1p), 762 nm (I₇₆₂) and 630 nm (I₆₃₀) emission was 2.3, 0.6 and 0.02 MR. The N₂ (B³Π_g) vibrational temperatures (T_v) were estimated to be 2800 K, 3200 K and 4300 K for multi-band emission ratios of I₆₃₀/ I_1p, I₆₃₀/ I₇₆₂ and I₇₆₂/ I_1p. For observed elves, the average brightness I_1p, I₇₆₂ and I₆₃₀ were 170, 50 and 3 kR. The estimated T_v values were 3700 K, 3700 K and 3800 K for ratios I₆₃₀/ I_1p, I₆₃₀/ I₇₆₂ and I₇₆₂/ I_1p. For observed gigantic jets, the derived T_v values were 3000 – 5000 K for a ratio I₇₆₂/ I_1p. Through N₂ (B³Π_g) T_v analyses from emission ratios of ISUAL multi-band observation, we derived the N₂ (B³Π_g) vibrational temperature that ranges between 3000 and 5000 K or higher in TLEs. Accuracy and variations of derived N₂ (B³Π_g) T_v are also discussed while the relative population of vibrational levels in the Boltzmann equilibrium are also compared with past spectra observation. The details are shown in the publication (https://doi.org/10.1029/2019JA027311).

How to cite: Kuo, C. L. and the ISUAL Science Team: Estimating Boltzmann vibrational temperature of N2 (B^3Pi_g) using ISUAL 630nm, N2 1P (623-754 nm) and 762 nm-filtered imager data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4333, https://doi.org/10.5194/egusphere-egu2020-4333, 2020.

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.

How to cite: Pizzuti, A., Soula, S., Mlynarczyk, J., Bennett, A., and Fullekrug, M.: Analysis of sprite events during small-scale winter thunderstorms in northern Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20065, https://doi.org/10.5194/egusphere-egu2020-20065, 2020.

During the last decade, a large number of sprites were observed thanks to low-light video cameras located in southern France, especially at Pic du Midi (2877 m) in the Pyrénées mountain range and at the Albion Plateau (1000 m) in the south-east of France. Sprites are Transient Luminous Events (TLEs) consisting of streamer discharges, that develop at the base of the ionosphere and whose structure, size and brightness are very variable according to the density and the dynamics of these streamers. The largest type is called jellyfish or « A-bomb » sprite, and it corresponds generally to a very impulsive return stroke. Among more than 3000 sprite events in the database, we selected a few cases with large size and very strong light emission. The goal is to determine the characteristics of the flashes that produced them and the storm context in which they occurred. Thus, we analyse the video imagery, the thundercloud structure, the current moment waveform of the lightning strokes, the radiations at various frequencies from the lightning flash. We show that such very bright sprites can occur above thunderstorms at any period of the year. The favourable conditions for their production seem to be stationary thunderstorms and one case of storm produced five of them. All cases of these sprite events are associated with a halo and they are produced with a very short delay after strong positive cloud-to-ground strokes. The peak current of these strokes is about 150 kA in average and their iCMC values can reach close to 2000 C km. The leader processes and the stroke location in the thundercloud are analysed in detail for some cases.

How to cite: Soula, S., Mlynarczyk, J., Pizzuti, A., Pedeboy, S., Gonneau, E., Gomez Kuri, Z., van der Velde, O., Montanya, J., Farges, T., Fullekrug, M., Bennet, A., Boyer, D., and Cavaillou, A.: Analysis of the lightning flashes associated with very large and luminous sprites in Western Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22263, https://doi.org/10.5194/egusphere-egu2020-22263, 2020.

In an article by Kostinskiy et al. (2019) proposed the mechanism of the origin and development of lightning from initiating event to initial breakdown pulses (termed the Mechanism). The Mechanism assumes initiation occurs in a region of a thundercloud of 1 km³ with electric field E > 0.3-0.4 MV/(m∙atm), which contains, because of turbulence, numerous small “E_th-volumes” of 0.001 m³ with E ≥ 3 MV/(m∙atm). The Mechanism allows for lightning initiation by two observed types of initiating events: a high power VHF event called an NBE (narrow bipolar event or CID), or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles passing through many of the E_th-volumes, thereby causing the nearly simultaneous launching of many positive streamer flashes.

