Archive for the ‘Natural Hazards’ Category

Something Swirling in the Sea

January 20th, 2017 by Kathryn Hansen

Almost every volcano is interesting from a scientific perspective, but there are just too many eruptions for us to cover every single one. Instead we tend to focus on eruptions that have the potential to affect people. Or, occasionally our satellites return images that simply look so unique that we find the time to cover them. The plume recently ejected from Alaska’s Bogoslof Volcano was noteworthy for both reasons.

Bogoslof, which has been erupting since mid-December 2016, gave rise to a compelling two-tone plume. Are materials being ejected from a vent that is still under water? (Most of the volcano is below the surface of the sea.) The volcano’s interaction with seawater explains the white steam. But if the vent is not yet above water, then how did such a large, dark plume of ash reach so high in the atmosphere? Scientists at the Alaska Volcano Observatory continue to monitor the remote volcano and perhaps answers will be forthcoming as the eruption evolves.

Also intriguing are the swirls of blue visible in the image above. The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured the image on January 7, 2017. My first thought was that the color was caused by a bloom of phytoplankton. The milky blue color looked about right. And iron from eruptions have previously been shown  to provide the nutrients needed for blooms to flourish. But when I asked the experts, the general consensus was that while you can’t rule out a bloom, there was another more likely explanation for the swirls.

According to ocean scientist Norman Kuring of NASA’s Goddard Space Flight Center:

“Phytoplankton don’t normally bloom in the Bering Sea during winter because there’s not a lot of sunlight and because winter storms deepen the mixed layer which also keeps the plankton more in the dark. Wave action can resuspend bottom sediments, and that may be happening farther east along the Aleutian chain in the January 7 image where the water is relatively shallow. Bogoslof Island is beyond the shelf break, however, so bottom resuspension is less likely. Ash in the water seems most probable…. I wouldn’t expect the Bering Sea to be nutrient limited in the winter, so I don’t expect an ash-based phytoplankton boost.”

In short, the swirls are probably ash in the water. The phenomenon is not unprecedented. We have previously published images of the occurrence here and here. But as Kuring reminds us, “the only way to know for sure would be to sample the water directly.”

 

Hurricane Roundup: Matthew from Above

October 14th, 2016 by Pola Lem

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After Hurricane Matthew ripped through Haiti, it blew through the Southeast. From above, NASA satellites, aircraft, and astronauts kept watch on the storm. The Earth Observatory published several images of the destructive storm (thumbnails above). The below includes a sampling of other notable images and maps related to the storm.

Soil Moisture
Matthew drenched the Carolinas, breaking records for single day rainfall in six places, The Washington Post reported. The Southeast received a total of 13.6 trillion gallons of water—that’s three-fourths the volume of the Chesapeake Bay. Hard-hit areas of North Carolina received 15 inches (38 centimeters) of rain.

That downpour saturated the area, causing values for soil moisture to increase substantially. The North American Land Data Assimilation System (NLDAS) mapped these values for October 1, 2016.

Even before the storm arrived, the ground in many areas was saturated. Eastern North Carolina and northeastern South Carolina have localized areas over the 98th percentile. That means that on October 1, the soil was wetter than it was on that date in 98 percent of previous years. The already-wet soils and heavy precipitation from Matthew led to significant flooding in these areas.

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Image: NASA

Temperature and Precipitation
The Jet Propulsion Laboratory (JPL) HAMSR instrument flew above Hurricane Matthew on October 7, 2016, aboard a NASA Global Hawk aircraft. The image below shows atmospheric temperatures overlaid atop ground-based radar and satellite visible images, according to a JPL release. Reds tones show a lack of clouds, whereas blue tones show ice and heavy precipitation. At the top left is an image taken from the Global Hawk.

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Image: JPL/NASA


Clouds Swirling from Above
Expedition 49 astronaut Kate Rubins took the photograph below from the International Space Station at 21:05 Universal Time, on October 4, 2016, as the hurricane approached the Florida coast. Hurricane clouds fill the shot, which includes the station’s solar arrays.

