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.



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