Introduction
Precipitation intensity is measured by a ground-based radar that bounces
radar waves off of precipitation. The Local Radar base reflectivity product is a
display of echo intensity (reflectivity) measured in dBZ (decibels).
"Reflectivity" is the amount of transmitted power returned to the
radar receiver after hitting precipitation, compared to a reference power
density at a distance of 1 meter from the radar antenna. Base reflectivity
images are available at several different elevation angles (tilts) of the
antenna; the base reflectivity image currently available on this website is from
the lowest "tilt" angle (0.5°).
The maximum range of the base reflectivity product is 143 miles (230 km) from
the radar location. This image will not show echoes that are more distant than
143 miles, even though precipitation may be occurring at these greater
distances. To determine if precipitation is occurring at greater distances, link
to an adjacent radar. In addition, the radar image will not show echos from
precipitation that lies outside the radar's beam, either because the
precipitation is too high above the radar, or because it is so close to the
Earth's surface that it lies beneath the radar's beam.
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. In clear air mode, the radar products update
every 10 minutes.
Precipitation Mode
When rain is occurring, the radar does not need to be as sensitive as in
clear air mode as rain provides plenty of returning signals. In Precipitation
Mode, the radar products update every 6 minutes.
The dBZ Scale
The colors on the legend are the different echo intensities (reflectivity)
measured in dBZ. "Reflectivity" is the amount of transmitted power
returned to the radar receiver. Reflectivity 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), is used.
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).
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 rain rates 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.
How Doppler Radar Works
NEXRAD (Next Generation Radar) can measure both
precipitation and wind. The radar emits a short pulse of energy, and if the
pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves
are scattered in all directions. A small portion 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 radar waves, time it took
to travel to the object and back, and frequency shift of the pulse. The ability
to detect the "shift in the frequency" of the pulse of energy makes
NEXRAD a Doppler radar. The frequency 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
NEXRAD strikes an object and is reflected back toward the radar. The radar's
computers measure the frequency change of the reflected pulse of energy and 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.
The National Weather Service's Doppler radars can detect
most precipitation within approximately 90 mi of the radar, and intense rain or
snow within approximately 155 mi. However, light rain, light snow, or drizzle
from shallow cloud weather systems are not necessarily detected.
Ground Clutter, Anomalous Propagation, and Other False Echoes
Echoes from objects like buildings and hills appear in almost all radar
reflectivity images. This "ground clutter" generally appears within a
radius of 25 miles of the radar as a roughly circular region with a random
pattern. An mathematical algorithm can be applied to the radar data to remove
echoes where the echo intensity changes rapidly in an unrealistic fashion. These
"No Clutter" images are available on the web site. Use these images
with caution; ground clutter removal techniques can remove some real echoes,
too.
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.
Radar returns from birds, insects, and aircraft 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 miles of the radar and produce
reflectivities of <30 dBZ.
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.
Radar Products Offered
The Weather Underground maintains a NOAAPORT
satellite dish which continuously ingests the Level III NEXRAD radar data
directly from the National Weather Service Doppler radars. Included in the Level
III NEXRAD data are the following products, all updated every 6 minutes if the
radar is in Precipitation
Mode or every 10 minutes if the radar is in Clear
Air Mode:
Note that zooming is currently allowed only into the center of the radar
image. Eventually, we hope to add an option to zoom into any region of the
image.
Base Reflectivity
This is a display of echo intensity (reflectivity) measured in dBZ.
The base reflectivity images in Precipitation
Mode are available at four radar "tilt" angles, 0.5�,
1.45�, 2.40�, and 3.35� (these tilt angles are slightly
higher when the radar is operated in Clear
Air Mode). A tilt angle of 0.5� means that the radar's antenna is
tilted 0.5� above the horizon. Viewing multiple tilt angles can help one
detect precipitation, evaluate storm structure, locate atmospheric boundaries,
and determine hail potential.
The maximum range of the "short range" base reflectivity product is
124 nautical miles (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.
Composite Reflectivity
This display is of maximum echo intensity (reflectivity) measured in dBZ
from all four radar "tilt" angles, 0.5�, 1.45�,
2.40�, and 3.35�. 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 "short range" composite 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.
Base Radial Velocity
This is the velocity of the precipitation either toward or away from the
radar (in a radial direction). No information about the strength of the
precipitation is given. This product is available for just two radar
"tilt" angles, 0.5� and 1.45�. Precipitation moving
toward the radar has negative velocity (blues and greens). Precipitation moving
away from the radar has positive velocity (yellows and oranges). Precipitation
moving perpendicular to the radar beam (in a circle around the radar) will have
a radial velocity of zero, and will be colored grey. The velocity is given in
knots (10 knots = 11.5 mph).
Where the display is colored pink (coded as "RF" on the color
legend on the left side), the radar detected an echo but was unable to determine
the wind velocity, due to inherent limitations in the Doppler radar technology.
RF stands for "Range Folding".
Determining True Wind Direction
The true wind direction can be determined on a radial velocity plot only
where the radial velocity is zero (grey colors). Where you see a grey area, draw
an arrow from negative velocities (greens and blues) to positive velocities
(yellows and oranges) so that the arrow is perpendicular to the radar beam. The
radar beam can be envisioned as a line connecting the grey point with the center
of the radar. To think of it another way, draw the wind direction line so that
the wind will be blowing in a circle around the radar (no radial velocity, only
tangential velocity).
