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How does the radar work?
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 (blue). 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. 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.
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 Reflectivity Mosaic.
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 Reflectivity
Mosaic.
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.
How often are the images
updated?
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 are updated every 10 minutes.
In Precipitation Mode, images are updated every five or six minutes.
The collection of radar data, repeated at regular
time intervals, is referred to as a volume scan.
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 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 a
real 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 less
than 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
United States 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 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. |
