Doppler Radar

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Throw a ball against a wall and when it bounces back towards you, you have the basic idea of radar Traditional radar shoots a beam of electromagnetic waves off into the atmosphere. If the beam hits something a certain percentage of the beam will be reflected back to the radar receiver making a spot on a screen.

Doppler radar uses the well-known Doppler effect to get more information than traditional radar. The Doppler effect is where the sound of an approaching car is higher pitched than the same car going away. The speed of the approaching car causes the sound waves in front of it to be squished together which is the same as raising the pitch. Moving away the waves get further apart lowering the pitch. So Doppler radar uses this effect to detect motion.

Doppler radar read-outs measure two things: reflectivity and velocity. The more solid an object, the more it reflects radar pulses in both traditional and Doppler radars. The problem is the radar doesn’t “know” if it is bouncing off a wall or a heavy thunderstorm. One way around this is to adjust the wavelength of the pulse to make it more responsive to certain size objects. Unfortunately, Doppler radar wavelengths can pick up anything from smoke to birds and no amount of tuning is going to make a wall invisible. This means that raw Doppler reflectivity scans have to be interpreted to avoid thinking a patch of trees is a patch of rain.

This Doppler radar scan shows Hurricane Katrina’s reflectivity pattern as it made landfall in South Florida. The heaviest rain is in red.

To overcome some of these observation problems Doppler radar sends pulses out at different angles. The lowest angle reaches out the greatest distance at the lowest level of the atmosphere while the successive layers aim progressively higher and have a shorter range. Most radars I’ve studied show returns from four “tilts.” Most of the time the Doppler you see on TV is a composite of all four tilts. This does a great job of showing the extent of a line of bad weather, but it might not show you whether it is raining in your backyard because the precipitation pictured in a composite may come from a higher tilt where the precipitation hasn’t yet reached the ground.

One thing you usually don’t see on TV is the other factor that Doppler radar can measure: velocity. The radar can’t measure wind speed directly. What it can measure is whether whatever the pulse is bouncing off is moving toward or away from the radar. Anything moving perpendicular to the radar can’t be measured at all. Unlike the usual color coding where cool colors represent little precipitation and warm colors heavier precipitation, cool colors represent stuff blowing towards the radar and hot colors stuff blowing away. This allows weather observers to determine the direction of the wind. Determining wind speed is a little more complicated and has to be calculated by analyzing the scan. This complication of calculation is why you don’t see this one on TV.

Just to make things even more complicated if the wind is blowing faster than the radar can measure, it shows on the screen as if it is blowing in the opposite direction. In other words you’ll get a big red dot in the middle of a patch of green or vice versa. These “swirlies” help track powerful storms like hurricanes, thunderstorms, and tornados. 

                        

This is a velocity scan of Katrina. Note location of highest winds

So, I hope, you can see that interpreting raw Doppler radar scans requires some study. How does this play into your choice of weather apps for your IPad or smart phone?  Tune in tomorrow!