Tracking Sea Ice at the Top of the Globe

In the summer of 2012, Arctic sea ice has broken the previous record for minimum extent (set in 2007), fallen below 4 million square kilometers, and, as of September 17, dropped below 3.5 million square kilometers in extent. Multiple studies indicate that the Arctic will eventually lose its sea ice during the summers of the future.

As summer sea ice shrinks, the ice edge—the perimeter of the ice pack that remains—will retreat farther north. Meanwhile, the satellites that observe the ice can see almost as far north as the North Pole—but not quite. As a result, scientists and data managers at institutions such as the National Snow and Ice Data Center (NSIDC) are looking for ways to cope with a situation that was once unexpected.

Since 1979, scientists have relied on a variety of satellite sensors, including the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), the Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E), and (most recently) the Special Sensor Microwave Imager/Sounder (SSMIS). The satellites carrying these sensors fly in near-polar orbits (see the Catalog of Earth Satellite Orbits for more information). This means that they pass close to the pole, but not quite over it. As a result, many sea ice images have a gap in the data extending from 87 degrees northward to the North Pole. The gap appears as a black circle over the pole in the image from August 2012.

Over the years, sensor capabilities have improved, shrinking the data gap over the North Pole. “For instance, SSM/I only saw as far north as 87 degrees, but AMSR-E and SSMIS can see up to 89 degrees north,” explains NSIDC’s Walt Meier. But to be consistent with satellite data from earlier years, NSIDC often stuck with the larger data gaps, treating the whole area the same way from year to year. This worked well for years because the entire region could be safely assumed to be ice-covered. “There might have been some small areas of open water, but nothing significant.”

The reliable presence of sea ice above 87 degrees north has long simplified how NSIDC measures sea ice. The smallest “unit” that a satellite sensor sees is known as a pixel, and the sensors measuring sea ice can have pixels as big as 25 by 25 kilometers. Sea ice concentration is the percentage of any pixel covered by ice. NSIDC uses a threshold of 15 percent and considers any pixel that is more than 15 percent ice covered completely ice filled. The resulting measurement is known as sea ice extent.

Historically, NSIDC scientists have handled the data gap over the North Pole by assuming it was ice-filled, and adding that area to the extent observed outside the data gap. But the continued retreat of sea ice presents a potential problem, especially if scientists stick with the historic data gap from 87 degrees northward to the pole. First-year, melt-prone ice is spreading northward. “In the past, we didn’t see first-year ice that far north,” Meier says. In addition, the 2012 melt season produced a surprising area of open water. “The open water is getting close to the 87-degree data gap.”

Sea ice concentration on September 10, 2012. Image courtesy NSIDC.

The data gap doesn’t pose an immediate problem for NSIDC’s sea ice estimates, but Meier acknowledges that changes may be advisable in the long run. “If the ice retreats past the current data gap, going to a smaller one may be beneficial. We won’t be able to just assume it’s filled with ice.”

This northward migration of the ice edge is another reminder—in an already historic summer—of how fast the Arctic is changing.

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Discovering Hot Towers

A satellite-radar view of Hurricane Isaac’s hot towers acquired on August 28, 2012. Data from the TRMM satellite provided by the Precipitation Measurement Missions science team at NASA and by JAXA.

Two hours before Hurricane Isaac made landfall, a satellite orbiting hundreds of miles above the storm used a radar instrument to map the storm’s inner structure. The instrument on the Tropical Rainfall Measuring Mission (TRMM) observed two extremely tall complexes of rain clouds called hot towers in the eyewall, a sign that Isaac was trying to strengthen. The towering clouds were so high that they punched through the troposphere (the lowest layer of the atmosphere where most weather occurs) and sent air loaded with ice crystals rushing into the stratosphere, a higher layer that normally contains very little moisture.

