Monday, March 20, 2017
In Praise of Doppler Weather Radars
On Mar. 28, 1920, a series of more than 30 deadly tornadoes made widespread paths of destruction through the central and southern U. S. and caused deaths from Michigan all the way to LaGrange, Georgia. Shortly before 6 P. M., that southern mill town was struck by a tornado with an estimated Fujita rating of F3, which means its winds probably ranged from 158 to 206 MPH. After wrecking a cluster of worker's houses, the town's main mill and factory were both destroyed. A total of twenty-seven people died in and around LaGrange as a result of this storm, which was the largest death toll caused by any tornado of the outbreak.
On Mar. 6, 2017, Oak Grove, Missouri was also hit by an F3 tornado, one of more than 30 tornadoes sighted that day which touched down during an outbreak that was similar in many respects to the one that happened 97 years earlier. Although over 400 homes sustained moderate to severe damage ranging from partially or completely destroyed roofs to almost complete destruction, only twelve people in the town were injured and there were no deaths reported as a result of the tornado.
Why did 27 die in the LaGrange tornado and no one died in the Oak Grove tornado? There are many reasons, but one I would like to focus on here is the existence and use of Doppler weather radars for tornado tracking and warning.
In The Tornado: Nature's Ultimate Windstorm, longtime weather researcher Thomas P. Grazulis describes the long process of discovery, research, technical advances, technology transfer from the research lab to National Weather Service and private users such as TV stations, and finally the incorporation of Doppler weather radar into the process of issuing official tornado warnings with their widespread dissemination through media outlets such as radio, TV, and now the Internet and mobile phones.
Although standards of building construction have certainly improved from the 1920s to now, it is likely that most people in LaGrange in the 1920 storm had no clue that a tornado was coming, and most people in Oak Grove in the 2017 storm did. Here in Texas, I grew up with the threat of tornadoes, and have memories of legendary weatherman Harold Taft of WBAP-TV in Fort Worth using one of the earliest weather radars installed at a TV station in the early 1960s to track dangerous storms.
Conventional (non-Doppler) radars are somewhat useful in tracking tornadoes, because a characteristic hook shape sometimes develops on the radar screen when a tornado forms. But it is by no means definitive, and if the storm is in a "cluttered" region such as a city, where radar returns from storms are masked by returns from tall buildings, you can lose the ability to track such a storm just where you need it most. Then came Doppler radar.
The basic Doppler principle has been used in simple police radars since the 1950s. The idea is that a moving object reflects radar waves at a slightly different frequency than the one that the transmitter emits. The frequency shift is directly proportional to the speed of the reflecting object with respect to the transmitter, so valuable information about wind speed is contained in the radar return from wind-whipped rain and hail in a thunderstorm.
Unfortunately, the technology of the 1950s was mostly too primitive to take advantage of the Doppler aspect of radar echoes, and early Doppler radars were so expensive that only the military could afford them (they were also good for detecting another class of dangerous moving objects, namely missiles). But as both radar and computer technology advanced, first adventurous weather researchers, then government labs, and finally the National Weather Service and private interests such as TV stations could afford to buy commercial versions of Doppler radars.
Grazulis describes how as early as 1958, experimental Doppler radars proved useful in measuring wind speeds associated with a Wichita, Kansas tornado. But it took another thirty years of research and development before the WSR-88D series of weather radars were produced commercially and installed in dozens of National Weather Service facilities across the country. With virtually every TV and cable TV outlet in "Tornado Alley now having its own Doppler weather radar, anyone with the slightest interest in what the weather is doing on a stormy day can tune in or look at a phone app to see extremely detailed maps of exactly where a suspected or verified tornado is headed, complete with extrapolations of likely travel directions and arrival times.
While we can't say we're not informed about tornadoes, there are still those who either can't receive such messages, or don't care. So public education is still an important aspect in the fact that although the U. S. population has increased from 1900 to 2000 by a factor of 3.5 or more, the annual death rate from tornadoes declined in that time from about 180 per year to less than 60 per year. That is still too many, but the combination of better-constructed housing, a better-educated populace, and vastly improved information networks that convey Doppler weather radar information virtually instantaneously to thousands of potentially endangered individuals no doubt helped to limit the casualties that resulted from the Oak Grove tornado earlier this month.
Still, Grazulis says there is much more to be done. The formation and life cycle of a tornado is one of the most physically complex weather events known. Although computer models can simulate many aspects of tornado formation, we still do not have either enough raw data or the computing power to predict exactly when and where tornadoes will form, or what they will do once they form. So we can presently track and describe tornadoes remotely once they show up. But it would be nice to be able to say on a minute-by-minute basis exactly which storm will produce a deadly tornado, and which will make only rain, hail, or strong straight-line winds.
Nevertheless, we can be grateful to the largely nameless teams of researchers, engineers, and administrators who together have provided the excellent warning system we have today. All we in Tornado Alley have to do now this tornado season, is to heed the warnings.
Sources: The Tornado: Nature's Ultimate Windstorm by Thomas P. Grazulis was published in Norman, Oklahoma by the Univ. of Oklahoma Press in 2001. For details of the Oak Grove tornado I used an Associated Press story carried on the U. S. News and World Report website at https://www.usnews.com/news/best-states/missouri/articles/2017-03-07/homes-damaged-as-severe-storms-tornadoes-hit-midwest. I also referred to the Wikipedia articles on LaGrange, Georgia and the 1920 Palm Sunday tornado outbreak.