Radar Imagery Basics
is NEXRAD Radar?
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.
Established in 1988 to replace
old radars around the country
Allows user more flexability and options
Cover 97% of the contiguous
does NEXRAD work?
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.
The Doppler effect
was named after the Austrian physicist, Christian
Doppler, who discovered it.
Allows for the determination
Doppler radar is now permitting the first ever
mappings of tornado and hurricane winds with
very high resolution.
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 the 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.
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."
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.
are many different weather phenomena that can be seen
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.
| 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,
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
from Minneapolis NEXRAD showing an outflow boundary
north of the radar site on June 11, 2001.
view of the same outflow boundary on June 11, 2001.
Note the red encircled outflow boundary.
Base 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
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.
|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.
from the same radar scan. Notice that the areas
of high reflectivity also show up on the Base
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 (example picture).
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
from Minneapolis, Minnesota on May 9, 2001
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.
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.(cone of silence picture)
|Echo Tops from the
Minneapolis NEXRAD site on May 9, 2001.
from the Minneapolis NEXRAD site on May 9, 2001.
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
Liquid from the Minneapolis NEXRAD site on May
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
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Mankato, MN 56001