How Autumn Leaves Could Help Pinpoint Tornadoes on Radar

Bob Henson
Published: November 7, 2018

In a major autumn outbreak, nine tornadoes moved through southwest Oklahoma on Nov. 7, 2011, including one that passed northwest of the town of Manitou.
(Wikimedia Commons/Chris Spannagle)

Although tornadoes are most likely to occur in the spring in the United States, radar is more likely to detect them in the fall — and autumn leaves might be a big reason why.

Tree leaves appear to be a major part of the debris lofted into tornadoes and detected by radar, especially in more heavily forested areas.

Leaf debris could explain the striking result of one study involving 119 tornadoes. On average, the twisters that formed in autumn took less than two minutes to generate a debris signature in dual-polarization Doppler radar data.

It took an average of at least four minutes for the debris signatures to form in all other seasons – and about seven minutes in winter.

“We speculate that the autumn peak may be due to the prevalence of natural debris, such as dry and loose leaves,” wrote the researchers, led by Matthew Van Den Broeke from the University of Nebraska-Lincoln.

“A tornado would more readily loft such light debris to the elevation of the radar beam.”

New work on tornado debris was presented in October at the American Meteorological Society’s 29th Conference on Severe Local Storms. The research, led by Steven Nelson of the National Weather Service in Peachtree City, Georgia, looks at how signatures of airborne tornado debris on Doppler radar vary by season and region.

Because there aren’t cameras inside tornadoes, it's impossible to know exactly how much of a given tornado’s debris is made up of leaves. However, Nelson and colleagues found some strong circumstantial evidence that leaves are among the biggest components of tornado debris, especially in the eastern U.S.

This fallen oak tree heavily damaged the home of Jennifer Lyles in Natchez, Mississippi, after a tornado hit early Thursday, Nov. 1, 2018.
(AP Photo/Rogelio V. Solis)

Nelson’s work, the most comprehensive such analysis to date, includes all 434 tornadoes rated EF2 or stronger from 2010 to 2017 that also had a tornado debris signature – the fingerprint that shows up in Doppler radar data when a sufficient amount of material is being tossed into the air by a tornado.

Such debris can range from fragments of destroyed buildings to blades of grass and clouds of dust.

Of all the EF2 tornadoes in the study period, about 75 percent produced a tornado debris signature, or TDS. Tornadoes in the more heavily forested mid-South were more likely to produce a TDS than those in more arid western and northern regions.

Typically, the stronger the tornado, the more debris it picks up and the higher that debris is lofted. The relationship varies by landscape, however. Nelson’s study found that the typical height of a TDS was only around 8,200 feet in the Northern Plains, but nearly 12,000 feet for tornadoes that moved across forested areas.

“There are more things for a tornado to 'hit' in densely-populated, forested areas such as the Southeast than in less-populated, grassland areas such as the High Plains,” Nelson said.

A total of 434 tornadoes rated at least EF2 from 2010 to 2017 (shown as dots on the map above) were classified by aspects of the debris signature they produced on dual-polarization Doppler radar, as well as by the type of land cover they passed over, ranging from grassland and forests to urban areas.
(Steven Nelson)

In areas that were at least 30 percent urban, TDSs were especially frequent, but the TDS heights were relatively low, around 7,800 feet, perhaps because those tornadoes were carrying larger, heavier pieces of debris.

The most damaging tornadoes often produce especially strong TDSs.

Sometimes a TDS is called a “debris ball” on broadcast and social media. However, they are not quite the same thing. A debris ball may be seen when very high reflectivity (a sign of heavy precipitation on radar) occurs at the end of the hook-type echo associated with a tornado.  However, a ball-like signal can also be produced by large hail and very heavy rain even in the absence of a tornado.

Other types of radar data can help alleviate this potential confusion. Since the early 2010s, the NWS’s U.S. network of Doppler radars has been equipped with dual-polarization (“dual-pol”) technology.

Dual-pol radar transmits and receives signals that are polarized in the horizontal and vertical, which allows scientists to use multiple ways of analyzing the resulting data.

Output from dual-polarization Doppler radar showing the signature of an EF3 tornado that struck Elmer, Oklahoma, on May 16, 2016. The tornadic circulation is evident in wind speeds (upper right), and is collocated with high reflectivity at the end of the "hook" (upper left). The low correlation coefficient (the blue spot also collocated at the center of the hook, lower left) indicates that radar is detecting a variety of shapes and sizes of hydrometeors (water droplets) and non-hydrometeors such as leaves and other debris. Low values of differential reflectivity (lower right) suggest that debris is present — tumbling, and on average as tall as it as wide — as opposed to water droplets, which are typically wider than they are tall.
(Steven Nelson)

Although leaf debris could help speed up the process of detecting a tornado on radar, it can also muddy the picture. In a small fraction of cases, very weak tornadoes (top winds less than 65 mph) can still produce TDSs. It’s possible that many of these cases are due to leaf debris, said Nelson.

Along with preparing a journal article on their new work, Nelson and colleagues will be working with local NWS offices and the Storm Prediction Center to incorporate weaker tornadoes (those rated EF0 and EF1) into their analysis.

Over time, tornado warnings and other alerts for the public may include details gleaned from tornado debris signatures, said Nelson.

The NWS is also working on improvements to the algorithm that uses radar-estimated wind speeds to identify which storms might soon produce a tornado. See the article at Weather Underground’s Category 6 for more details.

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