- Radar Background
- Understanding NEXRAD Operation
- Interpreting weatherTAP Radar Imagery
- Additional Sources of Information
What is Radar ?
The word "Radar" is derived from RAdio Detection And Ranging
and refers to the technique of using radio waves to detect the presence
and the distance of a target. Radar works by transmitting a radio signal
into the air and then listening for a returned signal. If the transmitted
radio signal reaches a target, then most of the energy is scattered, but
some will be reflected back to the radar receiver. The presence of the target
can be confirmed by the reflected signal. Since radio waves travel through
air at a known velocity, the distance of the target away from the radar
can be determined by measuring the amount of time between the transmission,
and the reception of the reflection. Radar was initially designed to detect
the presence of aircraft shortly before and during World War II, but today
radar is used for a wide array of applications. Since atmospheric moisture
will reflect a small amount of radar energy, radar can be used to detect
precipitation and other meteorological events.
What is Doppler Radar ?
"Doppler" radar is a special type of radar that makes use of
the Doppler Effect to measure not only the distance of a target, but its
velocity as well. It works by comparing the frequency of the transmitted
signal versus the frequency of the reflected signal. The classic example
of the Doppler effect is the sound of a train's whistle as it passes by
at high speed. As the train approaches, the pitch (frequency) of the sound
will rise until the train passes. After passage, the pitch will seem to
be lower. By measuring the change in "pitch" Doppler radar can
determine the speed of target. There is one caveat however, the speed is
relative to the radar location. For example, if an airplane is flying in
a perfect circle around a radar site at the center, the radar would show
the speed as 0 because the relative distance between the aircraft and the
radar site would be unchanged (there would be no motion relative to the
radar site). Using the train example, if an observer was standing in the
center of a circle with a train track around the perimeter, the sound of
the train's whistle would not change. The relative distance between the
observer and the train would always be a constant. If however, the target
where flying directly toward (or away from) the radar site, then the measured
velocity would be the actual velocity. The velocity component of a target
relative to the radar beam is referred to as the "radial velocity".
What is NEXRAD radar ?
"NEXRAD" is short for NEXt generation RADar and refers to the
nationwide network of Doppler radar sites installed by the National Oceanographic
and Atmospheric Administration (NOAA). These sites are specifically designed
to provide meteorological data so the official designation is WSR-88D (WSR88D
= Weather Surveillance Radar - 1988 - Doppler). The NEXRAD project was started
in the late 1980's and was designed to provide comprehensive radar coverage
of the United States and to replace older WSR-57 and WSR-74 radar systems.
The NEXRAD system is superior to the old ones because it is much more sensitive
and allows meteorologists to acquire a much wider range of data. All of
the radar images on weatherTAP are produced from data acquired from one
or more NEXRAD sites. More detailed information
is available on the NEXRAD radar system.
Scanning Patterns (VCPs)
Each NEXRAD site has a 28 ft diameter dish antenna which is used to transmit
and receive radio signals. This dish has the capability to rotate 360 degrees
in azimuth and up to 20 degrees in elevation allowing the radar to cover
a huge volume of atmosphere. The system normally collects data by rotating
the dish through 360 degrees at a prescribed elevation. When that scan is
completed, the elevation is increased slightly and another scan is performed
at the new elevation. This continues until all desired elevations have been
scanned. This pre-programmed set of scanning elevations is referred to as
the Volume Coverage Patterns (VCP). After the VCP has been completed, the
data is processed and images are generated. There are several VCPs that
are commonly used. VCP31 is a "Clear Air" pattern and is used
when little or no precipitation is detected in the coverage area. This VCP
consists of comprehensive scans of five low elevations (0.5,1.5,2.5,3.5,
& 4.5 degrees). It takes about 10 minutes to complete this VCP. Another
commonly used VCP is VCP21, a "Precipitation" VCP. This VCP consists
of 9 elevations starting at 0.5 degrees and ending at 19.5 degrees. This
mode allows meteorologists to see a vertical cross-section of a storm and
is only used if significant precipitation is detected in the area. VCP21
requires about 6 minutes to complete. Note that although several elevations
are scanned, the imagery on weatherTAP is from the lowest (base) elevation.
The table below summarizes the VCPs.
| VCP |
When Used |
Time to Complete |
Lowest
Elevation |
Highest
Elevation |
Number of Elevations |
| VCP31 |
Clear Air |
10 minutes |
0.5 (base) |
4.5 |
5 |
| VCP32 |
Clear Air |
10 minutes |
0.5 (base) |
4.5 |
5 |
| VCP21 |
Precipitation |
6 minutes |
0.5 (base) |
19.5 |
9 |
| VCP11 |
Severe Weather |
5 minutes |
0.5 (base) |
19.5 |
14 |
Operating Modes
Each NEXRAD site operates in one of two modes, "Clear Air" mode
or "Precipitation" mode.
Clear Air Mode
Clear air mode is the normal mode of operation and is used when there is
no significant precipitation in the area. In this mode the radar is VERY
sensitive and will detect even minute echoes. Clear air mode utilizes VCP31
or VCP32 and takes about 10 minutes to produce an image. Note that in the
winter months, some radar sites will go to Clear Air mode even if there
is light snow in the area. The added sensitivity of this mode allows the
detection of snow showers since snow generally reflects much less energy
than other forms of precipitation.
Precipitation Mode
When the radar detects significant precipitation in the area, it will automatically
change to Precipitation mode. This mode is designed to provide higher resolution
for relatively strong echoes so the radar becomes less sensitive. It generally
utilizes VCP21 and produces an image every 6 minutes. There is also a special
type of Precipitation mode sometimes called "Severe Weather" mode.
