Unveiling the secret of “Bolt from the Blue”: How does lightning transform into a “long-range sniper”?

The sky is clear, but a bolt of lightning suddenly strikes the ground like an "invisible assassin". This is the so-called "bolt-from-the-blue". Ordinary lightning often strikes the ground below rolling thunderclouds, but a bolt from the blue strikes the ground without warning in an area with no clouds or precipitation overhead. Its unpredictability poses a serious threat to people's lives and property. So how is this "long-range sniper" bolt from the blue formed?

Recently, Xie Xiushu, a researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, and his team published a breakthrough study in the Science Bulletin, revealing the mysterious cause of the blue-sky thunderbolt . The team used a lightning very high frequency interferometer independently developed with the ability to finely resolve discharge processes and channels to detect two blue-sky thunderbolts on the Qinghai-Tibet Plateau. The radio frequency dynamic positioning imaging results clearly gave a complete picture of the blue-sky thunderbolt starting from the thundercloud, transmitting horizontally for several kilometers after leaving the cloud, and then turning to the ground to develop and hit the ground (Figure 1).

Figure 1 Left: radar echo profile of thundercloud; Right: 3 negative ground-to-ground flash radiation source location results and inferred thundercloud charge structure. Negative ground-to-ground flash 1 is a normal lightning, and negative ground-to-ground flashes 2 and 3 are two thunderbolts from the blue sky, and their strike locations are 3.6 and 3.8 km away from the edge of the precipitation area, respectively.

Under certain dynamic and microphysical conditions, in the mixed phase region where ice and water coexist in thunderstorm clouds, charge transfer occurs after elastic collisions between ice-phase hydrometeors of different sizes (mainly graupel and ice crystals). Under the action of airflow and gravity, large and small ice-phase particles carrying charges of different polarities are macroscopically separated, and positive and negative charges are gathered in different regions in thunderstorm clouds, which is the prerequisite for lightning. The team developed a method to analyze the charge distribution of thunderstorm clouds based on the differences in the physical properties of lightning channel development. Based on this, the evolution characteristics of the charge structure of plateau thunderstorm clouds that produce thunderbolts from the blue are revealed. In the initial development stage, a negative dipole charge structure of "negative on top and positive on bottom" is first formed in the cloud. In the mature stage, an upper positive charge region is further developed, evolving into a "positive-negative-positive" tripole charge structure. The lightning discharge starts from between the positive and negative charge areas of the positive dipole above the cloud, and develops to the charge areas of opposite polarity in the form of a two-way leader. The negative polarity channel "breaks through" upward, and after breaking through the positive charge layer above the cloud, the lightning channel "turns" horizontally and rushes out of the cloud body, sneaking for several kilometers in the clear sky before turning and hitting the ground directly.

Figure 2 Schematic diagram of the evolution of the charge structure in a thunderstorm cloud over the Qinghai-Tibet Plateau when a "bolt from the blue" occurs

"In the past, we could only infer the charge structure and macroscopic characteristics of lightning in thunderstorm clouds by measuring the changes in electric fields. Now, through the lightning VHF interferometer independently developed by us, we can accurately distinguish the lightning process and discharge channel with a time resolution of microseconds or even nanoseconds, so as to study the characteristics of lightning development and transmission and the corresponding evolution of thunderstorm charge structure, which provides an important means to improve the understanding of lightning processes and mechanisms." Researcher Xie Xiushu said. Combining weather radar echoes and sounding data, the team found that the unbalanced electric dipole in which the upper positive charge area in the cloud is weaker than the lower negative charge area, and the small size of the thunderstorm cloud monomers are the main reasons for the two thunderbolts from the blue.

"In fact, thunderbolts from the blue occur not only on the plateau, but also in low-altitude areas. Thunderbolts from the blue can be several kilometers or even more than ten kilometers away from the thunderclouds. Since there is usually no rain overhead when thunderbolts from the blue occur, in addition to causing harm to life and property, the thermal effect of lightning strikes may also cause wildfires and forest fires, etc., and have an important impact on the earth's ecosystem and atmospheric environment. In the context of climate warming, thunderbolts from the blue are worthy of attention." Researcher Xie Xiushu further stated, "We would like to remind the public that when you are in a clear sky but dark clouds appear within your sight or you hear distinguishable thunder, you should also stay away from dangerous areas such as high ground and trees as soon as possible to avoid being hit by thunderbolts from the blue."

【Xiushu Qie is a researcher at the Institute of Atmospheric Physics, Chinese Academy of Sciences. He is a recipient of the Outstanding Young Scientist Fund of the National Natural Science Foundation of China, the Chairman of the International Commission on Atmospheric Electricity, and a fellow of the American AGU. He is mainly engaged in atmospheric electricity and mesoscale meteorology research. He has published 4 books and more than 300 papers. He was an expert of the National "863 Program" and the chief scientist of the National "973 Project". He has won the honorary titles of China Youth Science and Technology Award, National March 8 Red Flag Bearer, China's Top Ten Women, and Outstanding Mentor of the Chinese Academy of Sciences. As the first finisher, he won 3 first and second prizes in provincial and ministerial science and technology awards. He is the professional editor-in-chief of the journal JGR: Atmospheres, the deputy editor-in-chief of Atmospheric Research, and the deputy editor-in-chief or editorial board member of many domestic Chinese and English journals such as "Atmospheric Science", AAS, JMR, and EPP. 】

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