NASA Zooms in on “Night-Shining Clouds” of Earth’s Biosphere

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"These are the highest clouds in Earth's atmosphere, formed in the coldest place in Earth's atmosphere. Although the clouds occur only in the polar summer, they help us to understand more about the whole globe,"  says Charles Jackman, an atmospheric scientist at NASA's Goddard Space Flight Center.


Looking down from above, a NASA satellite captured this composite image of the silvery blue clouds shining brightly in the night — a noctilucent cloud cover some 50 miles above the Southern Pole.

First noticed in 1885, these clouds spawned over a century of research into what conditions causes them to form and vary –- questions that still baffle scientists to this day. Since 2007, a NASA mission called Aeronomy of Ice in the Mesosphere (AIM) has shown that the cloud formation is changing year to year, a process they believe is intimately tied to the weather and climate of the whole globe.

"The formation of the clouds requires both water and incredibly low temperatures," says Charles Jackman, who is NASA's project scientist for AIM. "The temperatures turn out to be one of the prime driving factors for when the clouds appear."

The appearance of the clouds, also known as polar mesospheric clouds or PMCs since they occur in a layer of the atmosphere called the mesosphere, can provide information about the temperature and other characteristics of the atmosphere, which in turn, helps researchers understand more about Earth's low altitude weather systems. Scientists have discovered that events in one hemisphere can have a sizable effect in another.

The clouds, exclusively a summertime phenomenon, light up because they're so high that they reflect sunlight from over the horizon. They are formed of ice water crystals most likely created on meteoric dust.

"The question people usually ask is why do clouds which require such cold temperatures form in the summer?" says James Russell, an atmospheric scientist at Hampton University in Hampton, Va., who is the Principal Investigator for AIM. "It's because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere."

Atmospheric scientist Bodil Karlsson, a member of the AIM team, has been analyzing why the start of the southern noctilucent cloud season can vary so dramatically. Karlsson is a researcher at Stockholm University in Sweden, though until recently she worked as a post-doctoral researcher at the University of Colorado. A change in when some pretty clouds show up may not seem like much all by itself, but it's a tool for mapping the goings-on in the atmosphere, says Karlsson.

"Since the clouds are so sensitive to the atmospheric temperatures, they can act as a proxy for information about the wind circulation that causes these temperatures. They can tell us that the circulation exists first of all, and tell us something about the strength of the circulation."

She says the onset of the clouds is timed to something called the southern stratospheric vortex –-  a winter wind pattern that circles above the pole. In 2010, that vortex lingered well into the southern summer season, keeping the lower air cold and interfering with cloud formation. This part of the equation is fairly straightforward, and Karlsson has recently submitted a paper on the subject to the Journal of Geophysical Research. But this is not yet the complete answer to what drives the appearance of these brightly lit clouds.

AIM researchers also believe there is a connection between seemingly disparate atmospheric patterns in the north and south. The upwelling of polar air each summer that contributes to noctilucent cloud formation is part of a larger circulation loop that travels between the two poles. So wind activity some 13,000 miles (20,920 km) away in the northern hemisphere appears to be influencing the southern circulation.

The first hints that wind in the north and south poles were coupled came in 2002 and 2003 when researchers noticed that despite a very calm lower weather system near the southern poles in the summer, the higher altitudes showed variability. Something else must be driving that change.

Now, AIM's detailed images of the clouds have enabled researchers to look at even day-to-day variability. They've spotted a 3- to 10-day time lag between low-lying weather events in the north –- an area that, since it is fairly mountainous, is prone to more complex wind patterns –- and weather events in the mesosphere in the south. On the flip side, the lower atmosphere at the southern poles has little variability, and so the upper atmosphere where the clouds form at the northern poles stays fairly constant. Thus, there's a consistent start to the cloud season each year.

"The real importance of all of that," says Hampton's Russell, "is not only that events down where we live can affect the clouds 50 miles (80 km) above, but that the total atmosphere from one pole to the next is rather tightly connected."

Hammering out the exact mechanisms of that connection will, of course, take more analysis. The noctilucent cloud season will also surely be affected by the change in heat output from the sun during the upcoming solar maximum. Researchers hope to use the clouds to understand how the sun's cycle affects the Earth's atmosphere and the interaction between natural- and humankind-caused changes.

The Daily Galaxy via nasa.gov

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Image credit: NASA/HU/VT/CU LASP 

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