NWS NEXRAD: Next Generation Radar obtains weather
information (precipitation and wind) based upon returned energy. The radar emits a burst of energy.
If the energy strikes an object (rain drop, bug, bird, etc), the energy is scattered in all directions.
A small fraction of that scattered energy is directed back toward the radar.
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 shift of the pulse. 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.
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.
Weather surveillance radars such as the WSR-88D can detect most
precipitation within approximately 80 nautical miles (nm) of the radar, and intense rain or snow within
approximately 140 nm. However, light rain, light snow, or drizzle from shallow cloud weather systems
are not necessarily detected.
Echoes from surface targets appear in almost all radar reflectivity
images. In the immediate area of the radar, "ground clutter" generally appears within a radius of 20 nm.
This appears as a roughly circular region with echoes that show little spatial continuity. It results
from radio energy reflected back to the radar from outside the central radar beam, from the earth's
surface or buildings.
Under highly stable atmospheric conditions (typically on calm,
clear nights), the radar beam can be refracted 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.
Returns from aerial targets are also rather common. Echoes from
migrating birds regularly appear during nighttime hours between late February and late May, and again
from August through early November. Return from insects is sometimes apparent during July and August.
The apparent intensity and areal coverage of these features is partly dependent on radio propagation
conditions, but they usually appear within 30 nm of the radar and produce reflectivities of <30
dBZ (decibels of Z).
However, during the peaks of the bird migration seasons, in April
and early September, extensive areas of the south-central U.S. may be covered by such echoes. Finally,
aircraft often appear as "point targets" far from the radar, particularly in composite reflectivity images.
The radar is also limited close in by its inability to scan directly
overhead. Therefore, close to the radar, data are not available due to the radar's maximum tilt elevation
of 19.5°. This area is commonly referred to as the radar's "Cone of Silence".
Though surface echoes appear in the base and composite reflectivity
images, special automated error checking generally removes their effects from precipitation accumulation
products. The national reflectivity mosaic product is also automatically edited to detect and remove most
nonprecipitation features. Even with limited experience, users of unedited products can differentiate
precipitation from other echoes, if they are aware of the general meteorological situation.
Base Reflectivity 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 base reflectivity image currently available on this website is
from the lowest "tilt" angle (0.5°). This means the radar's antenna is tilted 0.5° above the horizon.
The maximum range of the "short range" (S Rng) base reflectivity
product is 124 nm (about 143 miles) 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, select the "long range" (L Rng) view (out to 248
nm/286 mi), select an adjacent radar, or link to the National Reflectivity Mosaic.
Composite Reflectivity is a 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" (L Rng) composite reflectivity
product is 248 nm (about 286 miles) from the radar location. The "blocky" appearance of this product
is due to its lower spatial resolution on a 2.2 * 2.2 nm 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 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.
Because of this fact, 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.
Base reflectivity only shows reflected energy at a single
elevation scan of the radar. Composite reflectivity displays the highest reflectivity of ALL
elevations scans. 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 stronger dBZ level. 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.
Clear Air Mode: In this mode, the radar is in its most
sensitive operation. This mode has the slowest antenna rotation rate which permits the radar to
sample a given volume of the atmosphere longer. This increased sampling increases the radar's
sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode.
A lot of what you will see in clear air mode will be airborne dust and particulate matter. Also,
snow does not reflect energy sent from the radar very well. Therefore, clear air mode will occasionally
be used for the detection of light snow.
The radar continuously scans the atmosphere by completing volume
coverage patterns (VCP). A VCP consists of the radar making several 360° scans of the atmosphere,
sampling a set of increasing elevation angles. There are two clear mode VCPs.
In clear air mode, the radar begins a volume scan at the 0.5°
elevation angle (i.e., the radar antenna is angled 0.5° above the ground). Once it makes two full
sweeps (a surveillance/reflectivity sweep and a Doppler/velocity sweep) at the 0.5° elevation angle,
it increases to 1.5° and makes two more 360° rotations. For one of the clear air mode VCPs, two full
sweeps are also made at 2.5°. Otherwise, at the higher elevations (2.5°, 3.5°, and 4.5°) a single
sweep is made (reflectivity and velocity data are collected together).
This process is repeated at 2.5°, 3.5°, and 4.5°. Then the radar
returns to the 0.5° elevation angle to begin the next volume scan which will repeat the same sequence
of elevation angles. In clear air mode, the complete scan of the atmosphere takes about 10 minutes at
5 different elevation angles.
Precipitation Mode: (slower) When precipitation is occurring, the
radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning
signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation
is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch
the radar to precipitation mode using one of two volume coverage patterns.
Both precipitation VCP's begin like the clear air mode mentioned
above with the same evaluations scans as in the clear air mode. The difference is the radar continues
looking higher in the atmosphere, up to 19.5° to complete the volume scan. The time it takes to
complete the entire volume scan is also less. In the slower VCP, the radar completes the volume scan
of nine different elevations in six minutes. In the faster VCP, the radar completes 14 different
elevation scans in five minutes.
Precipitation VCP: (faster) Differences in the quality of
radar images between the two precipitation mode VCPs are relatively minor. Therefore, during severe
weather, the faster VCP is almost always used as it provides the meteorologists with the quickest
updates and most elevation slices through the storms.
Reflectivity Product Colors: The colors are the different
echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan.
"Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity
(designated by the letter Z) covers a wide range of signals (from very weak to very strong). So,
a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ),
The dBZ values increase as the strength of the signal returned
to the radar increases. Each reflectivity image you see includes one of two color scales. One scale
represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). The other
scale represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75).
Notice the color on each scale remains the same in both operational modes, only the values change.
The value of the dBZ depends upon the mode the radar is in at the time the image was created.
The scale of dBZ values is also related to the intensity of
rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ,
the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S.,
forecasters use a set of rainrates which are associated to the dBZ values.
These values are estimates of the rainfall per hour, updated
each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will
return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is
actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.
Authors: Dennis R. Cain and Paul Kirkwood, National Weather Service, NOAA (March 3, 2005)