Chapter 15, Weather Radar 15-1
15 Weather Radar
15.1 Introduction
The most effective tool to detect precipitation is radar. Radar, which is an acronym that stands for “radio
detection and ranging,” has been used to detect precipitation since the 1940s. Radar enhancements have
enabled more precision in detecting and displaying precipitation.
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15.2 Principles of Weather Radar
The radar used by the NWS is called the Weather Surveillance Radar —1988 Doppler (WSR -88D). The
prototype radar was built in 1988.
It is essential to understand some principles of weather radar. This will allow correct interpretation of
WSR-88D images. This chapter will also include a comparison between some WSR -88D principles and
aircraft radar principles. These comparisons will help explain the strengths and limitations of the WSR-88D
and aircraft radar.
15.2.1 Antenna
The antenna (see Figure 15-1) alternately emits and receives radio waves into the atmosphere. Pulses of
energy from the radio waves may strike a target. If they do, part of that energy will return to the antenna.
Figure 15-1. Radar Antenna
The shape of an antenna determines the shape of a beam. The WSR -88D has a parabolic-shaped antenna.
This focuses the radio waves into a narrow, coned -shaped beam. The antenna can be tilted to scan many
altitudes of the atmosphere.
15.2.2 Backscattered Energy
The amount of energy returned directly back to the radar after striking a target is called backscattered energy
(see Figure 15-2).
Chapter 15, Weather Radar 15-3
Figure 15-2. Backscattered Energy
Targets may include precipitation, clouds, dust, birds, insects, buildings, air mass boundaries, terrain
features, wind farms/turbines, etc. Reflectivity is a measurement of the amount of backscattered energy. An
echo is the appearance on a radar display of the backscattered energy (i.e., reflectivity).
15.2.3 Power Output
The WSR-88D has a peak power output of 750 kilowatts (kW). This allows for better detection of low
reflectivity (small) targets in the atmosphere, such as clouds, dust, insects, etc.
Most aircraft radars have a peak power output of less than 50 kW. Therefore, smaller targets are difficult
to detect with aircraft radar.
15.2.4 Wavelengths
The wavelength is the distance between two crests or two troughs within the radio wave emitted from the
radar (see Figure 15-3). The WSR-88D has a wavelength of 10 cm. Most aircraft radars have a wavelength
of 3 cm. Although shorter wavelengths are better at detecting smaller targets, they are significantly more
attenuated than longer wavelengths.
TARGET
TARGET
TARGET
BACKSCATTERED
ENERGY
TRANSMITTED
PULSERADAR
RADAR
RADAR
Chapter 15, Weather Radar 15-4
Figure 15-3. Wavelengths
15.2.5 Attenuation
Attenuation is any process that reduces energy within the radar beam. This reduces the amount of
backscattered energy.
15.2.5.1 Precipitation Attenuation
Precipitation attenuation (see Figure 15-4) is the decrease of the intensity of energy within the radar beam
due to absorption or scattering of the energy from precipitation particles.
Figure 15-4. Precipitation Attenuation
Precipitation close to the radar absorbs and scatters energy within the radar beam. Therefore, very little, if
any, energy will reach targets beyond the initial area of precipitation. Because of precipitation attenuation,
distant targets (i.e., precipitation) may not be displayed on a radar image.
The amount of precipitation attenuation is related to the wavelength of the radar (see Figure 15-5).
Chapter 15, Weather Radar 15-5
Figure 15-5. Precipitation Attenuation versus Wavelength
As the wavelength of the radar decreases, the amount of precipitation attenuation increases.
The WSR -88D’s 10 -cm wavelength is not significantly attenuated by precipitation. However, aircraft
radars, which typically have 3-cm wavelengths, have a significant precipitation attenuation problem. As a
result, aircraft weather radar typically only shows the leading edge of extreme intensity echoes.
15.2.5.2 Range Attenuation
Range attenuation is the decrease of the intensity of energy within the radar beam as the beam gets farther
away from the antenna. If not compensated for, a target that is farther away from the radar will appear less
intense than an identical target closer to the radar.
Range attenuation is automatically compensated for by the WSR-88D. However, most airborne radars only
compensate for range attenuation out to a distance of 50 to 75 NM. Targets beyond these ranges will appear
less intense than they actually are.
15.2.6 Resolution
Resolution is the ability of the radar to show targets separately.
15.2.6.1 Beam Resolution
Beam resolution is the ability of the radar to identify targets separately at the same range but with different
azimuths (see Figure 15-6).
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Figure 15-6. Beam Resolution
Two targets must be separated by at least one beam width (diameter) in order to be displayed as two separate
echoes on a radar image.
The WSR-88D has a beam width of 0.95°. Therefore, at a range of 60 NM, targets separated by at least
1 NM will be disp layed separately. At a range of 120 NM, targets separated by at least 2 NM will be
displayed separately.
Aircraft radars have beam widths that vary between 3° and 10°. Assuming an average beam width of 5° at
a range of 60 NM, targets separated by at least 5.5 NM will be displayed separately. At a range of 120 NM,
targets separated by at least 10 NM will be displayed separately.
The beam resolution is better for the WSR-88D than aircraft radar (see Figure 15-7).
Chapter 15, Weather Radar 15-7
Figure 15-7. Beam Resolution Comparison Between WSR-88D and Aircraft Weather Radar
In the example above, the targets (thunders torms) are at the same range in azimuths for both the aircraft
and WSR-88D radar. At 10 NM, the beam width is small enough for both the WSR-88D and aircraft radar
to display the thunderstorms separately. At 60 NM, the WSR -88D beam width is still small enough to
display both thunderstorms separately. However, the aircraft radar beam width is larger, which results in
the two thunderstorms being displayed as one echo.
Note that the beam becomes wider at greater distances from the radar. Therefore, the beam resolution
decreases with increasing range from the radar. As a result, lines of precipitation may appear to break up
as they move closer to the radar. In reality, the breaks in the precipitation were most likely always there.
15.2.7 Wave Propagation
Radar beams do not travel in a straight line. The beam is bent due to differences in atmospheric density.
These density differences, caused by variations in temperature, moisture, and pressure, occur in both the
vertical and horizontal directions and affect the speed and direction of the radar beam.
In a denser atmosphere, the beam travels slower. Conversely, in the less dense atmosphere, the beam travels
faster. Changes in density can occur over very small distances, so it is common for the beam to be in areas
of different densities at the same time as it gets larger. The beam will bend in the direction of t he slower
portion of the wave.