The United State's National Weather Service established its NEXRAD Program in 1988 to replace its 128 aging radar systems nationwide. NEXRAD is an acronym that is short for Next Generation Weather Radar and is now the radar system currently in use by all 137 National Weather Service sites in the United States and the Caribbean. The system was completed in 1997 and has served to improve meterological knowledge nation-wide. The system boasts new doppler-type radars that are signifcantly more advanced than their previous counterparts. Each site is equipped with special computers to handle and process the large amounts of information that these radars gather.
NEXRAD radar allows meteorologists to see precipitation and to detect motion by implementing the doppler effect. This doppler effect allows for the early warning of severe weather and allows for the identification of fronts and lines never before seen on conventional radar. The composite reflectivity feature allows for a volume scan that provides a three-dimensional view of the weather. Now with the ability of seeing the weather in 3 diminsions, meteorologists can now better identify severe weather areas, analyze the storm structure verticlly and gather upper air wind data. Each NEXRAD radar reading generates dozens of products, including Base Reflectivity, Base Velocity, Composite Reflectivity, Echo Tops, Total Storm Precipitation, and Vertically Integrated Liquid readings.
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NEXRAD Radar information:
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NEXRAD obtains weather information based upon returned energy. The radar transmitter emits a burst of electromagnetic energy. When the energy strikes an object, it is scattered in all directions. A very small fraction of that scattered energy is directed back toward the transmitter, which then serves as a receiver. Computers analyze the strength of the returned energy pulse, the time the beam was travelling, and whether there was a phase shift associated with the pulse (see Doppler Radar information below). 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 radars spend the vast amount of their 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. The phase of the returning signal typically changes based upon the motion of the raindrops or other objects in the atmosphere. The ability to detect the "shift in the phase" of the pulse of energy makes NEXRAD a Doppler radar.
The Doppler effect or doppler shift is simply an observed change or shift in the phase and frequency of light or sound waves due to the relative motion of the source and/or the observer. A simple way to demonstrate the doppler effect is to use a train's whistle. When the locomotive is far away from the observer, its whistle has a lower pitch. As the train gets closer to the observer, the pitch of the whistle begins to get higher. The pitch is at its highest as it passes the observer and then it begins to lower rapidly as the train travels away from the observer. The faster the train travels, the greater the change in the whistle's pitch will be as it passes by the observer. Doppler Radar works along the same lines. The electromagnetic energy that is emitted be the transmitter is scattered at different rates depending on the movement of the distant targets. This apparent shift in the waves are used to determine velocities
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Doppler Information:
•The Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it.
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Base
Reflectivity corresponds to the amount of energy that
is scattered or reflected back to the radar by whatever
targets are located in the radar beam at a given location.
Radar images are color coded to indicate the strength
of the reflected radar signal. A strong reflected
signal will be colored differently than a lower one.
The amount of reflective signal is a relative indicator
of rain or hail intensity. The greater the precipitation
intensity, the greater the downdraft and the greater
will be the updraft of the thunderstorm to support
it. Thus radar reflectivity gives a relative indication
of the intensity of the a thunderstorm and the potential
for severe weather.
Base Reflectivity images are available at several
different elevation angles of the antenna and are
used to detect precipitation, evaluate storm structure,
locate atmospheric boundaries and determine hail potential.
Base Reflectivity measures echo intensity readings
in dBZ. "dBZ" stands for decibels of Z, Z being the
reflectivity factor. The higher the dBZ level, the
more intense the precipitation. Reflectivity is the
amount of transmitted power returned to th
e radar
receiver. The maximum range of the short range Base
Reflectivity product is 124 nautical miles (nm), or
143 miles (mi), from the radar location. This view
will not display echoes that are more distant than
124 nm, even though precipitation may be occurring
at greater distances. To determine if precipitation
is occurring at greater distances long range Base
Reflectivity is used, which receives readings out
to 248 nm, or 286 mi.
The Base Reflectivity does have a few limitations. Anomalous propagation can contaminate some of the data. Also, since the Earth's surface is curved, the radar beam is continually climbing in altitude as it travels further from the transmitter. This can cause overshooting of distant targets resulting in missing data.
Here's a table showing how precipitation intensity corresponds with dBZ (a measure of reflectivity) :
15-30 dBZ = Light Precipitation
30-45 dBZ = Moderate Precipitation
45 and higher dBZ = Heavy Precipitation
The radar measures the amount of electromagnetic energy scattered back to the receiving antenna from distant targets. These targets include water droplets such as rain and snow, larger objects like hail stones, and other non-weather related objects like bugs, birds, airplanes, mountains and buildings. These "false" returns are known as Anomalous propagation or "ground clutter."
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| Anomalous Propagation or "ground clutter" from the Minneapolis, NEXRAD Site. These radar returns are not precipitation but more likely to be birds, bugs or other objects. |
The hook echo is a classical radar signature associated with the development of tornadoes. These "fish hook" looking features show up on radar and give an indication of rotation in parts of storm cells. The hook is formed from the cyclonic swirl of precipitation around an intense, rotating updraft of the tornado vortex. Please note that the radar does not detect the actual tornado, but gathers the reflected energy from the precipitation that is wrapping around the vortex. Not every tornado shows up nicely on radar nor does every hook echo indicate a tornado touchdown.
