Understanding Doppler Radar: Just What Do All Those Colors Mean!?

In the Tennessee Valley, it seems that there are 3 constants in life – death, taxes, and wild, dynamic weather scattered all throughout the year. With this in mind, the infrastructure to monitor this weather that’s been built up over the years has grown into a true force to be reckoned with. You can look at the network of weather sensors and cameras our team has installed, or the robust spotter network spread across the nation consisting of volunteer members watching the sky for signs of trouble when push comes to shove. But, if you ask me, perhaps the most foundational tool of them all is the Radar. From the hundreds of government NEXRAD radars across the country, to our X-band high-resolution doppler radar in Lawrenceburg, the first thing you’ll hear from many meteorologists when severe weather strikes is “what’s the radar look like?”

Perhaps the most basic (and most commonly seen) tool used on Radar is what’s known as “Reflectivity”, or dBZ (referring to the logarithmic scale, decibels relative to Z). That may sound like a bunch of weather jumbo, but this reflectivity is really quite easy to interpret – in most cases, you can think of the lighter colors as lighter rain, and the darker, more intense colors as indications of heavier rain, or even hail in the case of more intense severe weather. Most often, you’ll see these colors represented by greens, yellows, oranges, reds, and purple/pink colors, with those colors generally representing the least intense reflections, to the most intense reflections. While useful, reflectivity rarely tells the entire story, and this is where the “Doppler” part of “Doppler Radar” enters the picture.

Have you ever heard a siren pass by, and change in pitch while doing so? You just experienced the Doppler Effect! This effect has to do with the speed of sound, and the speed of an object. When passing by, it’s speed changes the relative speed of sound, so it can be a higher or lower pitch to you, a stationary observer. The cool thing is, you can do this with radars to measure wind speeds! Instead of sound, it’s the emitted light waves (in this case, radio waves) measuring the frequency change of moving objects… like rain, storms, tornadoes, or anything else it happens to bounce off of. Most often, you’ll see these doppler winds colored using green and red. In this product, green indicates winds moving TOWARDS the radar, and red indicates winds moving AWAY from the radar. Like reflectivity, the colors also get more intense, but in this case, they get intense with higher winds. In the photo to the right, can you spot where the tornado is? If you said where the greens and reds meet, you’d be correct – and this is why it’s such a big deal that we have our own radar in our backyard, here in Lawrenceburg. To be able to see this with your own nearby equipment is invaluable (and for the meteorologist, the more data, the better!)

Another term you may hear us throw around and use every now and then on air is “correlation coefficient” – again, it may sound like a big and intimidating phrase, but it’s actually something you can easily grasp! This tool is what we know as a “dual-polarization” tool; that is, the radar sends out two beams to gather the three-dimensional SHAPE of what it’s hitting. By doing this, it can differentiate and actually indicate to the user the shape and size of what it’s hitting. If all the objects its hitting in an area are very similarly sized and shaped, this is known as a high correlation (often reds or pinks) and if they’re very random and disorganized, this is known as a low correlation, or a “CC drop”, for short (often greens or blues). Think of rain, for example – raindrops have a lot of similarity, as there are only so many ways to make a raindrop, so rain will have a “high CC” appearance. But think about something like tornadic debris – you have wood, glass, tree limbs, shingles, and a whole assortment of random pieces of things being thrown about. The radar would see this as a “CC drop”, also referred to as a “Tornado Debris Signature”, or TDS. Take a look at this image here with our own Ben Luna pointing out a clear-cut TDS on our radar from earlier this year – this tool proved to us in real-time that we had trouble, and thanks to it, we were on 10 minutes before a tornado warning was issued.

I could go on and on, as there are so many different ways of producing data with just this one tool, but I’ll spare the details of the real guts and gore of Radar for another time. The point is this – as a safety-minded viewer, you’re doing the right thing by keeping tuned in when severe weather strikes. You’ll often see us using our array of tools and interpreting our data, but seeing what we’re seeing, and knowing what we’re calling out (and why) is a great skill to have, and may even motivate you to take action quicker if you truly know the implications of what you’re seeing. Hopefully this quick breakdown of some of our most used tools can help you stay informed when we use these products next time, and may even help spark a new interest. Take it from me… we could always use more radar nerds!

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