This report describes the method for the numerical calculation of the volume phase wave of ignition of streamer flashes in the turbulent region of a thundercloud, which is initiated by secondary particles of a extensive air shower (EAS).  The lateral distribution of energetic electrons and positrons, which are created by cosmic particles with an energy ε> 10¹⁵ eV, is described by the equation Nishimura-Kamata-Greizen (Kamata & Nishimura, 1958). When an EAS enters an electric field with an intensity of E> 400 kV /(m∙atm), which supports the movement of streamers, the electron runaway mechanism  is sure to start working (runaway threshold E> 280 kV/ (m∙atm), Dwyer, 2010). Each secondary electron and positron EAS initiates an avalanche of runaway electrons. The radial distribution of each avalanche was calculated in the diffusion approximation using the Dwyer-Babich approximation formulas (Dwyer, 2010; Babich & Bochkov, 2011). The model considered the effect of electrons of each such avalanche on the entire volume of a strong electric field.

The calculation showed that the EAS-RREA mechanism of almost simultaneous volumetric initiation of multiple streamer flashes can provide such NBE (CID) parameters as current and charge transfer at observation heights of 5–20 km above sea level.

References

Babich, L.P., Bochkov, E.I. (2011). Deterministic methods for numerical simulation of high-energy runaway electron avalanches. Journal of Experimental and Theoretical Physics, 112(3), 494–503, doi: 10.1134/S1063776111020014.

Dwyer, J. R. (2010), Diffusion of relativistic runaway electrons and implications for lightning initiation, J. Geophys. Res., 115, A00E14, doi:10.1029/2009JA014504.

Kamata, K., & Nishimura, J. (1958). The lateral and the angular structure functions of electron showers. Progress of Theoretical Physics Supplement, 6, 93. https://doi.org/10.1143/PTPS.6.93

Kostinskiy, A. Yu., Marshall, T.C., Stolzenburg, M. (2019), The Mechanism of the Origin and Development of Lightning from Initiating Event to Initial Breakdown Pulses, arXiv:1906.01033

Raizer Yu. (1991), Gas Discharge Physics, Springer-Verlag, 449 p.

How to cite: Vlasov, A., Fridman, M., and Kostinskiy, A.: Method for the numerical calculation of the mechanism of the origin the NBE (CID) due to the volume phase wave of synchronous ignition of streamer flashes by EAS-RREA , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22461, https://doi.org/10.5194/egusphere-egu2020-22461, 2020.

Measurements of the atmospheric electric field (or Potential Gradient, PG) in arid, desert regions are few in comparison to those in more wet/mid latitude regions, despite the fact that such measurements can provide important insights into dust charging processes. Dust charging is emerging as potentially important in sustaining the long range transport of particles, for which new charge and field data are essential. Here we present new PG data from an electric field mill at Al Ain international airport in the eastern part of the Abu Dhabi Emirate in the United Arab Emirates (UAE).  Measurements were made alongside a visibility sensor and ceilometer to provide information on the background meteorological conditions.  At Al Ain, the conditions are generally fair weather in mid-latitude terms (predominantly no clouds or precipitation), with very occasional fog or thunderstorms, but the PG still demonstrates considerable variability associated with local factors such as dust and aerosol content.  Throughout the data series, the PG is almost entirely positive, with the only negative values occurring during thunderstorms and violent dust storms.  The desert climate of the UAE lead to widespread uplift of dust on a regular basis, as evidenced by the generally low visibility measured at the airport (mean visibility = 9km).  The PG at Al Ain was found to be generally much larger than typical fair weather values at other sites, with a mean of 116 V/m, with 2 kV/m exceeded regularly.  The local influences on the PG at Al Ain are strongly apparent and the daily variation in PG was found to fall into two main categories: 1) convection dominated, 2) sea breeze dominated.   On the convection dominated days the PG followed the daily variation in temperature and wind speed closely, with very large maximum values of PG up to 4 kV/m in the mid afternoon.  The other regular daily feature in Al Ain PG was a sharp positive increase in PG up to several kV/m around 1800-1900 local time.  This feature is associated with the arrival of a sea breeze front, which originates more than 150 km away on the Abu Dhabi coastline.  The extremely large change in PG over a very short time scale (tens of minutes) is thought to be due to the action of dust pickup within the sea breeze front as it travels substantial distances over the flat arid landscape.  Overall, the electrical environment at Al Ain is found to be generally very highly charged and so the local effects (primarily from dust and aerosol) mask Global Electric Circuit influences in the surface data.

How to cite: Nicoll, K., Harrison, R. G., Marlton, G., and Airey, M.: Atmospheric electric field measurements in the central United Arab Emirates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5707, https://doi.org/10.5194/egusphere-egu2020-5707, 2020.

