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Using and Understanding Doppler Weather Radar |
Radar basics and the doppler shift NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy (green in the animated image). If the energy strikes an object (rain drop, snowflake, hail, bug, bird, etc), the energy is scattered in all directions (blue). Note: it's a small fraction of the emitted energy that is scattered directly back toward the radar. Learn about the Radar Beam here
NEXRAD spends the vast amount of time "listening" for returning signals it sent. When the time of all the pulses each hour are totaled (the time the radar is actually transmitting), the radar is "on" for about 7 seconds each hour. The remaining 59 minutes and 53 seconds are spent listening for any returned signals. Learn about the different scanning modes of the Radar here The ability to detect the "shift in the phase" of the pulse of energy makes NEXRAD a Doppler radar. The phase of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. You have most likely experienced the "Doppler effect" around trains. As a train passes your location, you may have noticed the pitch in the train's whistle changing from high to low. As the train approaches, the sound waves that make up the whistle are compressed making the pitch higher than if the train was stationary. Likewise, as the train moves away from you, the sound waves are stretched, lowering the pitch of the whistle. The faster the train moves, the greater the change in the whistle's pitch as it passes your location. The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the phase change of the reflected pulse of energy which then convert that change to a velocity of the object, either toward or from the radar. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to "see" the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.
Now, let's look at the radar data ![]() There are two main types of data, Velocity and Reflectivity. Reflectivity data shows us the strength of the energy that is returned to the radar after it bounces off precipitation targets. Other non-precipitation targets will return energy, but for now, we will only deal with the precipitation. In general, the stronger the returned energy, the heavier the precipitation. Learn more about Reflectivity here. Velocity data is derived from the phase, or doppler shift of the returned energy. The radar's computers will calculate the shift and determine whether the precipitation is moving toward or away from the radar, and how fast, then apply a corresponding color to those directions and speeds. Red is typically a target moving away from the radar, while green is applied to targets moving toward the radar. The intensity of these colors determines its estimated speed. Learn more about Velocity here. In the image above, you can see the velocity data that is associated with a strong storm depicted in the reflectivity data. This is a great example of what a tornado looks like in the velocity display. Click on the image for better detail. The radar is located to the southeast, or to the bottom right of the computer screen. Note the bright red, or strong outbound velocities right next to the bright green, or inbound velocities. This indicates a strongly rotating column of air. When coupled with a reflectivity pattern that exhibits a hook signature, as in this case, there is often a tornado occurring or about to occur. Sometimes the WSR-88D Doppler Radar sees non-precipitation targets If there is a "target" out there and it reflects radar energy back to the radar, the radar will display it as if it was precipitation. The radar does have some logic built in to help it discriminate between precipitation and non-precipitation targets. But, sometimes we see curious things on our radar display. Here are a few:
Anomalous Propagation. Based on our understanding of Radar Beam Characteristics, we expect the radar beam to leave the radar and propagate through the atmosphere in a standard way. Sometimes though, the atmosphere will cause the beam to superrefract or duct through the atmosphere. When this happens, the beam will sometimes bend downward causing some of the radar energy to hit the ground and return energy back to the radar, generating Anomalous Propagation (AP). The three images above show an interesting case. In the first image on the left, the circled area shows isolated AP. The middle image is a terrain map of southern Wisconsin. The image on the right shows the AP overlaid on the terrain map. Note how the high terrain of the Baraboo hills is highlighted by the radar. We know this is AP since we confirmed through satellite and other observations that skies were clear.
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