In order to determine the wind direction everywhere on the plot, a second
Doppler radar positioned in a different location would be required. Research
programs frequently use such "dual Doppler" techniques to generate a
full 3-D picture of the winds over a large area.
Finding Tornadoes
If you see a small area of strong positive velocities (yellows and oranges)
right next to a small area of strong negative velocities (greens and blues),
this may be the signature of a mesocyclone--a rotating thunderstorm.
Approximately 40% of all mesocyclones produce tornadoes. 90% of the time, the
mesocyclone (and tornado) will be spinning counter-clockwise.
If the thunderstorm is moving rapidly toward or away from you, the
mesocyclone may be harder to detect. In these cases, it is better to subtract
off the mean velocity of the storm center, and look at the Storm Relative Mean
Radial Velocity.
Storm Relative Mean Radial Velocity
This is the same as the Base
Radial Velocity, but with the mean motion of the storm subtracted out. This
product is available for four radar "tilt" angles, 0.5�,
1.45�, 2.40�, and 3.35�.
Vertically Integrated Liquid Water (VIL)
VIL is the amount of liquid water that the radar detects in a vertical column
of the atmosphere for an area of precipitation. High values are associated with
heavy rain or hail. VIL values are computed for each 2.2x2.2 nm grid box for
each elevation angle within 124 nm radius of the radar, then vertically
integrated. VIL units are in kilograms per square meter--the total mass of water
above a given area of the surface. VIL is useful for:
1) Finding the presence and
approximate size of hail (used in conjunction with spotter reports). VIL is
computed assuming that all the echoes are due to liquid water. Since hail has a
much higher reflectivity than a rain drop, abnormally high VIL levels are
typically indicative of hail. The following example
shows high VIL levels associated with a thunderstorm that produced 1.5 inch
hail and an F1 tornado near Lansing, Michigan in August 2003.
2) Locating the most significant
thunderstorms or areas of possible heavy rainfall.
3) Predicting the onset of wind
damage. Rapid decreases in VIL values frequently indicate wind damage may be
occurring.
A handy VIL interpretation guide is available from the Oklahoma
Climatological Survey.
Echo Tops
The Echo Tops image shows the maximum height of precipitation echoes. The
radar will not report echo tops below 5,000 feet or above 70,000 feet, and will
only report those tops that are at a reflectivity of 18.5 dBZ or higher. In
addition, the radar will not be able to see the tops of some storms very close
to the radar. For very tall storms close to the radar, the maximum tilt angle of
the radar (19.5 degrees) is not high enough to let the radar beam reach the top
of the storm. For example, the radar beam at a distance 30 miles from the radar
can only see echo tops up to 58,000 feet. The following example
taken from Hurricane Claudette in 2003 shows this limitation.
Echo top information is useful for identifying areas of strong thunderstorm
updrafts. In addition, a sudden decrease in the echo tops inside a thunderstorm
can signal the onset of a downburst--a severe weather event where the
thunderstorm downdraft rushes down to the ground at high velocities and causes
tornado-intensity wind damage.
Storm Total Precipitation
The Storm Total Precipitation 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.
1 Hour Running Total Precipitation
The 1 Hour Running Total Precipitation image is an estimate of one-hour
precipitation accumulation on a 1.1x1.1 nm grid. This product is useful for
assessing rainfall intensities for flash flood warnings, urban flood statements
and special weather statements.
Velocity Azimuth Display (VAD) Wind Profile
The VAD Wind Profile image presents snapshots of the horizontal winds blowing
at different altitudes above the radar. These wind profiles will be spaced 6 to
10 minutes apart in time, with the most recent snapshot at the far right. If
there is no precipitation above the radar to bounce off, a "ND"
(Non-Detection) value will be plotted. Wind are plotted in knots using the standard
station model.
Altitudes are given in thousands of feet (KFT), and the time is GMT (5 hours
ahead of EST). The colors of the wind barbs are coded by how confident the radar
was that it measured a correct value. High values of the RMS (Root Mean Square)
error (in knots) mean that the radar was not very confident that the wind it is
displaying is accurate--there was a lot of change in the wind during the
measurement.
Storm Attributes Table
The Storm Attributes Table is a NEXRAD derived product which attempts to
identify storm cells.
The table contains the following fields:
On the radar image, arrows show the forcast movement of storm cells. Each
tick mark indicates 20 minutes of time. The arrow length indicates where the
cells are forecast to be in 60 minutes.
When choosing the top 5 or top 10 storms from the "Show Storms"
select box, the top storms are based on Max DBZ.
Lightning Strikes
This should not be used for protection of life and/or property. It is
a new feature, and currently in an early "experimental" phase of
integrating StrikeStar into The
Weather Undergroud's NEXRAD Radar product.
StrikeStar is a network of Boltek
lightning detectors around the United States and Canada. These detectors all
send their data to our central server where the StrikeStar software developed by
Astrogenic Systems triangulates their data and presents the results in near
real-time.
Please note: Because of errors in sensor calibration and large
distances between some sensors, lightning data may display skewed or be missing
in certain regions.
If you have a Boltek detector and run Astrogenic's NexStorm software then we
would like to hear from you. There are a small number of simple criteria you
need to fulfill to join the network. You can email us at lorick@strikestarus.com
for further details.
Archived Historical Radar Data
The
National Climatic Data Center offers free U.S. mosaics for the past 5 years.
For a fee, they can retrieve any radar
image.
Plymouth State College
offers single-site radar images of all radar products going back 7 days.
Kudos