Chapter 15, Weather Radar 15-14
This angel was caused by bats departing Selman Bat Cave at
Alabaster Caverns State Park, OK, around sunset.
Figure 15-16. WSR-88D Weather Radar Angel Example
15.2.13 Anomalous Propagation (AP)
AP (see Figure 15-17) is an extended pattern of ground echoes caused by superrefraction of the radar beam.
Superrefraction causes the radar beam to bend downward and strike the ground. It differs from ground
clutter because it can occur anywhere within the radar’s range, not just at ranges close to the radar.
AP typically appears as speckled or blotchy high-reflectivity echoes. When animating images, AP tends to
“bloom up ” and dissipate, and has no continuity of motion. AP can sometimes be misinterpreted as
thunderstorms; differentiating between the two is determined by animating the images. Thunderstorms
move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly.
Chapter 15, Weather Radar 15-15
Figure 15-17. WSR-88D Weather Radar AP Example
15.2.14 Other Non-Meteorological Phenomena
15.2.14.1 Wind Farms
Wind farms can affect the return signal of the radar beam. Depending on the proximity of the wind farm to
the radar site (generally within 10 NM), wind farm turbines can result in beam blockage, false echoes, or
high reflectivity values (see Figure 15-18).
Figure 15-18. Wind Farms Can Make Benign Echoes Appear as Small Storms
Chapter 15, Weather Radar 15-16
15.2.14.2 Chaff
Chaff is a counter measure used by the military to distract enemy radars from their true targets (see Figure
15-19)Error! Reference source not found..
Figure 15-19. Radar Image of Chaff
15.2.15 Precipitation
15.2.15.1 Intensity of Precipitation
The intensity of precipitation is determined from the amount of energy backscattered by precipitation, also
known as reflectivity. Reflectivity is determined by:
• The size of precipitation particles.
• The precipitation state (liquid or solid).
• The concentration of precipitation (particles per volume).
• The shape of the precipitation.
15.2.15.1.1 Intensity of Liquid Precipitation
The most significant factor in determining the reflectivity of liquid particles is the size of the precipitation
particle (see Figure 15-20).
Chapter 15, Weather Radar 15-17
Figure 15-20. Reflectivity Associated with Liquid Targets
Larger particles have greater reflectivity than smaller particles. For example, a particle with a
1/4-in diameter backscatters the same amount of energy as 64 particles that each have a 1/8-in diameter.
Radar images/intensity scales are associated with reflectivities that are measured in dBZ. The dBZ values
increase based on the strength of the return signal from targets in the atmosphere.
Typically, liquid precipitation -sized particle reflectivities are associated with values that are 15 dBZ or
greater. Values less than 15 dBZ are typically associated with liquid cloud -sized particles. However, these
lower values can also be associated with dust, pollen, insects, or other small particles in the atmosphere.
15.2.15.1.2 Convective Precipitation
Convective precipitation (see Figure 15-21) is distinguished by the following radar characteristics:
• Echoes tend to form as lines or cells.
• Reflectivity gradients are strong.
• Precipitation intensities generally vary from moderate to extreme.
• Occasionally, precipitation intensities can be light.
• Echo patterns change rapidly when animating the image.
Numerous hazards are associated with convective precipitation. The se hazards include turbulence,
Low-Level Wind Shear (LLWS), strong and gusty surface winds, icing above the freezing level, hail,
lightning, tornadoes, and localized IFR conditions below the cloud base due to heavy precipitation.
Chapter 15, Weather Radar 15-18
Figure 15-21. WSR-88D Weather Radar Convective Precipitation on the 0.5° Base Reflectivity Product Example
15.2.15.1.3 Stratiform Precipitation
Stratiform precipitation (see Figure 15-22) has the following radar characteristics:
• Widespread in aerial coverage.
• Weak reflectivity gradients.
• Precipitation intensities are generally light or moderate (39 dBZ or less).
• Occasionally, precipitation intensities can be stronger.
• Echo patterns change slowly when animating the image.
Chapter 15, Weather Radar 15-19
Hazards associated with stratiform precipitation include possible
widespread icing above the freezing level, low ceilings, and reduced
visibilities.
Figure 15-22. WSR-88D Weather Radar Stratiform Precipitation on the 0.5° Base Reflectivity Product Example
15.2.15.1.4 Intensity of Snow
A radar image cannot reliably be used to determine the intensity of snowfall. However, snowfall rates
generally increase with increasing reflectivity.
15.2.15.1.5 Bright Band
Bright band is a distinct feature observed by radar that denotes the freezing (melting) level. The term
originates from a band of enhanced reflectivity that can result when a radar antenna scans through
precipitation. The freezing level in a cloud contains ice particles that are coated with liquid water. These
particles reflect significantly more energy (appearing to the radar as large raindrops) than the portions of
the cloud above and below the freezing layer.