Monthly Archives: March 2011

The Curious Case of Lake Superior's Shrinking Cloud Street Droplets

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Parallel lines of cumulus clouds often appear when frigid, dry winds rush over comparatively warm bodies of water. NASA satellites have observed the striking cloud formations – which atmospheric scientists call “cloud streets” — over the Hudson Bay, Greenland Sea, Bering Sea, and the Amery Ice Shelf a number of times in the past.

Recently, University of Wisconsin scientist Steve Ackerman was combing through data from NASA’s MODIS instrument as part of an effort to catalog and classify different cloud types. Something about the street clouds in this image of Lake Superior (above) struck him as peculiar. We caught up with him during a poster session at an American Geophysical Union meeting in San Francisco to find out more.

WoE: What are we looking at here?

Ackerman: These are cloud streets. They’re really quite interesting clouds. They occur when you get cold air blowing over warm water. You get them frequently over the Great Lakes and off the East Coast as well.

WoE: What was it about this particular cloud street set that you found notable?

Ackerman: We actually looked at a series of these, and what we found was that the clouds start small, grow in altitude, get thicker optically, and then do something quite strange and unexpected.

WoE: Strange and unexpected? Please explain…

Ackerman: Yes, often what happens is that the size of the cloud droplets grow as we’d expect at first, but then partway across the lake the size of the particles starts to decrease.

WoE: And that’s surprising?

Ackerman: Yes, we have no idea why they’d do that. They should be getting progressively bigger as they move across the lake and pick up moisture.

WoE: About how big are these cloud droplets, and how do they change over time?

Ackerman: They start off at about 5 microns. (For reference, human hair is about 100 microns.) They grow up to about 20 microns, and then they drop down to 10 microns.

WoE: How long does that process take?

Ackerman: About four hours.

WoE: Why do think it’s happening? 

Ackerman: We’re really not sure. Perhaps dry air is coming in from above.

WoE: Is this the only time you’ve observed this phenomenon?

Ackerman: It’s pretty rare. We found it in the MODIS imagery in the five years that we looked about 15 times.

WoE: What makes a peculiar phenomenon like this worth studying?

Ackerman: The next step is to work with cloud modelers and to see if they’re modeling things well enough to explain what’s going on. If the models can’t recreate unusual events like these cloud streets, we know they’re not getting things right. We need models to get the global climate right, and also the weather prediction right. 

The top image comes from the Moderate Resolution Imaging Spectroradiometer (MODIS).  The other two images are courtesy of Steve Ackerman.

–Adam Voiland, NASA’s Earth Science News Team

Can NASA Satellites Monitor Radiation Plumes from the Fukushima Disaster?

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NASA is using multiple satellites and sensors to monitor the aftermath of the devastating earthquake and tsunami that rattled Japan on March 11.

However, NASA’s Earth-observing satellites are unable to directly measure radiation-containing plumes, such as those experts fear may have wafted from a damaged Japanese nuclear plant in Fukushima prefecture.

We checked in with Robert Cahalan, the head of Goddard Space Flight Center’s Climate and Radiation Branch and the project scientist for the SORCE satellite, to find out why. Here’s how Cahalan explained it:

“NASA could fly a drone directly into a cloud to detect radioactivity, but it’s not easy to measure the damaging radiation from the Fukushima plant with a satellite. 

The radiation consists mostly of negatively charged electrons from so-called “beta decay” of radioactive products of the nuclear fission reactions, as well as positively charged alpha particles, which are identical to a helium nucleus (for example, two protons and two neutrons all bound together into a single particle). 



NASA does have detectors in space that measure such charged particles, but the great majority of these particles don’t come from Earth. Rather, they come from the sun, which emits a very large number of charged particles in what is called the “solar wind” — which is especially intense when the sun is active. The particles can also come from sources outside our solar system, so-called galactic cosmic rays, or GCRs, that become more detectable when the sun is less active.

Fortunately, we humans down on Earth are protected from a lot of this particle radiation by Earth’s magnetic field, which steers charged particles along the field lines toward Earth’s magnetic poles, and thus acts as a shield for the human population. 

Trying to pick out the Fukushima radioactivity from the huge number of charged particles in outer space would be like finding the proverbial “needle in the haystack.” So, unfortunately, we have to rely on ground-based particle detectors, like the common Geiger counters that have been shown in use by the workers in their white hazmat suits in the tragic scenes in Japan.
 


There is also high-energy gamma radiation, which is electromagnetic radiation. Again, NASA has had the Compton Gamma Ray Observatory (GRO) in space, and now has FERMI. But these look for extremely intense bursts of gamma radiation that come from colliding galaxies, quasars, and other extreme events in the universe. The low flux of gamma radiation from the nuclear power plant is all absorbed in the Earth’s atmosphere, and never makes it into space. The only way we might detect some gamma radiation from Earth’s surface would be if we created a gamma ray burst by detonating a large nuclear bomb. That kind of event cannot happen in a nuclear reactor, even in the worst case of a core meltdown.