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Photo: NASA/Kate Rubins

Louisiana

Heavy rains fell on Louisiana in August 2016, causing record-high crests for a number of rivers in the area. Map by Joshua Stevens/NASA Earth Observatory.

In the United States, we say “it’s raining cats and dogs” when we get a heavy downpour. In South Africa, it rains “women with clubs.” In Slovakia, a good soak means “tractors are falling.”

World languages brim with rainy day idioms. But when it comes to describing copious amounts of wet stuff, meteorologists do not encourage wordplay. Researchers are particularly adamant about one expression that does not work: the “rain bomb.”

The summer of 2016 brought extreme rain to multiple parts of the U.S., taking lives and causing billions  in property damage. In July, thunderstorms dumped more than six inches of rain on Elicott City, Maryland in roughly two hours, causing flash floods that upended cars and lives. In May, nearly eight inches of rain fell in two days, among a series of heavy rains to inundate Texas. Most recently in Louisiana, more than 30 inches of rain fell in three days, stranding 20,000 people and killing nine.

The Louisiana storm didn’t meet the criteria of a tropical depression as defined by the National Hurricane Center: a tropical cyclone in which the maximum sustained surface wind speed is 38 miles per hour (62 kilometers per hour) or less. In another instance of precise wording, 2012’s Hurricane Sandy technically ceased to be a “hurricane” a few hours before it made landfall, turning into a “post-tropical cyclone.”

For some in the media, “tropical depression” lacks pizzazz and conviction. It lacks the visceral pelting of tractors falling out of the sky or of women with clubs beating down on the Earth. Some news organizations referred to the Louisiana event as a rain bomb. So what should we call severe rain?

NASA scientists George Huffman and Owen Kelley parsed some of the commonly-used rain terminology.  

For one, there’s the “rain shaft.” A rain shaft is a centralized column of precipitationnot necessarily heavy rain. “The rain shaft […] is any rain event, no matter how modest or foreboding, that can be seen stretching from the cloud to the ground,” wrote Huffman, a research meteorologist at NASA’s Goddard Space Flight Center.

Then, there are “microbursts.” These are severe wind events caused by a “small column of exceptionally intense and localized sinking air that results in a violent outrush of air at the ground,” according to AccuWeather. Microbursts are smaller than 2.5 miles (4 kilometers) in size.

Be careful of mixing rain shafts with microbursts, Huffman cautioned.

“Just as you don’t have a microburst with every rain shaft, you don’t necessarily have an identifiable rain shaft with every microburst,” wrote Huffman in an email. “The really interesting dynamics of microbursts are a bit rare, and frequently not present in flooding rains.”

There’s also a size distinction between the different systems, NASA scientists said. A rain shaft comes out of an individual convective cell, making it roughly five to ten kilometers across. (By contrast, tropical depressions measure roughly 100 to 500 kilometers across.)

But in some cases, like Louisiana’s, the term “tropical depression” works, said Owen Kelley. “You don’t need to appeal to rain shafts, microbursts, or rain bombs to explain this system,” Kelley wrote in an email. The storm in Louisiana was “just a plain-old tropical depression that got stuck in one place for several days in a row and therefore dumped a lot of rain in one place.” That weather system did display some of the common signs of a tropical system. For instance, Huffman notes that it had low pressure at low and middle altitudes, and high pressure at the top, “implying some degree of warm core.” (Mid-latitude systems have a cold core, with the most negative pressure deviation at the system’s top.)

Researchers agree, though, about one term, “rain bomb,” which appeared in a couple of articles this summer in reference to extreme rainfall events. Don’t use it, scientists said. While it makes for a catchy headline, “rain bomb” is not an established meteorological term.  

For extreme rain, Kelley suggested yet another phrase: “vigorous convective cells.” These severe rainstorms can take on various forms: super-cells, squall lines, isolated cells.

