Morse Code of Weather: how Doppler radar is used to detect rotation within thunderstorms

BISMARCK, N.D. (KFYR) - Radar is one of meteorologists’ most valuable tools and the Doppler capabilities of weather radar allows meteorologists to see inside of storms to detect the speed and direction of winds/precipitation, which can help to determine rotation and identify where tornadoes may form.

This is possible because of the Doppler effect, which was discovered in 1842 by Austrian Physicist Christian Doppler. The Doppler effect is commonly demonstrated using a siren or car horn moving towards or away from a stationary observer. From the perspective of the observer, the sound is higher pitched as the vehicle is approaching them and lower pitched as the vehicle is moving away from them. This is because the sound waves bunch up in front of the moving object and stretch out behind it. In other words, due to the motion of the vehicle with respect to the stationary observer, the sound waves have a higher frequency as the vehicle approaches the observer leading to the higher pitch and a lower frequency as it moves away from the observer leading to a lower perceived pitch.

But the Doppler effect doesn’t just apply to sound waves. It also affects other types of waves, including those of electromagnetic energy. Weather radars use radio frequency energy (generally between 2.7 and 3 GHz) to send out signals and listen back for some of this radiation to bounce off of particles in the atmosphere, such as precipitation, and return to the radar. This radio frequency energy looks like sine waves.

The wavelength of this energy changes after it bounces off precipitation and is returned to the radar based on the direction of motion of the precipitation. If a storm is moving towards the radar (the stationary observer in this case), the wavelengths are compressed leading to a shorter wavelength. This is similar to when the vehicle is moving toward the stationary observer and the pitch is higher due to the shorter wavelengths. If a storm is moving away from the radar, the wavelengths are stretched out leading to a longer wavelength. This is similar to when the vehicle is moving away from the stationary observer and the pitch is lower due to the longer wavelengths.

Below is another example of how the Doppler effect is useful for weather radars. When precipitation is moving away from the radar, the wavelengths returning to the radar are longer than the ones the radar initially sends out and when precipitation is moving towards the radar, the returning wavelengths are shorter.

With this information, we can determine the velocity of particles, or precipitation. Velocity is a measure of both speed and direction of motion. Therefore, we can tell how fast and in what direction precipitation is moving inside of thunderstorms. Standard “reflectivity” mode of radars is shown on the left side of the examples below, where the radar is displaying the location and intensity of the precipitation. Velocity mode is on the right side of the examples below, with green denoting where winds (and precipitation) are going towards the radar and red indicating where winds are blowing away from the radar. The brighter these colors, the faster the wind (and precipitation) is moving in that direction.

This information is powerful and vital for meteorologists when assessing rotation within thunderstorms to determine if a tornado warning should be issued or not. Sometimes, as shown in the left example, it’s very clear that there’s strong rotation within the storm and a tornado is most likely occurring. On the other hand, a more subtle and weak signature of rotation within a storm, as shown in the right example, can lead to a tornado depending on other atmospheric parameters.

One example of how velocity mode on the Minot radar was used to detect a tornado was in 2010 in Burke County when an EF-3 tornado moved through. On radar, you can see the rotation with the bright red and green colors very close to each other indicating tight rotation and what meteorologists call a “velocity couplet.”

But radar’s velocity mode isn’t just useful for detecting rotation and tornadoes. It can also tell meteorologists information about the speed and direction that thunderstorms are moving in as well as areas of severe, damaging wind gusts. Below is an example of where the bright green (and even some blues) on the velocity mode of the radar in Burlington, Vermont were able to show where very strong winds (60-70+ mph) associated with a thunderstorm were moving towards the radar site.

But how, specifically, does all this velocity data get utilized by meteorologists at the National Weather Service to make decisions about severe thunderstorm and tornado warnings?

Chauncy Schultz, science and operations officer at the Bismarck National Weather Service, said: ”So when we say that there is a radar indicated tornado, what we’re really sensing is that there’s strong rotation in the clouds right near the ground. We can’t most of the time positively say that there is for sure a tornado. But what we can sense is that there is really strong rotation, a few thousand feet above the ground or as close to the ground as we can get with the radar. That very often is a tell that a tornado is going to form soon.

So we’re trying to gain lead time, advance warning on the tornadogenesis is what we would call it. And we can do that by sensing inside the clouds, seeing which direction those motions are within the clouds. If they’re rotating very quickly, then we can issue a tornado warning that is radar indicated that a tornado could form at any time.”

Next week on Morse Code of Weather on Wednesday’s First at Four newscast, we’ll talk about the dual-polarization capabilities of weather radars.

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