Mesospheric Bore

Nov. 29, 2016: This month, a lot is happening in the mesosphere. The mesosphere is a layer of Earth’s atmosphere above the stratosphere; it is the realm of sprites, noctilucent clouds (NLCs), and airglow. Starting on Nov. 17th, NASA’s AIM spacecraft spotted bright noctilucent clouds forming in the mesosphere above Antarctica. Then, in an apparently unrelated development on Nov. 24th, the normal dome of airglow over China split in two. Xiao Shuai photographed the event from Mount Balang in Sichuan:

This is called a “mesospheric bore”–and not because it’s dull.  A bore is a type of atmospheric wave with deep ripples at its leading edge.  Indeed, you can see the ripples in Shuai’s photo separating the zone of airglow from clear sky.

Bores fall into the category of “gravity waves”—so called because gravity acts as the restoring force essential to wave motion. Analogy: Boats in water. When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave.

In this case, instead of water, rarefied air is the medium through which the wave propagates.  The sudden boundary in the airglow layer is probably akin to a hydraulic jump.  But what created the disturbance in the first place?  (What is the ‘boat’?) No one knows.

“There may be updates in the coming days as scientists from NASA and the Chinese Academy of Science check data from satellites to learn more about this event,” says Jeff Dai, who has been helping Xiao Shuai process and communicate his extraordinary images. “Also, we encourage other photographers from Thailand, Myanmar, Bangladesh and India to submit their images of the wave.”

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Climate Change at the Edge of Space

by Dr. Tony Phillips (Spaceweather.com)

In the summer of 1885, sky watchers around northern Europe noticed something strange. Sunsets weren’t the same any more.  The red and orange colors they were used to seeing were still there—but those familiar colors were increasingly joined by rippling waves of luminous blue.

At first they chalked it up to Krakatoa, which had erupted just two years earlier. The explosion of the Indonesian super volcano hurled massive plumes of ash and dust into the atmosphere more than 50 miles high, coloring sunsets for years after the blast.

Eventually Krakatoa’s ash settled, yet the rippling waves of luminous blue didn’t go away.  Indeed, more than 100 years later, they are shining brighter than ever.

Ruslan-Merzlyakov-2_1465805905_stripAbove: Noctilucent clouds over Nykøbing Mors, Denmark, on June 13, 2016. Photo credit: Ruslan Merzlyakov

Today we call them, “noctilucent clouds” (NLCs). They appear with regularity in summer months, shining against the starry sky at the edge of twilight. Back in the 19th century you had to go to Arctic latitudes to see them. In recent years, however, they have been sighted from backyards as far south as Colorado and Kansas.

Noctilucent clouds are such a mystery that in 2007 NASA launched a spacecraft to study them. The Aeronomy of Ice in the Mesosphere satellite (AIM) is equipped with sensors specifically designed to study the swarms of ice crystals that make up NLCs.  Researchers call these swarms “polar mesospheric clouds” (PMCs).

A new study published in the Journal of Geophysical Research (doi:10.1002/2015JD024439) confirms what some researchers have long suspected:  PMCs in the northern hemisphere have become more frequent and brighter in recent decades—a development that may be related to climate change.

The story begins long before the launch of AIM.

sbuvicemassThe paper’s lead author Mark Hervig, an AIM scientist with GATS, Inc., explains: “Thanks to decades of data from the Solar Backscatter Ultraviolet (SBUV) instrument on NOAA weather satellites, we know that PMCs have become thicker and more frequent.”

Right: According to data from SBUV, the ice mass of PMCs has increased since 1980.

“The question we’ve been grappling with is why?” says co-author David Siskind of the Naval Research Lab in Washington, DC. “Why did the upper mesosphere (the atmospheric layer where PMCs form) become icier?”

The ingredients for PMCs are simple enough. Ice requires water molecules + freezing temperatures.  However, SBUV could not tell researchers if the mesosphere was getting wetter or colder–or both.

Fortunately, AIM has an instrument onboard named SOFIE that can unravel the water-temperature knot.  Hervig, Siskind, and another co-author, Uwe Berger of the Leibniz-Institute of Atmospheric Physics in Germany, recently interpreted the 36-year SBUV record using data from SOFIE, and this is what they found:

At altitudes where PMCs form, temperatures decreased by 0.5 ±0.2K per decade. At the same time, water vapor increased by 0.07±0.03 ppmv (~1%) per decade.

current_daisyAbove: AIM data taken on June 21, 2016, show noctilucent clouds ringing the north pole.

“These results settle the decades old question of whether or not the observed long-term change in PMCs is an indicator of changing temperature or humidity,” says James Russell, AIM Principal Investigator. “It’s both.”

