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Images
Storm of 2003
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What is a radar?
What is the Doppler
effect?
How accurate
is weather radar?
What is "base
reflectivity"?
What do the colors
mean?
Question:
What is radar?
Answer: The word radar is actually an acronym for “Radio
Detection and Ranging”. A weather radar emits a burst of energy
lasting around one millionth of a second, around 1000 times per second.
This energy is in the form of radio waves at a frequency above that used
by radio stations. For example, the US NEXRAD radar operates at around
3000 Mhz or 3x109 Hz.
When the radio wave strikes an object (rain drop, bug, bird, etc), the
energy is scattered in all directions. A small fraction of that scattered
energy is directed back toward the radar, and its characteristics depend
on the properties of the object that scattered it. In addition, the presence
of objects in the air modifies the properties of a radio wave even if
they let it pass by – they cause a decrease in power (attenuation)
and a phase shift.
This signal is received by the radar during its listening period. Computers
analyze the strength of the returned pulse, the time it took to travel
to the scattering object and back, and when available, the phase shift
of the pulse. A weather radar spends much more of its time “listening”
(99.8%) than it does emitting signals (0.2%).
The US NEXRAD radar is a Doppler radar – this is a type of radar
that can measure the phase shift, and from this determine the motion of
cloud particles and rain drops. This allows a wind speed to be inferred.
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Question:
What is the Doppler effect?
Answer: To understand the Doppler effect, consider the following:
As a fast moving car passes you, you may have noticed that the pitch in
the car’s engine changes from high to low. As the car approaches, the
sound waves that are generated by the engine are compressed, making the
pitch higher. As the car proceeds away, the sound waves are stretched,
lowering the pitch of the whistle. The faster the car moves, the greater
the change in the engine 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 US National Weather Service
to detect the formation of tornados which, in turn, allows them to issue
tornado warnings with more advanced notice.
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Question:
How accurate is weather radar?
Answer: Most weather radars such can detect precipitation within
around 150 km of the radar, but it should be remembered that the width
of the beam is around 1 km at a distance of 60 km. The wider the beam,
the more likely it is that is will sample more than one type of precipitation.
Also, light drizzle and snow from shallow cloud weather systems are not
necessarily detected, and nothing at all can be detected directly overhead.
Finally, the beam can be blocked by obstacles on the ground (trees, buildings,
ocean waves, the earth’s surface.) This is known as ground clutter,
and generally appears within a radius of 40 km. Other spurious signals
include reflections from migrating birds and aircraft.
Although these artifacts appear in the images, automated error checking generally
removes their effects from precipitation accumulation data. The US national
reflectivity mosaic product is also automatically edited to detect and
remove most spurious features. Even with limited experience, users of
unedited products can differentiate precipitation from other echoes, if
they are aware of the general meteorological situation.
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Question:
What is "base reflectivity"?
Answer:This is a display
of backscattered radio wave energy, measured in dBZ (decibels). 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.
In your reading you may also come across the term composite reflectivity,
which shows the 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, when available, can reveal important storm
structure features and intensity trends of storms.
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Question:
What do the colors mean?
Answer: The colors are the different echo intensities (reflectivity)
measured in dBZ during each elevation scan. Reflectivity covers a wide
range of signals (from very weak to very strong) and hence it is more
convenient number to use a decibel, or logarithmic, scale.
| dBz |
Rain rate (in/hr) |
| 65 |
16.00+ |
| 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.
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