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| Hook Echo from May 9, 2001 of a tornado touchdown near Northfield, Minnesota. (Radar reflectivity slightly transparent) | Hook Echo from June 11, 2001 located 2 miles north west of Willmar, Minnesota. |
An outflow boundary is the leading edge of the horizontal airflow resulting from the downdraft of a dying thunderstorm. This is caused when the cooler and denser air sinks and pushes warmer and less dense air up into the atmosphere. This interaction can create cloud development which can lead to further precipitation. Thus, this precipitation can show up on radar reflectivities as higher reflectivities. Outflow Boundaries are useful in predicting further thunderstorm development, wind shear and strong straight line winds.
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| Base Reflectivity from Minneapolis NEXRAD showing an outflow boundary north of the radar site on June 11, 2001. | Base Reflectivity from the same radar scan. Notice that the areas of high reflectivity also show up on the Base Velocity image. |
Velocity is the average radial velocity of the targets in the radar beam at a given location. The radar can only determine true velocities that are moving directly up or down the radar beam. If the wind is perpindicular to the beam, then no velocity is determined. The image still shows reflective images of precipitation, but the color indicates the speed and direction the wind is blowing. Positive Base Velocity values (warm colors) denote outbound velocities that are directed away from the radar. Negative Base Velocity values (cool colors) are inbound velocities that are directed towards the radar. Purple shows what is called "range folding," which means that the wind direction is not detectable in those locations, while gray shows either winds blowing parallel to the radar station or weak winds.
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| Velocities use different colors to represent different velocities either toward or away from the radar site. In the image above, blue and green represent velocities moving toward the radar while yellow and red show velocities moving away from the site. | Base Reflectivity from Minneapolis, Minnesota on May 9, 2001 |
Composite Reflectivity displays the maximum echo intensity (reflectivity) from any elevation angle at every range from the radar. This reading 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. Although the Composite Reflectivity product is able to display maximum echo intensities 248 NM from the radar, the beam of the radar at this distance is at a very high altitude in the atmosphere. Thus, only the most intense convective storms and tropical systems will be detected at the longer distances. While the radar image may not indicate precipitation, it is quite possible that the radar beam is overshooting precipitation at lower levels, especially at greater distances.
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| Composite Reflectivity from Minneapolis, Minnesota on May 9, 2001 | Base Reflectivity from Minneapolis, Minnesota on May 9, 2001 |
The data gathered from the composite reflectivity is compiled from multiple slices of the atmosphere. As stated in the previous section, base reflectivity gathers data from only one angle. Composite reflectivity takes all the data from all the single slices of the base reflectivity and then shows the entire sample. Each angle is analyzed and then most severe reflectivity reading is plotted. This shows the viewer the most severe reflectivity regardless of the angle from which the dBZ reading came from. Thus, more data is shown and data that may be missing from the base reflectivity can be seen on the composite reflectivity.
The Echo Tops product shows the elevations that the precipitation echoes, or reflectivities, extend up to in the atmosphere. Echo Tops are similar to cloud tops, but usually the top of the cloud will be somewhat higher than the top of the precipitation echoes. Echo Tops are measured in thousands of feet above mean sea level. The lowest detectable tops are those at 5,000 feet, while the highest detectable tops are at heights of 70,000 feet. Echo Top readings are extremely valuable for the aviation industry. They also contain information about individual thunderstorms and their potential for producing severe weather. For example, echo tops information can be useful in identifying areas of strong updrafts. The higher the echo tops, generally speaking, the stronger the updrafts within the thunderstorm. In addition, severe weather events often coincide with the collapse of the top of the echo. The echo top heights will be misrepresented if the true top of the reflectivity echo is above the height of the highest elevation angle scan. The NEXRAD Doppler radars never scan directly overhead, so they are unable to detect the true top of the reflectivity echoes directly over the radar site.
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| Echo Tops from the Minneapolis NEXRAD site on May 9, 200 | Base Reflectivity from the Minneapolis NEXRAD site on May 9, 2001 |
Liquid The Vertically Integrated Liquid evaluates the received data to show the amount of liquid water contained in a vertical column over each point on the display. This data is measured in kilograms per square meter, a measurement of volume. Hail has an extremely high reflectivity, much higher than the largest raindrops, causing an overestimatation of the amount of liquid water actually contained in the clouds. For this reason, very high Vertically Integrated Liquid values in thunderstorms are a good indication that hail may be occurring. Usually, areas with the highest Vertically Integrated Liquid readings are those that have the strongest echoes and are receiving the highest precipitation totals within the coverage range.
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| Vertically Integrated Liquid from the Minneapolis NEXRAD site on May 9, 2001. | Base Reflectivity from the Minneapolis NEXRAD site on May 9, 2001 |
Notice the storm cells that have higher Vertical Liquid components are also the same cells that have higher reflectivities.
