Archive for the ‘Storms’ Category

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.

While I was interviewing University of North Carolina climate scientist Wei Mei about his new research that shows a significant increase in the intensity of land-falling typhoons in the western Pacific, the strongest storm of the 2016 season (Super Typhoon Meranti) was on the verge of slamming into China after grazing Taiwan.

“Meranti fits the trend,” said Wei. “In 2016 so far, there have been six typhoons in the northwestern Pacific. Three have already made it to category 4 or 5. In the late 1970s, only about one-quarter of typhoons reached that strength. Now about half do.”

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NASA Earth Observatory MODIS image of Super Typhoon Meranti.

Some meteorologists have mused that with sustained winds of 165 knots (190 miles per hour), Meranti would have been the equivalent of a Category 6 storm—if the Saffir-Simpson scale actually went that high. (It maxes out at 5). Even though Meranti only grazed southern Taiwan, it still knocked out power to 500,000 households and produced giant waves along the coast.

The focus of Mei’s research, however, is not Meranti or the 2016 typhoon season. Working with colleague Shang-Ping Xie of Scripps Institution of Oceanography, Mei has been digging through records that detail every typhoon in the northwestern Pacific since 1977 and looking for changes in the intensity of storms. What they found was a strong increase in typhoon intensity. Overall, landfalling storms strengthened by about 15 percent over the past four decades, with the proportion of typhoons reaching categories 4 and 5 more than doubling. Mei and Xie showed that storms that passed over waters relatively near to land and moved toward land (red and green dots in the chart below) have strengthened the most. Those that stayed out over the open ocean (black and blue dots) did not strengthen by a significant amount.

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Figure from Mei and Xie, 2016.

“Elevated rates of warming in coastal seas (in comparison to the open ocean) are the reason for the intensification of land-falling typhoons,” said Mei. Between 1977 and 2013, many coastal areas in Asia have warmed by upwards of 0.20 degrees Celsius (0.36 degrees Fahrenheit) per decade along the coasts—more than twice as much as open ocean areas.  In the chart below, notice all the deep reds (more warming) near the coasts; farther out to sea tends to be yellow and orange (less warming).

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Figure from Mei and Xie, 2016.

“We are not arguing that the warming of the coastal seas is due to greenhouse gas-driven climate change; that would require attribution studies that we have not conducted yet,” he said. “But we feel confident that land-falling storms are getting stronger because of rising sea surface temperatures, particularly in a band off the coast of East and Southeast Asia.” A related 2015 study led by Mei argued that sea surface temperatures are a more important factor in controlling long-term variations in typhoon intensity than other factors, such as vertical wind shear.

In this study, Mei and Xie did not look at the frequency of storm development. Some storm researchers have argued that a warming world may make hurricanes and typhoons stronger but less frequent.

For more details about Mei and Xie’s latest study, read more from Scripps Institution of Oceanography, The Verge, and Nature Geoscience.

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Himawari-8 image of Super Typhoon Meranti and night lights (in orange) of Asia via Colorado State University and Mashable.