How to Interpret Doppler Velocities
Radar is one of the most valuable tools in the Meteorologists toolkit - besides the standard reflectivity (which measures rainfall rates) and the relatively new dual-polarity functions that can help radars detect tornadic debris, one of the most vital and longest utilized tools is the "doppler" part of "doppler radar" - the doppler velocities. This tool measures wind speed of reflected objects relative to the radar, permitting the radar to see downbursts, rotation, cold fronts, and more. All and all though, sometimes it just seems like radar screens are just a bucket of spilled paint. Today, we're going to take a look at how to interpret these colors, and how they work.
The first thing to understand is how it MEASURES the winds: the Doppler effect. You are familiar with this phenomenon, even if you don't know it. When a siren passes by you, you almost certainly have noticed how, when it's coming towards you, it's high pitched, and as it drives away, it becomes noticeably low pitched. This is the doppler effect in action - as the motion of the object changes RELATIVE to you, the FREQUENCY of the sound or light waves coming towards you increases or decreases. This same phenomenon is responsible for doppler wind measurements - if a tornado is moving at 40mph, the frequency of the emitted beam is shifted DOWN because of the fact the object is moving away, thus taking longer for the beam to return. This is the core idea behind velocities, and this raw data is converted into blue and green color schemes.
Circling back to a real-world example, this tornado is easily visible on Radar, you just need to know what to look for when analyzing radar - remember, the wind measurements are all RELATIVE to the radar. GREEN shaded velocities indicate wind is moving towards the radar, and the brighter the green, the FASTER the motion is towards the radar. RED shaded velocities indicate wind moving AWAY from the radar, and likewise, the brighter the red, the faster the motion is away from the radar. Knowing these two crucial things open up a world of observational possibilities. Think about what a tornado is - a rapidly rotating column of air. Therefore, you should be looking for a very compact area of RED and GREEN in contact, as well as considerations as to how bright the colors are. This is called a couplet, and is often a predecessor or indicator to a tornado touching down. Without it, we'd truly be in the dark!
The possibilities don't stop there, either - with velocities, you can also measure intense lines of wind, such as derechos. Take this example from Iowa in 2020, where a Derecho is moving through. You can see the substantial winds in green (and even blue, because they're so intense!) moving TOWARDS the radar (the black dot), and you can see the initial gust front moving AWAY from the radar on the opposite side. With all of this in mind, hopefully you have the knowledge to know exactly what we're talking about during severe weather coverage, if and when push comes to shove.