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

This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned pulse, time it took to travel to the object and back, and phase, or doppler shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around 1300 times each second!

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.

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:

Bird Roost Rings.  These are most common in the fall around bodies of water that typically have temperatures warmer than the surrounding land at night.  It is also the time birds are gathering for the seasonal migration.  At night, birds rest/nest in and around the lakes.  Just before sunrise, there is often a coordinated lift off and dispersion of the birds out into the surrounding fields for feeding during the day.  Click on the image to the left for a quick animation of the bird rings.

Anomalous Propagation

Terrain

AP & Terrain

Wind Farm Interference. Wind farms can impact Doppler radars in three ways if the turbine blades are moving and they are within the radar’s line of sight. If close enough (within a few kilometers) they can partially block a significant percentage of the beam and attenuate data down range of the wind farm. They can also reflect energy back to the radar and appear as clutter (AP) on the radar image and contaminate the base reflectivity data. The reflectivity data is used by radar algorithms to estimate rainfall and to detect certain storm characteristics. Finally, they can impact the velocity data, which are also used by radar operators and by a variety of algorithms in the radar’s data processors to detect certain storm characteristics, such as mesocyclones, relative storm motion, turbulence, etc.  Learn more here.

Sun Interference. Twice a day, at sunrise and sunset, the radar experiences interference from the electromagnetic energy emitted by the sun.   There is a point at sunrise and sunset where the radar dish points directly at the sun and is hit with this energy.  This is then displayed as a spike of returned energy on our display.  It is brief, typically only occurring during one volume scan.  Notice in the image to the left that sunset is slightly south of due west.  The date is March 11, 2009.  In less than 2 weeks, we will be at the Spring Equinox.  The sun will set due west of the radar. 

Smoke Plumes. During dry periods, when there is controlled burning or uncontained wildfires going on, our radar will detect smoke plumes associated with the fires. Many of the big smoke plumes are from prescribed, or controlled burns. These are fires intentionally set by Federal/State/Local officials for land management purposes. Other fires may be on private lands. The two plumes in this example (click on image for an animation) were prescribed burns by the Wisconsin DNR.