Microbursts, rain shafts, vigorous convective cells. At the end, isn’t it all just wet stuff coming out of the sky? Yes and no, scientists say. Terms used to describe extreme rain should be used with an eye on precision. As extreme rains (and extreme weather, in general) become more frequent, so will the terms we use to describe them.

Julia: A Drenching, Perplexing Tropical Storm

September 28th, 2016 by Kathryn Hansen

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Tropical Storm Julia made headlines in September 2016, but not in the usual way. It wasn’t a particularly strong or destructive storm, although it brought heavy rainfall to coastal areas of the eastern United States from Florida to Virginia. The unusual aspect was where it formed: Julia became a tropical storm while over land, not over the ocean.

The image above, acquired at 11:55 a.m. Eastern Daylight Time (15:55 Universal Time) on September 14, 2016, with the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, shows Julia over the southeastern United States. At the time, the storm was moving toward the east and had maximum sustained winds of about 55 kilometers (35 miles) per hour.

A NASA-funded study in 2013 by meteorologists Theresa Andersen and Marshall Shepherd described a new category of storm—one that draws energy from water on land. The research showed that some storms can derive energy from the evaporation of abundant soil moisture. Since publishing the research, Shepherd and colleagues have received frequent inquiries as to whether a particular storm was influenced by this “brown ocean” effect. In the case of Julia, the answer is not clear.

“I personally find it overly speculative to make that linkage right now,” Shepherd said. “Frankly, the Baton Rouge floods may have more of a link to the brown ocean than this event, which experienced quite a bit of moisture advection from the ocean.”

The uncertainty in Julia’s case comes from its position. Although the storm developed its center while over land, it was still too close to the ocean for scientists to distinguish an influence from land-based water. Read Shepherd’s full explanation here.

Just How Bad was the 2015 Fire Season in Indonesia?

August 5th, 2016 by Adam Voiland
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Image from MODIS/NASA Earth Observatory.

The 2015 fire season was the most severe ever observed by NASA Earth Observing System satellites, a new study shows. As we reported in December, 2015 was an intense fire season in Indonesia because the drying effects of El Niño exacerbated seasonal fires lit by growers. Many farmers lost control of fires, which then spread through dried-out peat deposits. Peat fires produce thick, acrid smoke rich with greenhouse gases.

Since our story was published, scientists tracking fire activity with several satellite sensors have further analyzed the 2015 data and compared the 2015 fire season with 2006, another severe burning season. The group of scientists looked at measurements of carbon monoxide from the Measurement of Pollution in the Troposphere (MOPITT), the Microwave Limb Sounder (MLS), and the Atmospheric Infrared Sounder (AIRS). They tracked aerosol pollution with Moderate Resolution Imaging Spectroradiometer (MODIS) and the Ozone Monitoring Instrument (OMI). They also used MODIS to track the number  of actively burning fires. Finally, they used the Tropical Rainfal Measuring Mission (TRMM) to track rainfall.

Some of the results from their analysis are shown in the chart below. Note that red lines indicate trends in 2006 (also a severe fire year); blacks lines indicate 2015. The tick marks on the X-axis indicate the month of the year. Comparing the two years, it is clear that 2015 was the more severe fire season. The sensors generally detected higher levels (or longer duration of emissions) of each pollutant in 2015. The peak number of fires observed by MODIS was slightly higher in 2006, but the sensor detected more fires overall in 2015. In both 2006 and 2015, fire activity increased rapidly as rainfall decreased.

 

 

To see how the 2015 fires compared to severe fire seasons before the Earth Observing System satellites were in space, Goddard Institute for Space Studies scientist Robert Field looked back at longer-term records of visibility collected at Indonesian airports. The chart below compares visibility in 2015 with 1997 and 1991—two other years that were dry because of El Niño. (Note: Bext stands for extinction coefficient; a higher extinction coefficient means more smoke was in the air. The upper part of the chart shows how much rain fell. In that chart, “CPC pcp” stands for precipitation from the Climate Prediction Center, a NOAA research group.) By that measure, 1997 was a far more severe fire season. In Sumatra, visibility was also lower in 1991, though in Kalimantan. visibility was about the same in 2015 and 1991.

