Morphologie de sprites et conditions de productions de sprites et de jets dans les systèmes orageux de méso-échelle

Ce document décrit l'analyse des conditions de production de phénomènes lumineux transitoires dans la mésosphère, produits en réponse à des décharges électriques énergétiques orageuses localisées au-dessous. Pendant les campagnes d'observation EuroSprite, quelques centaines d'images de sprites ont été obtenues, fournissant des informations sur la morphologie, la localisation et le moment de leur production. Des données issues de radars météorologiques, de satellite météosat, de deux types de système de détection d'éclairs, et de récepteur radio large bande ont été analysées. Des études de cas et une étude statistique sur un grand nombre de cas de sprites produits par 7 orages distincts sont réalisées. L'analyse porte sur le rôle de la composante intranuage des éclairs nuage-sol positifs à l'origine des sprites et notamment le lien avec leur morphologie, sur la relation avec le stade d'évolution des orages, et enfin sur les conditions associées à la production d'un jet géant aux Etats-Unis. Les sprites observés ont été produits par des systèmes convectifs de moyenne échelle (MCS) lorsque la partie stratiforme était en phase d'expansion. Les séquences des éclairs nuage-sol et l'activité intranuage observées au moment des sprites confirment une propagation horizontale importante (convective-vers-stratiforme). Les sprites de type colonne sont produits avec des délais plus courts que les sprites de type carotte. Plus le délai est court plus le nombre d'éléments est grand et plus leur luminosité est concentrée à une altitude élevée. Le jet géant semble avoir été favorisé par la configuration de charge et l'activité d'éclairs plutôt que l'altitude du sommet du nuage.This dissertation is devoted to the description of the conditions of production of transient luminous phenomena (sprites, jets, elves) in the mesosphere, which occur in response to energetic lightning discharges in thunderstorms underneath. During EuroSprite observation campaigns, a few hundred images of sprites have been obtained, providing information about event morphology, location and timing. Precipitation data from weather radar and cloud top altitude from Meteosat, as well as two lightning detection networks and a wide-band radio receiver have been analyzed. The methodology includes case studies and a statistical study over a large number of sprites produced by 7 different storms. The work focuses on the aspect of the intracloud lightning component associated with positive cloud-to-ground flashes, the link with the morphology of sprites, and the life cycle of thunderstorm systems. Additionally, a storm which produced a rare gigantic jet observed in the United States is analyzed in detail. The observed sprites were produced by mesoscale convective systems (MCS) during the expanding phase of the stratiform region. The cloud-to-ground flash sequences and the intracloud lightning component observed at the time of sprites confirm a large horizontal convective-to-stratiform propagation, as mechanism of charge collection, explaining displaced sprites. Sprites of column-type are produced with shorter delays than carrot sprites, and the shorter the delay, the more elements, their luminosity concentrating at greater altitudes. The gigantic jet appears to have been promoted by a certain charge configuration and lightning activity pattern, rather than a high cloud top altitude

Statistics and variability of the altitude of elves

From June 2008 to January 2016 nearly 800 elves have been recorded by a low-light camera in northeastern Spain. Elves occur in this region mainly over the lower-topped cold airmass maritime thunderstorms, peaking from November to January. Cloud-to-ground strokes still produce elves when maritime winter storms are carried inland, suggesting the cold season thunderstorm charge configuration favors strokes with large electromagnetic pulses. Altitudes of 389 elves were determined using optical data combined with a lightning location network. The overall median altitude was 87.1 km, near the typical OH airglow height, but average heights during individual nights ranged between 83 and 93 km. The lower elve nights (~84 km) occurred during slightly elevated geomagnetic conditions (Kp>3-, ap-index >10). Elve altitude often shifts by several kilometers during the night, apparently in response to changing background conditions in the upper mesosphere.Postprint (author's final draft

Turbines shoot upside-down lightning

Wind turbines emit lightning flashes upwards, producing these electrical discharges at regular intervals relative to the turbine’s rotation, and can do so tens of kilometres away from an active thunderstorm area.Peer ReviewedPostprint (published version

Evolution of gigantic jets at high temporal and spatial resolution

Gigantic Jets (GJ) are lightning discharges between the cloud and the lower ionosphere, which manifest as leaders at low altitude and transition to streamers as they rise in altitude. The evolution has never been imaged before at rates faster than 1/60th second. We ran a campaign in northern Colombia with an intensified high-speed camera at 900 images per second to study the development. The image sequence reveals a weakly luminous stepwise upward propagation from the cloud top to 40 km altitude, after which continuous and much brighter branches grow to the ionosphere in 2 ms. We show the stepping to be consistent with pilot system propagation in streamers.Postprint (author's final draft

Automated analysis and statistics of lightning leader speed, local flash rates and electric charge structure in thunderstorms

We introduce new methods for the analysis of complex three-dimensional lightning data produced by Lightning Mapping Arrays and illustrate them by cases of a mid-latitude severe weather producing thunderstorm and a tropical thunderstorm in Colombia. The method is based on the characteristics of bidrectional leader development as observed in LMA data (van der Velde and Montanyà, 2013), where mapped positive leaders were found to propagate at characteristic speeds around 2 · 10^4 m s-1, while negative leaders typically propagate at speeds around 10^5 m s-1. We fit slopes (t-x, t-y, t-z) to approximate leader speed, which can be used to determine the polarity, and summarize flash rate and polarity characteristics in a 3D grid over time. The summarized data can be used to follow charge structure evolution over time, as well as climatological studies comparing lightning parameters with the meteorological environment of storms.Postprint (author's final draft