Interestingly, the “hot” in the name comes not from the temperature of the air that hot towers loft high into the atmosphere, but because of the latent heat the rain clouds release. “The latent heat is an important ingredient in fueling the updrafts that allow the towers to rise to such icy heights, so ‘in honor’ of the role that latent heat plays, the towers are called hot towers,” explained Owen Kelley, a research scientist based at NASA’s Goddard Space Flight Center.

Early radar instruments first confirmed the existence of hot towers in the 1950s, but such instruments (and those that followed in subsequent decades) did not measure their vertical structure precisely and could not track and catalog the features in a uniform way. That’s no longer a problem. Since TRMM launched in 1997, the satellite has been monitoring hot towers over land and open ocean throughout the tropics in a consistent fashion. And when the follow-on mission to TRMM (the Global Precipitation Mission) launches in 2014, it will continue to expand TRMM’s catalog of radar-observed hot towers further north and south to the edge of Arctic and Antarctic circles. “Before TRMM, we didn’t have enough of a sample size to study where, when, and why hot towers form and how hot towers relate to larger systems like tropical cyclones,” noted Kelley.

Read this Earth Observatory feature to learn more about how the late Joanne Simpson pioneered the study of hot towers. The video below, produced by NASA Goddard’s Scientific Visualization Studio, offers a dramatic view of how hot towers work. There’s also a great deal more information about TRMM on NASA’s Precipitation Measurement Missions website that’s well worth checking out.

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Another kind of image of the day

Every day we bring you a different view of Earth, as only NASA can see it…from high above, and usually from space.

One of our colleagues has also been working since 2000 to bring you breathtaking views of Earth…albeit, from a perspective that’s a bit closer to the ground. Together with a few professional helpers and hundreds of amateur photographers, atmospheric scientist Jim Foster of NASA’s Goddard Space Flight Center offers up the Earth Science Picture of the Day. Here is a recent offering that caught my eye:

Foster and photographer Jimmy Hamilton wrote: “Airplanes are great places to observe sunsets as long as you have a window seat…The photo above was taken mid-flight somewhere over the Atlantic Ocean, on a flight from the Dominican Republic to London, England. I was presented with this startling view when I popped open the window blind. The Sun was illuminating the base of the clouds producing a seemingly upside down sunset.”

Check out more Earth photos, and learn how to submit your own, by visiting EPOD.

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Q & A: The Midwest Drought with Richard Seager

This drought severity map was updated on August 21, 2012. It was generated by the National Drought Mitigation Center. For more information about the map, please visit this site.

NASA Earth Observatory writer Adam Voiland spoke with Columbia University climatologist Richard Seager about the widespread drought currently affecting North America. 

The current dry spell has been called a “flash drought.” Has it really come on that quickly or as a surprise?  
No, I wouldn’t say it has been a surprise to those of us who closely watch the seasonal annual forecasts and the La Niña and El Niño forecasts. I don’t think that’s something that has come across in a lot of the news coverage. It’s not like this has come on that quickly.

Remember all of those stories last summer about the drought in Texas? The drought did ameliorate some in the winter, but then the interior Southwest dried out, which is exactly what you would expect during a La Niña winter.  And then it really expanded north in the late winter, which was more surprising.  Now it’s gotten up into the Midwest and even up into New York where I am. I think the extension toward the Northeast, which is remarkable, has been caused by some random atmospheric variability that’s being added on to the forcing coming from the tropical Pacific Ocean.

A recent IPCC report on extreme weather events and climate change indicated that the link between climate change and droughts in North America isn’t straightforward. Do you think there is a link between the two?
I don’t think you can make a direct link because the natural variability is so tremendous. It is really hard to definitively detect human-induced changes for a specific region. Climate forecasts do predict that much of the southwest and the southern plains will get drier, but the human-induced component should be relatively small in comparison to the natural variability. However, we do have a background warming trend in the Southwest, so when these droughts come along, they happen in a warmer environment. That adds an extra layer of water stress. I do think that climate change plays an important role by making these La Niña-induced droughts worse than they otherwise would be.