These results are consistent with a simple model linking PMCs to two greenhouse gases. First, carbon dioxide promotes PMCs by making the mesosphere colder. (While increasing carbon dioxide warms the surface of the Earth, those same molecules refrigerate the upper atmosphere – a yin-yang relationship long known to climate scientists.) Second, methane promotes PMCs by adding moisture to the mesosphere, because rising methane oxidizes into water.

methane_stripAbove: A graphic prepared by Prof. James Russell of Hampton University shows how methane, a greenhouse gas, boosts the abundance of water at the top of Earth’s atmosphere. This water freezes around “meteor smoke” to form icy noctilucent clouds.

However, the simple model may not be enough:

“Our study shows that PMCs may be tied to changes in the temperature of the stratosphere as well,” says Hervig. “This complicates things because the stratosphere is governed by a wide range of phenomena including ozone concentration, greenhouse gases, and volcanic aerosols.

“While we have finally quantified the underlying temperature and water vapor changes related to PMCs,” he adds, “there is still work to be done in understanding the details of what caused these changes.”

Summer is the season for PMCs and noctilucent clouds.  As June turns into July, observers in Europe are already reporting some displays, and they should appear over the northern USA within weeks.

Observing tips: Look west 30 to 60 minutes after sunset when the sun has dipped ~10 degrees below the horizon. If you see blue-white tendrils spreading across the sky, you may have spotted a sign of climate change.  It happens, even at the edge of space.

Noctilucent Cloud Season Begins (May 2015)

by Dr. Tony Phillips (Spaceweather.com)

May 23, 2015: NASA’s AIM spacecraft has spotted a luminous patch of electric-blue drifting across the Arctic Circle. The sighting marks the beginning of the 2015 season for noctilucent clouds (NLCs). “The first clouds appeared on May 19th–a bit earlier than usual,” reports Cora Randall, AIM science team member at the University of Colorado. They are located at longitude +90o in this polar image recorded by AIM’s CIPS instrument:


The first northern-hemisphere NLCs of 2015, recorded by AIM/CIPS on May 19th

“It is always good to see the beginning of another season,” says James Russell of Hampton University, principal investigator for the AIM mission. “What surprises will it bring? We will see. The clouds have never disappointed us.”

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the planet’s surface. The clouds are very cold and filled with tiny ice crystals. When sunbeams hit those crystals, they glow electric-blue.

Noctilucent clouds first appeared in the 19th century after the eruption of super-volcano Krakatoa. At the time, people thought the clouds were caused by the eruption, but long after Krakatoa’s ash settled, the clouds remained. In those days, NLCs were a polar phenomenon confined mainly to the Arctic. In recent years they have intensified and spread with sightings as far south as Utah and Colorado. This could be a sign of increasing greenhouse gases in Earth’s atmosphere.

Data from AIM have shown that NLCs are like a great “geophysical light bulb.” They turn on every year in late spring, reaching almost full intensity over a period of 5 to 10 days. News flash: The bulb is glowing. Stay tuned for sightings.

Noctilucent Clouds, Behaving Strangely

by Dr. Tony Phillips (Spaceweather.com)

March 2, 2015: The southern season for noctilucent clouds (NLCs) has come to an end. NASA’s AIM spacecraft observed the last wisps of electric-blue over Antarctica on Feb. 20, 2015. The end of the season was no surprise: The polar clouds always subside in late summer. Looking back over the entire season, however, reveals something unexpected. In an 8-year plot of Antarctic noctilucent cloud frequencies, the 2014-2015 season is clearly different from the rest:

These data come from the AIM spacecraft, which was launched in 2007 to monitor NLCs from Earth orbit. The curves show the abundance (“frequency”) of the clouds vs. time for 120 days around every southern summer solstice for the past 8 years.

“This past season was not like the others,” notes Cora Randall, a member of the AIM science team and the chair of the Department of Atmospheric and Oceanic Sciences at the University of Colorado. “The clouds were much more variable, and there was an enormous decrease in cloud frequency 15 to 25 days after the summer solstice. That’s when the clouds are usually most abundant.”

What does this mean? Previous research shows that NLCs are a sensitive indicator of long-range teleconnections in Earth’s atmosphere, which link weather and climate across hemispheres. The strange behavior of noctilucent clouds in 2014-2015 could be a sign of previously unknown linkages. “Preliminary indications are that it is indeed due to inter-hemispheric teleconnections,” says Randall. “We’re still analyzing the data, so stay tuned.”

Now attention turns to the northern hemisphere, where the season for NLCs typically begins in May. Will the northern season ahead be as strangely variable as the southern season, just concluded? Says Randall, “I can’t wait to find out.”