Deciphering precipitation & tornado debris- dual pol radar explained

A Moment of Science

Radar has come a long way over the decades with monumental upgrades in resolution, range, and capabilities!

Early National Weather Service Doppler Radar in the 1980s. Credit: NOAA

The 80s. Reagan. Rob with a mustache. Doppler radar!

Meteorologists could now see a storm’s motion and any possible rotation. Cut to the early 2000s. Radar could now see precipitation in 3D! It’s called dual pol, or dual polarization, and it begins with the radar’s transmitter.

Previously this transmitter would only send and receive horizontal pulses of radio waves, with this upgrade, it could now simultaneously send both horizontal AND vertically oriented, or polarized, pulses. This literally opened a new dimension for meteorologists who now have more in-depth information about the size and shape of objects inside a storm.  

A visualization of dual polarization radar viewing a raindrop with different cross sections. Credit: NOAA

Take for example a storm with high reflectivities, indicating either very heavy rain or possibly hail. Without dual pol radar, we would have to make an educated guess what was falling until we got reports of hail or flooding. But this valuable radar addition can distinguish between the two by looking at the difference in horizontal and vertical measurements. Big, heavy raindrops are surprisingly flat and wide in shape, while hail is much more spherical. 


This same principle allows us to determine whether a radar return is non-meteorological through a dual pol product called “correlation coefficient.” This radar product highlights objects that look different from normal precipitation in size and shape. It could be birds, bugs, ground clutter, or something more much worse, debris from a tornado.

Top left: radar reflectivity, top right: doppler radar velocities, bottom left: correlation coefficient from the EF-2 tornado that struck Johns Island in 2015.

Take this example, with our “correlation coefficient” or “debris detector” at the bottom left, it’s easy to notice that blue dot. That is a “debris ball,” a sure sign that a tornado is violently throwing destruction hundreds of feet in the air!  We can also see a strong tornado signature with strong rotation and the characteristic hook echo through doppler radar but it’s only this dual pol data that confirms a dangerous tornado is on the ground & actively causing damage. 

Dual pol has proven its worth in winter weather as well. It makes identifying frozen precipitation much clearer- despite dual pol data often looking quite messy. But don’t worry, because in the end, this data, like all of our other radar products, can be complicated. It’s up to  trained, knowledgeable meteorologists to interpret these colorful displays and relay that weather information to you.

Storm Team 2 Meteorologist David Dickson

Copyright 2021 Nexstar Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

DOWNLOAD THE APP!

WCBD News app graphic_1558160087567.png.jpg

Click for latest news and information

TRENDING HEADLINES