The Huygens probe to Titan in 2005 was the first planetary probe or lander to feature ELF electric field sensing and atmospheric conductivity measurements. The atmospheric electricity community showed great interest in the claimed detection of a Schumann resonance signal on another world (despite its unexpected dominant frequency of 36 Hz), and the planetary science community embraced an interpretation of the altitude dependence of the signal as evidence of a theoretically-anticipated internal water ocean beneath an ice crust many tens of km thick.

Quantitative scrutiny suggests that prospects of detecting a Schumann signal at Titan with the Huygens experiment were in fact very poor, due to short measurement time, a horizontal antenna orientation, a lack of lightning, and the likely presence of severe dynamical effects on the probe. Although the latter objections were considered, and arguments developed against them (notably the novel postulated Saturn-magnetospheric excitation of the resonance), we have re-examined the data in the light of a better understanding of the probe dynamics. The evolution of the 36Hz power shows a very strong correlation with accelerometer records of short-period motions of the probe under its small stabilizer parachute, suggesting that mechanical oscillations of the probe and/or the antenna booms were actually the cause. The ‘signal’ ramped up just as the probe accelerated from the much more quiescent main parachute, and ceased abruptly a couple of seconds after impact.

While the Huygens signal may therefore have been an artifact, this does not mean that a Schumann resonance does not occur on Titan. Most likely if it occurs, it may be very sporadic, responding to the infrequent rainstorms on Titan. A search for such signals should therefore be a long-duration monitoring exercise (not unlike listening for seismic events that could also probe Titan’s interior). The Dragonfly mission to Titan, recently selected for launch in 2026 with arrival planned in 2034 and over two years of surface operation, provides an opportunity to perform such monitoring.

How to cite: Lorenz, R. and Le Gall, A.: Schumann Resonance on Titan : Huygens Observations Critically Re-Assessed and prospects for the Dragonfly Mission, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2217, https://doi.org/10.5194/egusphere-egu2020-2217, 2020.

In [1] it was established that collective dynamics of charged hydrometeors that involved in turbulent motion play a fundamental role in thundercloud electrostatic energy redistribution and dissipation. The main reservoirs for accumulating electrostatic energy in thunderclouds are i) the large-scale field of the main charged layers that appear due to the large-scale separation of oppositely charged hydrometers, ii) the intermediate-scale field of charged hydrometeors distributed in the turbulent flow, and finally iii) the small-scale field of net and polarization charges on the surface of individual solid and liquid water particles. Since three different spatial scales are involved into the process of electrostatic energy dissipation, we represent the lightning initiation scenario as a sequence of two transitions of discharge activity to progressively larger spatial scales: the first one is from small-scale avalanches to intermediate-scale streamers; and the second one is from streamers to the lightning seed. At the first stage of the proposed scenario, the essentially non-conducting cloud becomes seeded by elevated ion conductivity regions with spatial extent of 0.1 - 1 m and a lifetime of 1 - 10 s. These regions can serve to promote the intermediate electric field enhancements and increase in pre-ionization level that is sufficient for the initiation and development of streamers. Due to the positive the proposed streamer generation mechanism has an important feature: streamers in our scenario are not exponentially rare events, but continuously fill the entire volume. The collective dynamics of such a nearly continuous, volume filling streamer network appears to be very sensitive to both the magnitude of external large-scale electric field and longitudinal extent of the region occupied by the field. Moving in the course of its development along the external field, a positive streamer can get into the negative trails left by other streamers (relay race effect). In this way, the size of the streamer discharge along the external field can grow, providing the emergence of a kind of streamer trees, thereby tapping electrostatic energy from a relatively large cloud volume. Over time, many streamer trees are feeding their current into narrow channels, where the heating occurs (the bottleneck effect). The hot segments of the network can get polarized and grow within its overall channel system even if the ambient field amplitude is much smaller than the critical field of streamer propagation. Successful initiation of lightning also requires that potential difference across the layer occupied by the large-scale electric field makes about three megavolts. The proposed scenario can possibly lead to a paradigm shift in our approaches to the still unsolved mystery of lightning initiation, because it does not require the presence of super-energetic cosmic ray particles, unrealistic potential difference inside the cloud, or unrealistically large hydrometeors.

1. Iudin, D.I., Rakov, V.A., Syssoev, A.A. et al. Formation of decimeter-scale, long-lived elevated ionic conductivity regions in thunderclouds. npj Clim Atmos Sci 2, 46 (2019) doi:10.1038/s41612-019-0102-8.

How to cite: Iudin, D., Rakov, V., Syssoev, A., and Bulatov, A.: Streamer network critical behavior and lightning initiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12799, https://doi.org/10.5194/egusphere-egu2020-12799, 2020.

