Exaedre project: forecasting severe thunderstorms?

Lightning has been shooting electric darts through terrestrial skies since the beginning of time. On Earth, it undoubtedly enabled humankind to tame fire. An attribute of the Greek god Zeus, it also inspired some of the greatest myths. Yet the violence of this meteorological phenomenon is such that we are still as always faced with the challenge of protecting ourselves from it. Furthermore, thunderstorms are far from having yielded all their secrets. Atmospheric physicists still do not fully understand the process of cloud electrification or the way in which a lightning discharge is triggered and then spreads. The aim of the European Exaedre project is to gain a better understanding of the different processes at work in storm clouds, since lightning is the culmination of a multitude of dynamic and electrical microphysical phenomena of great complexity. The results of the project, conducted in Corsica in October 2018, should contribute to improving real-time storm monitoring and weather forecasts. The first findings of the research are being analyzed.

Storm chasing in Corsica

For the first time, a sensor-equipped aircraft penetrating into the heart of the cumulonimbus has enriched analyses made from the ground.

Severe thunderstorms could increase as a result of global warming. Verifying this hypothesis requires precise descriptions of storm-related phenomena. This is the aim of Exaedre (EXploiting new Atmospheric Electricity Data for Research and the Environment), a European project led jointly by CNRS, Cnes and Météo France. In October 2018, French scientists set out in search of thunderstorms in the Corsican sky. We followed them on mountain tops where the network of lightning observation stations are located and onboard the CNRS aircraft. Equipped with a variety of sensors to collect data on the composition and distribution of particles in storm clouds. Indeed, particles and their brutal collisions are what generate the static electricity of thunderstorms. The ideal flight plan involves heading toward the top of the cumulonimbus and then approaching the core of convection as the storm develops. Another scientific project, South American this time (Relampago - Cacti), also studies the processes at work in the cumulonimbus, but using drones that can only fly above the clouds, not inside them. It cannot therefore yield images of particles suspended in the cloud. During the Exaedre mission, 10,000 lightning discharges were measured during 15 thunderstorms. The average diameter of storm cells: 25 to 30 kilometres, compared to 1,000 kilometres for those measured by the South American Relampago campaign.

From cumulonimbus to lightning

At the core of a storm cloud, the lightning flash, which is called a lightning strike when it hits the ground, is the result of complex microphysical and electrical phenomena.

Thunderstorms are linked to the presence of a particular cloud type: the cumulonimbus. But how does electrical activity manifest itself? The cumulonimbus is a natural capacitor, whose electrostatic force essentially derives from the force of the oppositely charged currents traveling through it. The more intense the heat on the ground, the more powerful the upward convection motion and the greater the risk of thunderstorms. Air carrying water vapour and dust accumulates ice crystals as it rises in altitude. Aggregations of ice form at the anvil-like top of the cloud, more than 10 kilometres high. When they become too heavy, they fall and collide with rising droplets and crystals. Since these two opposing currents move at high speed, the friction causes electrons to be ripped off the suspended particles of dust, water and ice. As a result, the particles become electrically charged, in much the same way as a balloon rubbed against a synthetic fabric generates static electricity. But the cumulonimbus is a monster of millions of volts of potential energy! The dynamics at work in the storm cloud generally lead to an accumulation of negative charges at the bottom and positive charges at the top. Although air is an insulator, when the imbalance between charges reaches a certain point, the electricity opens a pathway through which the discharge occurs. It is estimated that most lightning happens inside the cloud or between two clouds. Only 20% of discharges reach the ground: these are the lightning strikes.

Victims of lightning strikes

Lightning strikes cause an estimated 6,000 deaths per year in the world and 240,000 injuries, often with severe consequences.

Extremely powerful meteorological phenomenon, lightning sparks a great many fires throughout the world – more than 22,000 a year on average in the United States alone – and it is estimated to be responsible for about 6,000 deaths per year in the world and 240,000 injuries, leaving survivors with often permanent disabilities. Yet, contrary to popular belief, the vast majority of people struck by lightning survive. In France, according to Inserm, there are around 15 to 25 deaths by lightning per year and a hundred lightning-strike injuries. How and why do most people survive lightning strikes? A research centre in Toulouse, the Institut de Médecine Environnementale Kéraunique (“keraunós” means lightning in Greek and Roman mythology) has been set up to study this unexplored area of medicine. Lightning-caused injuries vary from the very benign to the extremely severe: from bruises or fractures to arrhythmia, neurological disorders or organ damage from the shock wave. Interestingly, a group of people hit by lightning will exhibit very different symptoms, for yet unknown reasons. Starting in October 2019, a research protocol will be used in France to monitor metal nanoparticles in survivors. The hypothesis is that people may more or less “attract” lightning, and that these metal nanoparticles, once they are altered by an electric current running through the body, can have a lasting impact on the organism.

First Exaedre findings

A year after the Exaedre mission, the team is still far from having processed all the data collected in the eight flights of the flying laboratory.

Air and ground measurements conducted in Corsica have demonstrated that, as thunderstorms develop, hydrometeors (particles of water and ice) inside the cloud increase in number, and the lightning lasts longer and travels longer distances. This finding confirms the crucial role of suspended particles in storm processes. The team also observed that aggregations of ice (called graupels) are more numerous in the cloud’s core of convection and are clearly associated with the development of the electric field. Monitoring them should make it possible to improve severe thunderstorms and lightning forecasts. But a sensor-equipped aircraft cannot be flown during every storm! Remote instruments of observation (satellites, ground radars, etc.) will have to be developed, as will algorithms (even artificial intelligence) capable of processing new data. In the meantime, the information collected on the graupels during the Exaedre mission is already being integrated into Metéo France weather models. In the longer term, scientists hope to be able to simulate thunderstorms by integrating data (on the size and concentration of these aggregations, for example) monitored in real time. This should improve the coordination of severe storm-related warnings and preventive measures.