NASA’s Earth-observing satellites monitor many health related quantities including aerosols and ozone, nitrous oxides, and other constituents in the air we breathe, as well as fires, floods, and other events that impact life on Earth; however, near-Earth radioactivity can only be detected near the radioactive source, not by satellites.”

Visualization of solar wind and Earth’s magnetosphere courtesy of Steele Hill and NASA’s SOHO team. Visit this page for more information.

–Adam Voiland, NASA’s Earth Science News Team

A Moment for Glory

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NASA held a press conference about its soon-to-launch Glory satellite on January 20 in Washington, DC. The  mission will advance understanding of the energy budget and climate change by taking critical measurements of aerosols and total solar irradiance.

Want to learn more about Glory? Read an overview of the mission, view one of these two image galleries, brush up on aerosol science, take a look at this Q & A (pdf), follow along on Twitter, or browse the mission websites. Also, see what Nature, Discovery, and SpaceFlight Now have to say about Glory.

–Adam Voiland, NASA’s Earth Science News Team

Is Coagulation Geoengineering's Achilles' Heel?

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Jason English, a graduate student at the University of Colorado at Boulder and a participant in NASA’s Graduate School Researchers Program, chats with us about some of his recent research into geoengineering.

WoE: Do you find there is a taboo of sorts against studying geoengineering among Earth scientists? It’s fairly unusual to see the topic come up at conferences, so your poster caught our eye.

English: There is more acceptance of studying it in just the last couple of years. I think scientists are facing the reality that countries aren’t doing much to slow the emissions of greenhouse gases. Eventually, we may have to choose between the risks and consequences of climate change and the risks and consequences of climate engineering. The only way to make an educated decision about that is to study it.

WoE: What type of geoengineering are you focusing on?

English: For my PhD, I have been looking at stratospheric aerosols.

WoE: Hold up. What are stratospheric aerosols?

English: They’re the tiny particles that are aloft in the atmosphere about 20 kilometers above the surface of the Earth. One of the leading geoengineering ideas is to inject aerosols into the stratosphere. I decided, after getting help and input from colleagues such as Michael Mills and Brian Toon, to set up a computer model that would analyze exactly how something like that would work.

WoE: And what do stratospheric aerosols have to do with climate?

English: People have suggested we could use a type of a particle for geoengineering that is actually composed of tiny droplets of sulfuric acid. Those are called sulfates. Sulfates reflect sunlight. If you have a layer of these particles up in the stratosphere they reflect part of the incoming solar radiation from the sun back to space. Overall, they have a cooling effect.


WoE: And sulfates can make it all the way up to the stratosphere?

English: Yes, some of the stronger volcanic eruptions can send particles into the stratosphere. They take a couple of years to settle back down to the surface. Very tiny amounts from power plants and other sources can also make it up that far.

WoE: I get that you modeled what might happen if humans decided to inject sulfates into the stratosphere, but what was the precise question you set out to answer?

English: There have been a few other scientists who have looked at geoengineering using stratospheric aerosols, but they didn’t simulate all the processes that can affect the particles. Recently, a team led by Patricia Heckendorn, a researcher based in Zurich, simulated all of these processes in a 2D model and found that the effectiveness of sulfate geoengineering diminished as more sulfate was added. I wanted to use a 3D model that looked at all of the processes, and I wanted to compare our results to Heckendorn’s.

WoE: What processes did you include that others didn’t?


English: For example, our model simulates coagulation, the process by which multiple particles can combine to become one. We also included nucleation – that’s when tiny gas molecules condense on each other to form liquid droplets. Also condensational growth. If you watch, say, water drops grow bigger and bigger on a piece of grass on a foggy morning you’re looking at condensational growth.

WoE: What did you find when you included all of that in your model?

English: What we found was that effective geoengineering required injecting larger masses of sulfuric acid than some have hoped because the particles coagulate and get much bigger than thought. Larger particles fall out of the stratosphere faster to the surface, so they’re not as effective at reflecting light. This matched Heckendorn’s results.

WoE: How much less effective?

English: It depends on how much sulfate we add. The more we add the less effective they become.

WoE: That’s the opposite of what people probably think…

English: It still gets more effective as you add more, but it has a diminishing return. We haven’t done a detailed assessment yet, but the group led by Heckendorn did, and they had a similar result. They found that you would need to inject more than 10 million metric tons of sulfur into the stratosphere per year if you wanted to offset the current forcing from greenhouse gases. People used to think it could be done with about 3 million metric tons.

WoE: Ten million metric tons sure sounds like a lot.

English: It is. Mount Pinatubo released about 10 million metric tons, but that was a one-time shot. Basically, we would need one or two Mount Pinatubo’s every single year.

WoE: Where do we go from here?

English: These results were surprising. If geoengineering is going to work, I think we’re realizing that scientists will need to look at new and creative ways to add particles to the stratosphere in such a way that they don’t grow too big and fall out too quickly.

Image Information: Astronauts took this image of Mount Etna erupting in 2002. Credit: NASA/JSC/Gateway to Astronaut Photography. The lower image is courtesy of Jason English.

–Adam Voiland, NASA’s Earth Science News Team