We have developed a 3D fluid model to simulate streamer discharges in unsteady air flow. The model couples the drift-diffusion equations for the charged particles, the Navier-Stokes equations for the air and the Poisson equation for the electric field. It allows to study electrical discharges at different timescales defined by light and heavy particles and to investigate the effects of unsteady airflow. The model treats the time integration in an implicit manner to allow longer time steps, which makes the simulation of long duration discharges feasible. Moreover, the model uses an unstructured mesh with adaptive refinement allowing the incorporation of solid bodies with complex geometries. The accuracy of the model has been verified by comparing its results with a test case from the literature comparing simulation in steady air from five different streamer codes. Our results were consistent and among the most accurate. We present results from a simulation of long duration discharges, in which a series of successive positive streamers are initiated from a positive polarity electrode in a transverse airflow condition. It shows that the impact of a low speed air flow on the streamer comes essentially from the ions being blown away by the wind.

How to cite: Niknezhad, M., Chanrion, O., Köhn, C., Holbøll, J., and Neubert, T.: Simulation of electric discharges in unsteady airflow using a 3D fluid model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7944, https://doi.org/10.5194/egusphere-egu2020-7944, 2020.

How to cite: Yucemoz, M.: Backward peaking radiation pattern from a relativistic particle accelerated by lightning leader tip electric field , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10585, https://doi.org/10.5194/egusphere-egu2020-10585, 2020.

Terrestrial Gamma-ray Flashes (TGFs) are short (~20 us to ~2 ms) flashes of high energy (< 40 MeV) photons, produced by thunderstorms When interacting with the atmosphere, the TGF’s photons produce relativistic electrons and positrons at higher altitudes, and a fraction is able to escape the atmosphere [1,2,3]. The electrons/positrons are then bounded to Earth's magnetic field lines and can travel large distances inside the ionosphere and the magnetosphere. This phenomenon is called a Terrestrial Electron Beam (TEB).

The Atmosphere-Space Interactions Monitor (ASIM), dedicated to the study of TGF and associated events, started to operate in June 2018. ASIM contains an optical instrument (MMIA) made of micro-cameras and photometers, as well the Modular X and Gamma-ray Sensor (MXGS) for high energy radiation. MXGS is composed of the low energy detector (LED, 50 keV to 400 keV) and the High Energy detector (HED, 300 keV to 40 MeV). 

This presentation is focused on a new event which was detected on March 24, 2019. The TEB originated from rainbands produced by the tropical cyclone Joaninha, in the Indian Ocean, close to Madagascar. This observation shows, for the first time to our knowledge: (1) the low energy part (>50 keV) of the TEB spectrum, using the LED, (2) an estimate of the incoming direction (to ISS) of the electron Beam from recorded data.

References:

[1] J. R., Dwyer, B. W., Grefenstette and D. M. Smith. High-energy electron beams launched into space by thunderstorms. DOI: 10.1029/2007GL032430. Geophysical Research Letters, 2008.

[2] B. E. Carlson T. Gjesteland N. Østgaard. Terrestrial gamma-ray flash electron beam geometry, fluence, and detection frequency. DOI: 10.1029/2011JA016812. Journal of Geophysical Research (Space Physics), 2011.

[3] D. Sarria, P. Kochkin, N. Østgaard et al. The First Terrestrial Electron Beam Observed by the Atmosphere-Space Interactions Monitor. DOI: 10.1029/2019JA027071. Journal of Geophysical Research (Space Physics), 2019.

How to cite: Sarria, D., Kochkin, P., Østgaard, N., Mezentsev, A., Lehtinen, N. G., Marisaldi, M., Maiorana, C., Neubert, T., Reglero, V., Carlson, B. E., Ullaland, K., Yang, S., Genov, G., Qureshi, B., Budtz-Jørgensen, C., Kuvvetli, I., Christiansen, F., Chanrion, O., Navarro-Gonzales, J., and Connell, P. H.: Observation of a Terrestrial Electron Beam during the tropical cyclone Joaninha in March 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3899, https://doi.org/10.5194/egusphere-egu2020-3899, 2020.

In an article by Kostinskiy et al. (2019) proposed the mechanism of the origin and development of lightning from initiating event to initial breakdown pulses (termed the Mechanism). The Mechanism assumes initiation occurs in a region of a thundercloud of 1 km³ with electric field E > 0.4 MV/(m∙atm), which contains, because of turbulence, numerous small “E_th-volumes” of 0.001-0.0001 m³ with E ≥ 3 MV/(m∙atm). The Mechanism allows for lightning initiation by two observed types of initiating events: a high power VHF event called an NBE (narrow bipolar event or CID), or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles passing through many of the E_th-volumes, thereby causing the nearly simultaneous launching of many positive streamer flashes, Kostinskiy et al. (2019).