Is the cause of this drought a lack of precipitation this summer or the lack of snowpack from the winter?
The lack of snowpack has definitely played an important role. Partly because of the La Niña, there was very little snow across the United States this past winter.  We had very little snow melt in the spring, so we entered the summer with much drier soil to begin with. That’s probably  contributing a positive feedback, in addition to the fact that the storm track has shifted way north.

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Moonlit Clouds Obscure Indian Blackout

Earlier this week, as India endured the worst blackout in its history, many readers and members of the media asked us to show the vast country without lights. Just a few weeks ago, Earth Observatory showed the world what the Mid-Atlantic United States looked like before and after a massive storm-induced blackout. (We also showed the city lights of London.)

But this week, there was nothing to see…at least from space. Here is India before the blackout, as viewed through the day-night band of the VIIRS instrument on the Suomi-NPP satellite:

July 30, 2012

One night later, the view was not much better…

July 31, 2012

What’s the problem? It’s monsoon season in India, which means near-constant clouds and rain…and very little visibility for satellite instruments observing visible wavelengths of light.

Surely there is a blackout under those clouds, as nearly half of the nation (and 600 million people) went without power for parts of two days. You can catch a faint hint of more or less light between July 30 and 31 in the northeastern portion of the images. But mostly, the view is clouds, clouds, and more clouds. They are lit by moonlight, at least.

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Twelve Years of Fires

Besides acquiring photo-like images of the surface of Earth, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua satellites can detect the anomalously high temperatures associated with actively burning fires. Using this “hotspot” data, John Nelson of IDV Solutions made a map of major fires in the contiguous 48 United States from 2001 through early July 2012.

Image courtesy John Nelson, IDV Solutions.

This map shows not just the locations, but also the intensity of major fires. Nelson has scaled the fires by “units of the typical American nuclear power plant’s summertime capacity.” The most intense fires are yellow, and less intense fires appear in shades of magenta and purple. Graphs in the lower left corner show the proportion of fires by year and by month.

Jessica McCarty, who studies U.S. fire patterns at Michigan Tech Research Institute, observes that the most intense blazes are usually wildfires in forested or peatland areas. Prescribed fires to benefit agriculture and ranching are generally less intense.

A high-resolution version of this image is available here.

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Help Find an Alphabet in the Sky

NASA image courtesy Jeff Schmaltz, LANCE MODIS Rapid Response. This image was acquired on July 11, 2012, by the Aqua satellite.

One of the first things that caught my eye when I started checking for interesting satellite imagery yesterday was this: an enormous “V” of smoke draped over northern Canada, as seen by the MODIS instrument on the Aqua satellite. The plume was caused by numerous wildfires burning in the Caribou Mountains of northern Alberta.

At first glance, what looks like a decorative swash on the upper left of the V even reminded me of the look of the N we use on the Earth Observatory to indicate the orientation of an image. It made me think the two might in essence share the same typeface. In fact, the bottom point of the capital V of Adobe Jensen Pro (the typeface of our N) is much wider and curvier than the point in the smoke above. (Wired points out it also looks like the letter Z, which is true if you rotate the image clockwise a bit.)

Still, it’s a memorable image. And it made me wonder: how many other letters have satellites captured momentarily gracing Earth’s atmosphere and oceans? This is the first that I’ve noticed, but I have no doubt there are many more to find given the ceaseless mixing and swirling of clouds, smoke, dust, ice, and even phytoplankton that constantly occurs across our planet.

I think it would be fun to compile a gallery of them, so if you’ve seen a letter (or other typographical mark) in a satellite image, please let me know. Just leave a comment on the thread below. Send a link to what you’ve found, and explain what letter or other typographical mark you think you see.

If you’re feeling especially ambitious, mention what typeface it reminds you of as well. I’ll update this post as new letters come in, and perhaps we’ll eventually have the whole alphabet (plus a good collection of numbers and symbols).  Sending non-English characters is ok: just note what the character is and what it’s called.