Fair weather induced charges and currents on tall wind turbines and experiments with kites

Earth’s atmospheric potential rapidly increases up to few tens of kilovolts below 200 m altitude. This potential drop will induce charge to tall objects at ground by virtue of electrostatic induction. In this work we investigate the induced electric charges in fair weather to a 1.5 MW and 5 MW wind turbines. The effect of rotation is included and the current calculated result in currents of few micro-amps. The production of point discharge and corona is investigated and some experiments are conducted by means of instrumented kites.Preprin

Meteorological factors in the production of gigantic jets by tropical thunderstorms in Colombia

Gigantic jets are electric discharges that on rare occasions can be seen at night shooting out of the top of tropical thunderclouds, reaching the ionosphere (90 km). Using sensitive camera systems and detection software, we recorded 70 events over northern Colombia and adjacent seas, most of them captured between 2016 and 2022. This is the first study to compare the meteorological background conditions for thunderstorms that produced gigantic jets in 48 nights against 83 reference cases with monitored thunderstorms that did not produce jets, using vertical profiles from ERA5 reanalysis near the event location. From the vertical profiles, various meteorological parameters are calculated, grouped by metrics of the low level convective parcel, instability, humidity, warm cloud and mixed phase parameters, and vertical wind shear, not limited to default levels. We report statistically significant differences and effect sizes (Cohen's d) for gigantic jet producing environments compared to null environments. Gigantic jets are produced in conditions with reduced low level temperatures in combination with warmer mid levels. This causes a lower cloud base and higher -10 °C isotherm altitude, thus a greater warm cloud depth, as well as reduced updraft and downdraft buoyancy. Over northern Colombia the non-GJ producing storms tend to grow in an environment that supports more vigorous, multicellular convection by enhanced low-level storm-relative winds and stronger downdrafts. Over western Colombia, the non-GJ cases tend to have a lower equilibrium level while having favorable warm cloud parameters. No evidence is found for hypotheses that upper level vertical wind shear enables gigantic jet production, nor are overshooting tops larger. The findings can be used for forecasting gigantic jets and their climatologically optimal regions on Earth. We speculate that the environmental conditions shift the droplet size distribution towards larger drops at the cost of cloud droplets, with enhanced droplet shattering ice multiplication processes as they freeze. Depending on convective evolution, low rime accretion rates could briefly expand the negative charge region downward by inverse polarity charging while the upper positive charge concentration may weaken at the same time, which could lead to a temporary negatively imbalanced electric potential distribution in the cloud needed for gigantic jet emission.Peer ReviewedPostprint (published version

Polygyny and extra-pair paternity enhance the opportunity for sexual selection in blue tits

Polygyny and extra-pair paternity are generally thought to enhance sexual selection. However, the extent to which these phenomena increase variance in male reproductive success will depend on the covariance between success at these two strategies. We analysed these patterns over four breeding seasons in facultatively polygynous blue tits Cyanistes caeruleus. We found that both polygyny and extra-pair paternity increased variance in male reproductive success and that standardised variance in annual number of genetic fledglings was 2.6 times higher than standardised variance in apparent success when assuming strict monogamy. Nevertheless, male success at securing within-pair paternity was unrelated to success at gaining extra-pair paternity and, when considering the positive effect of age on extra-pair success and attracting a second female, polygynous males were no more likely to sire extra-pair fledglings. Overall, polygynous males fledged more genetic offspring than monogamous males, but first-year polygynous males lost a greater share of within-pair paternity. A literature review suggests that this adverse effect of polygyny on within-pair paternity is frequent among birds, inconsistent with the prediction that females engage in extra-pair copulation with successful males to obtain good genes. Furthermore, a male's share of paternity was repeatable between years, and among females of polygynous males within years, such that a compatibility function of extra-pair copulations was likewise unsupported. Instead, we suggest that the observed patterns are most consistent with a fertility insurance role for extra-pair copulations, which does not exclude the greater opportunity for sexual selection through differential ability of males to gain paternity

Gigantic jet discharges evolve stepwise through the middle atmosphere

In 2002 it was discovered that a lightning discharge can rise out of the top of tropical thunderstorms and branch out spectacularly to the base of the ionosphere at 90¿km altitude. Several dozens of such gigantic jets have been recorded or photographed since, but eluded capture by high-speed video cameras. Here we report on 4 gigantic jets recorded in Colombia at a temporal resolution of 200 µs to 1¿ms. During the rising stage, one or more luminous steps are revealed at 32-40¿km, before a continuous final jump of negative streamers to the ionosphere, starting in a bidirectional (bipolar) fashion. The subsequent trailing jet extends upward from the jump onset, with a current density well below that of lightning leaders. Magnetic field signals tracking the charge transfer and optical Geostationary Lightning Mapper data are now matched unambiguously to the precisely timed final jump process in a gigantic jet.Peer ReviewedPostprint (published version