What is Doppler Radar?
NEXRAD (Next Generation Radar) obtains precipitation and wind information based upon returned energy. The radar emits a pulse of energy, called a signal, in a certain direction. If the energy strikes an object (rain drop, bug, bird, etc.), the energy is scattered in all directions. A portion of that scattered energy is reflected back toward the radar and is received ("heard") by the radar during its listening period. Thus, the radar acts as both a transmitter and a receiver. http://www.srh.noaa.gov/radar/radinfo/radarops.gif
Computers analyze the strength of the returned signal, the time it took to travel to the object and return to the radar, and the phase shift of the signal. This process of emitting a signal, listening for any returned signal, then emitting the next signal, occurs about 1300 times each second.
The ability to detect the signal's "phase shift" (frequency change) 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.
What is the Doppler Effect?
You have most likely experienced the Doppler effect. As a train passes your location, you may have noticed the pitch of 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 more the pitch drops as the train passes you.
The same effect takes place in the atmosphere as a signal from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the phase change of the reflected signal and from that are able to estimate the velocity of the object and determine whether it is moving toward or away from the radar. Information on the movement of objects either toward or away from the radar can be used to estimate the speed and direction 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 the NWS to issue tornado warnings with more advanced notice.