MELBOURNE, Australia: Satellite images have captured a usually invisible phenomenon known as atmospheric gravity waves pulsing through clouds off Western Australia’s north-west.
Thunderstorms in the Pilbara and Kimberley on Monday and Tuesday triggered the waves. These waves spread out over the Indian Ocean and combined with a plume of dust to create a spectacular and rarely seen display.
Bureau of Meteorology (BOM) senior meteorologist Adam Morgan said atmospheric gravity waves were basically ripples in the sky.
“When you think of waves in the ocean, they’re a type of gravity wave,” he said.
“When you throw a rock in a pond and you see the ripples flowing out, they’re gravity waves as well.
“Essentially gravity waves are disturbances in any sort of fluid, so we see them often in water, but the atmosphere is a fluid as well — so any sort of disturbances in there can generate waves.”
On this occasion, the disturbance was the cold air flowing out from a thunderstorm on a very hot day.
“There was a big thunderstorm over the north-west of WA and the disturbance in this case was the cold air falling out of the thunderstorm and into the warmer air near the surface,” Dr Morgan said.
“The difference in density there causes the disturbance and then the gravity wave can travel out as the cold air spreads out.
“The disturbance will exist until everything rebalances itself, that’s why they can travel a long way.
“They travel a really long distance until they peter out. If you’ve ever been to the swimming pool and someone jumps in and does a dive bomb at one end of the pool, you might get the wave right at the other end of the pool after a bit of time.”
Although the gravity waves themselves are invisible, these were picked up by satellite imagery because bands of cloud formed along the wave crests.
Dr Morgan said these waves were very common and happen all the time, but they are usually invisible.
A gusty south easterly surge over NW WA – perhaps enhanced by outflow from last night's thunderstorms over the Northern Interior – carries a plume of dust out over the Indian Ocean. pic.twitter.com/TqySYPa3NL
— Andrew Miskelly (@andrewmiskelly) October 21, 2019
“When we do see them it’s usually when there’s enough moisture in the air to form a bit of cloud along the leading edge of the gravity wave.”
The atmospheric gravity waves that showed up on the satellite this week were also enhanced by a large plume of dust blowing off the north-west coast in offshore winds, following a dust storm in the Pilbara.
“The brown, orange tinge is dust being pushed off the coast because of the windy conditions,” Dr Morgan said.
“The cold outflow that falls out of that thunderstorm is quite gusty and those gusts have picked up a lot of dust and pushed it offshore.
“So when you look at the satellite image, it actually looked quite impressive.”
Dr Morgan said while atmospheric gravity waves did not pose a danger on the ground, they could create problems in the air.
“They’re not travelling at huge speeds. Where they can cause an impact though is in aviation,” he said.
“For planes flying around, gravity waves can cause a bit of turbulence and sometimes it can be severe turbulence, but for anyone standing on the ground, there’s nothing severe about gravity waves.”