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The team confirmed the rare event using satellite images, social-media posts and a visit to the affected area.

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People inspect storm damage at Bhaluhi village in Bara district, Nepal.

A deadly storm that tore through Nepal almost two weeks ago was the country’s first ever recorded tornado, say researchers there. A team identified the extremely rare event in southeast Nepal without the aid of typical tornado-detecting instruments, instead relying on satellite images, analysis of social-media posts and a visit to the affected area.

The government says 28 people died and more than 1,100 were injured in the storm on 31 March, which also damaged about 2,600 buildings and a national park that is listed as a World Heritage Site.

The storm shifted slabs of concrete 50 metres, which requires a massive amount of power not typical of storms observed in Nepal, says Dhiraj Pradhananga, a meteorologist and president of The Small Earth Nepal, a non-governmental organization in Kathmandu. “We don’t even have a Nepali word for tornado,” he says.

Reports of the storm’s damage took many meteorologists by surprise. A team of researchers at The Small Earth Nepal and the country’s Department of Hydrology and Meteorology (DHM) initiated an investigation into the nature of the storm the morning after it struck.

Windstorms and thunderstorms are common during the pre-monsoon months from March to May, says Archana Shrestha, a meteorologist at the DHM in Kathmandu. But she suspected something unusual had happened after hearing accounts from locals of spinning winds. Violent, whirling funnels of air that follow a path are characteristic of tornadoes, she says.

Twisted trees

Nepal lacks the scientific infrastructure to identify a tornado easily, says Shrestha.

In countries such as the United States, which is hit by some 1,200 tornadoes every year, these rotating columns of wind are identified using Doppler radar imagery, says Leigh Orf, an atmospheric scientist at University of Wisconsin–Madison.

To determine whether the storm was in fact a tornado, the Nepali research team analysed high-resolution images from the European Earth-observing satellite pair Sentinel-2, taken before and after the event. The researchers also looked at social-media posts and plotted them onto Google Maps using the posts’ geolocation data or locations mentioned in the text. During a four-day visit to the affected towns, they took measurements of the trail of damage left by the storm and the distance that debris had moved. The team also collected surveillance footage of the storm.

The researchers saw a 90-kilometre-long, 200-metre-wide path of destruction carved out by the tornado. The trail began at Chitwan National Park near the border with India and smashed through homes, groundwater pumps and power lines. The researchers estimate that the tornado reached wind speeds of between 180 and 330 kilometres per hour.

Locals reported seeing a vertical funnel-shaped structure reaching from the clouds to the ground, which lifted anything that crossed its path, according to a report for the government by Piyush Dahal, a research coordinator at The Small Earth Nepal in Kathmandu. Images also showed trees and corrugated metal sheets that had been twisted and distorted. The researchers will submit a full report of their investigation to the government next week.

“Looking at that satellite image, it’s definitely a tornado that hit — there is no other atmospheric phenomenon that produces that kind of damage path,” says Orf. “Damage on the ground appears consistent with a weak to moderately strong tornado.”

Tornadoes are typically formed by thunderstorms known as supercells, which are not usually found in Nepal, he adds.

Unsurprised

Some scientists are not surprised to see a tornado in Nepal. The Ganges river basin in Bangladesh, southeast of where the storm hit, has frequent tornadoes, and a few cases have been reported in northeastern India, says Hirohiko Ishikawa, a meteorologist at the Disaster Prevention Research Institute of Kyoto University in Uji, Japan. It is possible the southeastern plains of Nepal have previously been hit by minor tornadoes that went unrecorded, he says.

The Nepali government is improving its weather instrumentation and has installed one radar device in western Nepal, which is still in the testing phase, says Shrestha. Two more are planned for the central and eastern regions. A weather balloon for measuring vertical atmospheric conditions is also being tested. But Pradhananga says more resources are needed to study extreme weather events in the country.

“The government needs to set up a separate unit to conduct research, not only of tornadoes, but of other water-related disasters such as flooding, drought and heavy rainfall,” he says.

doi: 10.1038/d41586-019-01159-w
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Article credit to: http://feeds.nature.com/~r/nature/rss/current/~3/GtbdPIANCnw/d41586-019-01159-w

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