This mode operates like normal precip mode, but utilizes VCP11 and produces
an image about every 5 minutes. This mode is only used for research or for
extreme weather events like hurricanes or tornadoes.
Reflectivity
We are able to obtain useful information from the NEXRAD site by transmitting
a radio signal and analyzing any echoes that may be reflected back. "Reflectivity"
is the amount of transmitted power returned to the radar receiver and is
often designated by the symbol 'z'. Reflectivity is calculated by a complex
empirical equation which involves the amount of power received (in Watts),
the distance and nature of the target, and other factors. z is in units
of mm^6/m^3 and can actually be defined in terms of the density of water
droplets which would return a similar amount of power. z covers a wide range
of values from very weak to very strong signals, so to produce a more convenient
number for calculations and comparison, a decibel scale is often used. Mathematically:
| dBz= 10 * log (z/z0) |
Where |
z = reflectivity factor |
| |
and |
Z0 is defined to be 1 mm^6/m^3 |
dBz is the symbol used to denote z in decibels and is the units used on
weatherTAP radar maps. Note that if z is less than 1 mm^6/m^3, then dBz
will actually be negative. This is why Clear Air mode scales go below zero.
Relationship between reflectivity and Precipitation
NEXRAD radar is very sensitive and can detect reflections as small as -28
dBZ, however, most significant precipitation events will reflect much more
power, usually in the neighborhood of 15dBz or more. In general the more
energy returned to the dish, the more intense the precipitation. WeatherTAP
represents reflectivity by using a color scale plotted on a geographic map.
A legend on the right side of the image shows the relationship between the
colors and the amount of reflected energy. Since Clear Air mode is more
sensitive than Precipitation mode (although over a smaller reflectivity
range), two different scales are utilized.
Clear Air Mode Scale:
Precipitation Mode Scale:
In addition, the header at the top right corner of the image provides additional
information about the site.
Non-Precipitation Echoes and Clutter
Sometimes weak echoes will appear on the radar image even though there
is no precipitation. These type of echoes are particularly common when the
radar site is in the extremely sensitive "Clear Air" mode. There
are three general classes of non-precipitation echoes: atmospheric effects,
ground clutter, and false echoes.
Atmospheric Effects
It is possible (and even likely) that small echoes will be returned even
on a clear day. Many things can reflect small amounts of radar energy including
clouds, smoke, and fog. Even atmospheric effects like inversion layers and
the variation in air density introduced by temperature variation can produce
echoes. These echoes are not errors or problems with the radar, they are
legitimate meterological, non-precipitation phenomenon detected by the radar.
Echoes from atmospheric phenomenon are usually characterized by large uniformly
colored areas usually centered around the radar site.
Ground Clutter
Another class of non-precipitation echo is "ground clutter".
These reflections are usually caused by obstacles on the ground, including
buildings, mountains, antenna towers, etc. "Ground" clutter does
not necessarily have to be ground based. Aircraft, birds, and insects may
also clutter a NEXRAD image. Clutter echoes are usually characterized by
small points of reflected energy distributed in seemingly random pattern.
False Echoes
The third class of non-precipitation echoes is false echoes. These can
occur when variations in air density cause the radar signal to refract (or
bend) into the earth. The reflected signal is then refracted back to the
dish where it shows up as a very strong echo. This situation is rare, but
it has been observed.
Identifying Precipitation
The first clue to identifying precipitation is to determine the operating
mode of the radar, Clear air mode or Precipitation mode. If the radar is
in Precipitation mode, then there is a good chance that all of the visible
echoes are indeed precipitation. Examining the strength of the echoes will
not only indicate whether it is precipitation, but also the intensity of
the precipitation (see above). Precipitation usually reflects at least 15
dBz. Precipitation usually occurs in large clumps of activity with strong
echoes in the middle and weaker echoes at the perimeter. Precipitation has
a very characteristic look and is easy to identify with a little practice.
Perhaps the best way to distinguish precipitation from non-precipitation
echoes is to animate the radar image. Precipitation will move whereas most
non-precipitation echoes will be stationary.
Radar Mosaics
National, regional, and state radar images on weatherTAP are produced by
compiling data from many NEXRAD sites. All of these products use the Precipitation
mode scale, even if the individual sites are in Clear Air mode. This has
the effect of cleaning up the mosaic and removing most of the non-precipitation
echoes. The images are further enhanced by applying anti-clutter algorithms
to remove non-precipitation data. In the winter months, the algorithms are
adjusted to make the mosaics more sensitive. This is done to detect snow
showers that could otherwise go undetected. Snow generally reflects much
less energy than other forms of precipitation.
Additional Sources of Information
Information on the NEXRAD radar system can be obtained from goverment publications
or from numerous sites on the world wide web.
Publications
Details about the NEXRAD radar system can be obtained from the four volumes
of Federal Meteorological Handbook No. 11 (FMH-11).
The four volumes and approximate cost are:
A - System Concepts, Responsibilities, and Procedures - $8.00
B - Doppler Radar Theory and Meteorology - $16.00
C - Products and Algorithms - $20.00
D - Unit Description and Operational Analysis - $20.00
This handbook can be ordered from the NOAA
National Data Centers' Online Store.
Links
Radar
Meteorology Guide from University of Illinois
WSR-88D and
It's Products
Fort Dodge
Iowa Automated FSS NEXRAD Radar Discussion
Radar
Resources from The National Climatic Data Center
NOAA Glossary
of Weather Terms
NEXRAD Operational Support Facility
