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Denver
NEXRAD Regonal Radar (30 Min Loop)
NEXRAD
(Next Generation Radar) obtains weather information (precipitation and wind)
based upon returned energy. The radar emits a burst of energy (green). 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.
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.
The ability to detect the "shift in the phase" of the pulse of energy makes
NEXRAD 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 NEXRAD 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.
Doppler
Radar - Short Range Composite Revlectivity
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 composite reflectivity product is 248
nm (about 285 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.
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), is used. dBZ Rainrate (in/hr) 65 16+ 60 8.00 55
4.00 52 2.50 47 1.25 41 0.50 36 0.25 30 0.10 20 Trace 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 (far left) represents
dBZ values when the radar is in clear air mode (dBZ values from -28 to +28).
The other scale (near left) 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.
Latest
Storm 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.
