Introduction to the WSR-88D
The WSR-88D is the most powerful and advanced Weather Surveillance Doppler Radar
in the world. Since first being built and tested in 1988, it has been installed
and used operationally at over 160 locations
across the United States, including Alaska and Hawaii. The WSR-88D has also been
installed in Puerto Rico and several islands in the Pacific. The NWS Northern Indiana
radar began warning operations on March 17th, 1998.
The WSR-88D is considered by many to be the most powerful radar in the world, transmitting
at 750,000 watts (an average light bulb is only 75 watts)! This power enables
a beam of energy
generated by the radar to travel long distances, and detect many kinds of weather phenomena. It
also allows energy to continue past an initial shower or thunderstorm near
the radar, thus seeing additional storms farther away. Many other radar
systems do not have this kind of power, nor can they look at more than one
"slice" of the atmosphere. During severe weather, the NWS
WSR-88D is looking at 14 different elevations every 5 minutes, generating
a radar image of each elevation. That's about 3 elevations per
minute, or one radar image every 20 seconds! What other operational
weather radar can do that??
How does the radar work?
The WSR-88D obtains weather information (precipitation and wind) based upon returned
energy generated and received at the Radar Data Aquisition (RDA) unit (see animated diagram below). The
radar emits a burst of energy (green), from a 28 foot diameter antenna
inside the radome (the white, soccer ball covering). If the energy strikes any object (rain
drop, snow, hail, bug, bird, dust, etc), the energy is scattered in all directions (blue).
A small fraction of that scattered energy is directed back toward the radar.
The reflected signal is then received by the same antenna that sent the signal, during its listening
period. This signal is then sent to a computer system located in a small building at the base of the radome.
These computers analyze the strength of the returned pulse, time it took
to travel to the object and back, and phase 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
The WSR-88D 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.
The ability to detect the "shift in the phase" of the pulse of
energy makes the WSR-88D 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
the radar 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.
Is everything I see on the images an
accurate picture of my weather?
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
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
Returns from aerial targets are also rather common. Echoes from migrating
birds regularly appear during nighttime and early morning 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. The WSR-88D is also able to detect sunrise
and sunset. As the sun sets and rises on the horizon, solar radiation
becomes concentrated, and the 88D picks this up as an intense and narrow
area of reflectivity. Finally, aircraft often appear as "point
targets" far from the radar, particularly in composite reflectivity
The radar is also limited close in by its inability to scan directly
overhead. Therefore, close 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
non-precipitation features. Even with limited experience, users of unedited
products can differentiate precipitation from other echoes, if they are
aware of the general meteorological situation.
What are the different types of radar images?
- Base Reflectivity
- 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 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
- Composite Reflectivity
- 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" (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. To determine if
precipitation is occurring at greater distances link to an adjacent
radar or link to the National
For a higher resolution (1.1 * 1.1 nm grid) composite
reflectivity image, select the short range (S Rng) view. The
image is less "blocky" as compared to the long range image.
However, the maximum range is reduced to 124 nm (about 143 miles) from
the radar location.
- One-hour Precipitation
- This is an image of estimated one-hour precipitation accumulation on
a 1.1 nm 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 124
nm (about 143 miles) from the radar location. This product will not
display accumulated precipitation more distant than 124 nm, even
though precipitation may be occurring at greater distances. To
determine accumulated precipitation at greater distances you should
link to an adjacent radar.
- Storm Total Precipitation
- This image is 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 124 nm (about 143 miles) from the
radar location. This product will not display accumulated
precipitation more distant than 124 nm, even though precipitation may
be occurring at greater distances. To determine accumulated
precipitation at greater distances link to an adjacent radar.
Image updates are based upon the operation mode of the radar at the time
the image is generated. The WSR-88D Doppler radar is operated in one of
two modes -- clear air mode or precipitation mode. In clear air mode,
images you see are updated every 10 minutes. In precipitation mode, images
you see are
updated every five or six minutes. The collection of radar data, repeated
at regular time intervals, is referred to as a volume scan. Meteorologists
at the NWS have access to many more products than those available on the
internet. Our warning meteorologists are looking at new products
continuously, and at several different levels in the atmosphere.
this mode, the radar is actually 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 often 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
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.
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.
In summary, when the radar is in clear air mode, radar images on the
internet will be
updated approximately every ten minutes. In precipitation mode, the
updates will occur around five to six minutes apart.
What do the colors mean in the reflectivity