Wondering where you can look for imagery besides the EO archives?  Here are a few places to try:

1) NASA Visible Earth
2) The Gateway to Astronaut Photography
3) Jet Propulsion Laboratory Photojournal
4) Scientific Visualization Studio Archives
5) MODIS Image Gallery
6) Landsat Imagery Gallery

Please note: Our gallery won’t likely include many of the high-resolution commercial satellite images that you may have seen on Google Earth because we cannot reproduce those images on our website without buying them. Besides, medium-resolution and low-resolution satellite instruments are actually better for observing large features such as clouds and smoke plumes. Here’s a list of some of the high-resolution instruments that we’ll only be using sparingly, if at all.

Tags: alphabet, smoke, typography
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A Unique View of Wildfire Smoke

We publish a lot of wildfire imagery on our natural hazards page (particularly after the wildfire season ramped up recently in Colorado and other states in the western U.S.)  Most of the imagery is acquired during the day by instruments on polar-orbiting satellites: the Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua and Terra, the Advanced Land Imager (ALI) on EO-1, or the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on Terra.

So it caught our eyes when this image of wildfire smoke at night—captured by the Expedition 31 crew on the International Space Station—turned up on Marshall Space Flight Center’s Flickr page. Faint smoke is visible drifting near Ciudad Juaréz, Mexico, and El Paso, Texas — neighboring border cities along the Rio Grande. It isn’t possible to distinguish the U.S./Mexico border, but a line of lights along Interstate 10—which is slightly north of the border—is visible.  (See this story about city lights viewed from space for a clearer view of Interstate 10 and the border). It’s likely the smoke originated from the Whitewater-Baldy wildfire, a large blaze that has been burning 225 miles to the northeast of El Paso in Gila National Forest.

The photo was taken on June 2, 2012, with a Nikon D3S, a digital single-lens reflex camera identical to what’s available to consumers. A Russian spacecraft  docked to the station is visible on the left side of the image.

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Derecho Downs Trees from Indiana Eastward

On June 29, 2012, a long-lived, fast-moving windstorm blew over the eastern United States. The storm started in northwestern Indiana and, over the next 10 hours, traveled roughly 600 miles (1,000 kilometers) toward the East Coast. Wind speeds matched those of an EF-1 tornado in places. The storm uprooted trees, damaged homes, smashed cars, downed power lines, and left more than a million residents of the Washington, D.C. area without electricity.

This photo shows an unusual cloud formation on the leading edge of the June 29 windstorm. Image courtesy Kevin Gould and NOAA.

Meteorologists classified the storm as a derecho—a windstorm associated with fast-moving thunderstorms. A derecho is not a new phenomenon; the term was coined in the late nineteenth century. These storms can rival tornadoes in terms of the damage they cause but, unlike tornadoes, derecho winds generally cause damage in one direction.

The derecho that struck on June 29, 2012, occurred along the boundary of a stable, dry air mass to the north, and a moist, unstable air mass to the south. Areas affected by the southern air mass were suffering a severe heat wave, including record-breaking temperatures in multiple locations. The high heat and humidity provided energy to the strong winds. The thunderstorms sucked up warm, moist air and subsequently returned it in downdrafts. Hitting the ground at tremendous speed, the downdrafts fanned out over the surface, sometimes picking up speed.

The U.S. National Oceanic and Atmospheric Administration (NOAA) provides a primer on derechos, and an overview of the June 29 event. The Capital Weather Gang also provides a summary of the event.

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National Research Council Video Series Summarizes the State of Climate Science Research

A  new video series from the National Research Council  summarizes what scientists have learned about global warming and climate change.  It’s difficult to pack decades of complex research into short video snippets, but the makers of these videos have done an excellent job. As you watch, keep an eye out for mentions of the key role that remote sensing has played in advancing climate science. Also, look for the numerous data visualizations produced by the Scientific Visualization Studio at Goddard Space Flight Center that made the cut.

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