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.
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.
How does Doppler Radar Scan the Atmosphere?
Doppler radar operates in two different modes, depending on the weather conditions. One is clear air mode and the other is precipitation mode.
Clear Air Mode
A Doppler radar is usually set to clear air mode when the weather is fair. In this mode the radar is in its most sensitive operation and revolves at its slowest rate. This slower sweep rate permits the radar to sample the atmosphere longer and increases the radar's ability to detect smaller objects in the atmosphere than when in precipitation mode. Much of what you see on a radar image set to clear air mode will be airborne dust, particulate matter, flocks of birds or bats, and even insect swarms. Also, snow does not reflect radar energy very well, so clear air mode will occasionally be used for the detection of light snow.
In clear air mode the radar performs two complete revolutions (a surveillance/reflectivity sweep and a Doppler/velocity sweep) each at angles of 0.5°, 1.5°, and 2.5° above the horizon. It then performs a single revolution at angles of 3.5° and 4.5° to the horizon (reflectivity and velocity data are collected together).
The complete set of sweeps at five different angles to the horizon is called a volume scan, which essentially gives a three dimensional picture of the weather. In clear air mode a volume scan of the atmosphere takes about 10 minutes. This explains why, when the weather is fair, the radar images are updated approximately once every 10 minutes.
Because the radar easily detects rain and hail, the radar does not need to be as sensitive in precipitation mode as in clear air mode. However, during rain events and storms, meteorologists want to see higher into the atmosphere to analyze the structure of the storms. This is when the radar is switched to precipitation mode using one of two volume coverage patterns.
Both patterns begin like the clear air mode mentioned above wih complete sweeps at 0.5°, 1.5°, 2.5°, 3.5°, and 4.5° to the horizon. Additionally, the radar continues looking higher into the atmosphere, up to 19.5° above the horizon, to complete the volume scan.
In the slower of the two precipitation patterns, the radar completes the volume scan at nine different angles in about six minutes. In the faster of the two precipitation patterns, the radar completes the volume scan at 14 different angles in about five minutes. This explains why, when the weather is stormy, the radar images are updated approximately once every 5 to 6 minutes.
Since differences in the quality of radar images between the two precipitation patterns are relatively minor, during severe weather the faster pattern is almost always used as it provides the meteorologists with the quickest updates and the most elevation slices through the storms.
In summary, when the radar is in clear air mode, radar images will be updated approximately every ten minutes but may show more ground clutter. In precipitation mode, radar images will be updated approximately every five to six minutes.
Echoes from surface targets appear in almost all radar images. In the immediate area of the radar, "ground clutter" generally appears within a radius of about 25 miles. This appears as a roughly circular region with echoes that show little spatial continuity and appear nearly stationary. It results from radio energy being reflected back to the radar from outside the central radar beam, from the earth's surface or buildings.
When in clear air mode (when the radar is in its most sensitive state), the radar may detect echoes from targets on or near the ground. Such targets could include buildings, hills, waves on large bodies of water, migrating birds, swarms of bats and even insects.
Under highly stable atmospheric conditions (typically on calm, clear nights), the radar beam can be refracted (bent) almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. This anomalous propagation phenomenon (commonly known as AP) is much less common than ground clutter. Certain sites situated at low elevations on coastlines regularly detect sea return, a phenomenon similar to ground clutter except that the echoes come from ocean waves.
What are the Different Types of Radar Images?
Base Reflectivity (BR)
This is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). "Reflectivity" is the amount of transmitted power returned to the radar receiver. Base reflectivity images are available at several different elevation angles (tilts) of the antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential. The maximum range of the base reflectivity scan is about 143 miles.
Composite Reflectivity (CR)
This display is of maximum echo intensity (reflectivity) from any elevation angle at every range from the radar. This product is used to reveal the highest reflectivity in all echoes. When compared with base reflectivity, the composite reflectivity can reveal important storm structure features and intensity trends of storms.
The maximum range of the "long range" composite reflectivity product is about 286 miles from the radar location. The "blocky" appearance of this product is due to its lower resolution on a larger grid. It has one-fourth the resolution of the base reflectivity and one-half the resolution of the precipitation products.
Although the composite reflectivity product is able to display maximum echo intensities about 286 miles from the radar, the beam of the radar at this distance is at a very high altitude in the atmosphere. Thus, special care must be taken interpreting this product. While the radar image may not indicate precipitation, it's quite possible that the radar beam is overshooting precipitation at lower levels, especially at greater distances.
This is an image of estimated one-hour precipitation accumulation on a 1.27 miles by 1 degree grid. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements and special weather statements. The maximum range of this product is about 143 miles from the radar location. This product will not display accumulated precipitation more distant than that, even though precipitation may be occurring at such distances.
Storm Total Precipitation
This is an image of estimated accumulated rainfall, continuously updated, since the last one hour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff, and provide rainfall accumulations for the duration of the event.
The maximum range of this product is about 143 miles from the radar location. This product will not display accumulated precipitation more distant than that, even though precipitation may be occurring at such distances.
What is the difference between base and composite reflectivity?
The main difference is that base reflectivity only shows reflected energy at a single scan angle, whereas composite reflectivity displays the highest reflectivity of all scan angles (it is a composite of all scan angles). So, if heavier precipitation is higher in the atmosphere over an area of lighter precipitation (the heavier rain that has yet to reach the ground), the composite reflectivity image will display the greater echo strength.
This occurs often with severe thunderstorms. The updraft, which feeds the thunderstorm with moist air, is strong
enough to keep a large amount of water aloft. Once the updraft can no longer support the weight of suspended water
then the rain intensity at the surface increases as the rain falls from the cloud.