Ground-to-cloud lightning recorded for the first time in Brazil

Atmospheric discharges moving from the ground toward the clouds can cause great damage to elevated structures such as wind energy generators

By Elton Alisson

Agência FAPESP – A type of lightning that, instead of descending from the clouds to reach the ground (as occurs in most atmospheric discharges), begins on the surface of the earth and propagates toward the clouds has begun to attract attention in countries such as the United States and Japan in recent years because it can cause damage to elevated structures such as wind energy generators.

Researchers from the Atmospheric Electricity Group (ELAT) at the National Institute for Space Research (INPE) recorded the occurrence of this type of lightning for the first time in Brazil.

Using normal and high-speed cameras, they observed and recorded ground-to-cloud lightning on the Pico do Jaraguá—the city of São Paulo’s highest point at 1,135 meters above sea level—at the end of January and beginning of March.

Also known as upward-moving lightning, these atmospheric discharges originate from elevated structures such as telecommunications towers or lightning rods on tall buildings.

Because of their altitudes, these structures can concentrate strong electrical charges at their tops induced by and of opposite charge to the base of a storm cloud passing over it. When this event occurs during a storm, a discharge begins in the structure and propagates in the direction of the cloud.

“Usually, they are clouds that are negatively charged at their bases attract these bolts of lightning from the ground, generally from high places,” said Marcelo Fares Magalhães Saba, ELAT researcher and project coordinator, to Agência FAPESP.

To find the most likely place for this type of lightning in São Paulo State, the researchers used an atmospheric discharge detection system implemented by INPE with FAPESP funding. The system showed that nearly three times as many lightning strikes occur on the Pico do Jaraguá than in other parts of the city.

Using a high-speed camera able to take 4,000 frames per second, the researchers recorded the formation of three ground-to-cloud bolts during a storm at the end of January. The lightning was discharged from a 130-meter transmission tower on the Pico de Jaraguá during a period of less than six minutes.

At the beginning of March, the researchers again recorded three more upward-moving bolts at the same location in only 7 minutes, a number considered to be very high, especially considering the short time period. For example, at the Empire State Building in New York City, which is 410 meters high, an average of 26 ground-to-cloud lightning bolts occur over a one-year period.

“It can take 10 or 20 years for a tall building to produce three upward-moving lightning bolts,” estimated Saba.

The researchers still can not explain why the Pico de Jaraguá has such a high frequency of ground-to-cloud lightning. They also intend to investigate whether the intensity of these bolts is greater than that of normal cloud-to-ground lightning, and to estimate the minimum height of a structure that can promotes such discharges. They aim to answer these and, other questions during the coming years.

The group has already observed that although the impact of a cloud-to-ground (CG) lightning bolt is widely distributed, with half of the discharges touching different places on the ground, the ground-to-cloud lightning bolts always impact a single point—the point from which they depart.

“In general, there are many more descending bolts than ascending ones. However, the descending bolts impact many different points on the ground, whereas the ascending bolts always depart from the same place. For this reason, they impose substantial stress on this location, which can be a TV or mobile phone tower, for example,” said Saba.

Reviewing safety norms

According to Saba, observations made by the group on ground-to-cloud lightning could help improve safety norms to protect against lightning in Brazil. The existing norms are based on CG lightning.

As the tendency in cities such as São Paulo is always to build taller and taller buildings, the researcher believes that it will be necessary to review safety norms to protect these new structures against lightning. “The taller the building, the more likely it is to originate upward-moving lightning,” he said.

Saba says that in countries such as the United States and Japan, efforts are underway to increase knowledge about the physics and characteristics of upward-moving lightning bolts to protect the turbines in wind energy generators, which can be more than 150 meters high and which can be destroyed by ground-to-cloud lightning.

As this type of energy generation is becoming more widely used in Brazil, the researcher believes it is important to intensify studies on this type of lightning in the country, which has the highest rate of lightning strikes in the world.

Through the use of high- velocity cameras and the atmospheric discharge detection system used in São Paulo, the researchers intend to visualize the trajectories of ground-to-cloud lightning in three dimensions. With this approach, they seek to estimate the frequency and conditions under which the phenomenon occurs and, to analyze the velocity and propagation of the electrical discharge more accurately.

The study could also help to improve atmospheric discharge detection systems that monitor the frequency of lightning in Brazil.

“If we know exactly where these upward-moving lightning bolts leave the ground, it could help these detection systems monitor the location of lightning with more accuracy,” said Saba.