In this report, based on the Meek’s criterion for the initiation of streamers (Raizer, 1991) at different heights of lightning initiation and taking into account the number of all background electrons, positrons and photons of cosmic rays with energy ε < 10¹² eV (Sato, 2015) crossing E_th-volumes sizes of E_th-volumes are specified (3∙10^-4-3∙10^-5 m³). The report also showed that synchronous injection with a high probability of relativistic electrons into such small E_th-volumes requires of relativistic runaway electrons avalanches to be initiated by extensive air showers with energies ε > 10¹⁵ eV, which would supply (injected) 10⁵-10⁷ secondary electrons into a turbulent region of a thundercloud with a strong electric field.

References

Kostinskiy, A. Yu., Marshall, T.C., Stolzenburg, M. (2019), The Mechanism of the Origin and Development of Lightning from Initiating Event to Initial Breakdown Pulses arXiv:1906.01033

Raizer Yu. (1991), Gas Discharge Physics, Springer-Verlag, 449 p.

Sato T. (2015), Analytical Model for Estimating Terrestrial Cosmic Ray Fluxes Nearly Anytime and Anywhere in the World: Extension of PARMA/EXPACS, PLOS ONE, 10(12): e0144679.

How to cite: Kostinskiy, A., Marshall, T., and Stolzenburg, M.: The mechanism of the origin the NBE (CID) and the initiating event (IE) of lightning due to the volume phase wave of EAS-RREA synchronous ignition of streamer flashes , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11487, https://doi.org/10.5194/egusphere-egu2020-11487, 2020.

A broad-band analyzer of the IME-HF instrument (“Instrument de Mesure du champ Electrique Haute Frequence”) is prepared for the TARANIS (Tool for Analysis of RAdiation from lightNIng and Sprites) micro-satellite of the French space agency CNES. The spacecraft is based on the MYRIADE series platform. It will be launched on a Sun synchronous polar orbit at 700 km altitude. TARANIS will carry a complex payload of six scientific instruments to study radiation from lightning and optical phenomena (Transient Luminous Events) observed at altitudes between 20 and 100 km (blue jets, red sprites, halos, elves). The scientific instruments onboard TARANIS will detect electromagnetic radiation from very low frequencies up to 37 MHz, optical radiation, X rays (with an aim to study the Terrestrial "Gamma-ray" Flashes), and energetic electrons.

The IME-HF instrument will record waveform measurements of fluctuating electric fields in the frequency range from a few kHz up to 37 MHz, with the following scientific aims: (i) Identification of possible wave signatures associated with transient luminous phenomena during storms; (ii)    Characterization of lightning flashes from their HF electromagnetic signatures; (iii) Identification of possible HF electromagnetic or/and electrostatic signatures of precipitated and accelerated particles; (iv) Determination of characteristic frequencies of the medium using natural waves properties; (v) Global mapping of the natural and artificial waves in the HF frequency range, with an emphasis on the transient events. The instrument will be also able to trigger and record interesting intervals of data using a flexible event detection algorithm.

How to cite: Santolík, O., Kolmašová, I., Lán, R., Uhlíř, L., Rauch, J.-L., Millet, A.-L., and Pincon, J.-L.: Broad-band electric field measurements above thunderstorms by the IME-HF instrument prepared for the TARANIS mission, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15159, https://doi.org/10.5194/egusphere-egu2020-15159, 2020.

A new computational approach, based on treating an electric streamer as a nonlinear instability, allows to determine unambiguously its parameters, for a given streamer length and external electric field, which may be nonuniform. Among the determined parameters are the speed, current and conductivity inside the streamer. These parameters may vary over orders of magnitude, depending on external conditions.

We use these parameters to calculate the radio emissions which would be observed on the ground from fast discharges produced in lightning, in which streamer velocities approach a significant fraction of the speed of light. Fast discharges play an important role in lightning initiation and may be responsible for production of Terrestrial Gamma Flashes (TGF). They manifest themselves in ground-based radio observations as Narrow Bipolar Events (NBE), to which the calculation results are compared.

We will discuss conditions, the effect of which on streamer propagation (and therefore electromagnetic radiation) may be quantified with the used computational method. These include (i) the external electric field modification due to charges deposited by previous streamers; and (ii) electron attachment inside the streamer channel, which is strongly affected by cloud humidity.

How to cite: Lehtinen, N.: Radio emission from fast streamers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9912, https://doi.org/10.5194/egusphere-egu2020-9912, 2020.