Still, greenhouse gas emissions from the 2015 Indonesian fires were considerable. Using the Global Fire Emissions Database, the scientists estimate that 2015 released 380 teragrams of carbon—which is roughly more than the annual fossil fuel emissions of Japan.

“Without significant reforms in land use and the adoption of early warning triggers tied to precipitation forecasts, these intense fire episodes will reoccur during future droughts, usually associated with El Niño events,” the authors emphasized.

+Read a NASA press release about the study here.
+Read a more detailed story about the 2015 fires here.

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In Indonesia, dry weather can mean fire. September 2015 data from the TRMM satellite shows lack of rainfall in the areas where fire broke out. Image by NASA Earth Observatory.

 

 

Holuhruan: Still Erupting

November 14th, 2014 by Adam Voiland
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On September 6, 2014, the Operational Land Imager (OLI) on Landsat 8 captured this view of the eruption.

In some ways, the ongoing fissure eruption at Holuhraun lava field in Iceland is an ideal one. Unlike the lava pushing through the Hawaiian village of Pahoa, Holuhraun’s lava is nowhere near any towns or villages. And the eruption has had little effect on air traffic–unlike the Eyjafjallajokull eruption in 2010--because Holuhraun has produced little of the volcanic ash that can damage propellers and gum up engines.

What Holohruan has produced in vast quantities is lava. As a stream of eye-popping field videos—and satellite images—have shown, lava has gushed from Holuhraun almost continuously since late August. Now covering more than 70 square kilometers and filling a volume of one cubic kilometer, Holuhuraun has proven to be Iceland’s largest effusive eruption since 1784.

Holohruan also has been a reliable source of sulfur dioxide, a colorless but toxic gas present within magma. In addition to triggering health problems if inhaled, sulfur dioxide can lead to acid rain and it can react with other substances in the air to produce sulfate aerosols that cool the atmosphere by deflecting sunlight.

Fossil-fuel burning releases sulfur dioxide, but volcanoes do so on a grander scale. Nature reported that an average of 35 tonnes of sulfur dioxide are streaming from Holohuraun each day, more than twice the amount that all of Europe’s factories produce in the same period. “Those levels of sulfur dioxide emissions are quite typical—even low—for an erupting volcano,” said Simon Carn, a volcanologist at Michigan Technological University. “The most interesting aspect of the Holuhruan eruption is its duration. It could go on from months, so the total sulfur dioxide emissions could end up being large.”

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For a sense of the daily ebb and flow of sulfur dioxide emission, see the chart above, which is based on observations from the Ozone Monitoring Instrument (OMI) on NASA’s Aura spacecraft. The image below shows OMI’s observation of a sulfur dioxide plume drifting over Iceland on September 4, 2014, one of the days the sensor observed a peak in sulfur dioxide emissions. High concentrations of sulfur dioxide are shown in red.

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In at least one case, gases from Holuhruan have entered the atmosphere with remarkable style. On September 3, engineers with Nicarnica Aviation were monitoring the eruption from the ground with an infrared camera. The camera is designed to detect ash, but they ended up capturing a view of something unusual: a spiraling plume that was likely dominated by sulfur dioxide and other gases. The structure was reminiscent of a dust devil, a small, rotating column of air that forms when one part of a surface heats up much faster than the surrounding area.

In the video below, colors correspond to the temperatures observed by the camera. Red depicts the hottest temperatures, yellow moderate temperatures, and blue the coolest. “The general mechanism is probably the same as that for dust devils, but in this case the funnel is most likely filled with sulfur dioxide, gas, and volcanic ash,” Prata told New Scientist.