Chapter 15, Weather Radar 15-8
15.2.7.1 Normal (Standard) Refraction
Under normal (i.e., standard) conditions, the atmosphere ’s density gradually decreases with increasing
height. As a result, the upper portion of a radar beam travels faster than the lower portion of the beam. This
causes the beam to bend downward (see Figure 15-8).
The radar beam curvature is less than the curvature of the Earth. Therefore, the height of the radar beam
above the Earth’s surface increases with an increasing range.
Figure 15-8. Normal Refraction
15.2.7.2 Subrefraction
Atmospheric conditions are never normal or standard. Sometimes, the density of the atmosphere decreases
with height at a more -than-normal rate (actual density is less than normal) . When this occurs, the radar
beam bends less than normal and climbs skyward. This phenomenon is known as subrefraction (see Figure
15-9).
Subrefraction may cause the radar beam to overshoot objects that would normally be detected. For example,
distant thunderstorms may not be detected with subrefraction. Subrefraction may also cause radar to
underestimate the true strength of a thunderstorm. Thunderstorms may appear weaker on radar because
subrefraction causes the radar beam to strike the thunderstorm near the top of the cumulonimbus cloud,
where the precipitation particles tend to be smaller.
Chapter 15, Weather Radar 15-9
Figure 15-9. Subrefraction
15.2.7.3 Superrefraction
Conversely, sometimes the density of the atmosphere decreases with height at a less -than-normal rate
(actual density is greater than normal) or even increases with height. When this occurs, the radar beam will
bend more than normal toward the Earth’s surface. This phenomenon is called superrefraction (see Figure
15-10).
Superrefraction causes the radar beam to travel closer to the Earth ’s surface than what would occur in a
normal atmosphere. This can lead to overestimating the strength of a thunderstorm, as the beam would
detect the stronger core of the storm, where precipitation-sized particles are larger.
Figure 15-10. Superrefraction
15.2.7.4 Ducting
If the atmospheric condition that causes superrefraction bends the beam equal to, or greater than, the Earth’s
curvature, then a condition called ducting (or trapping) occurs (see Figure 15-11).
When ducting occurs, the radar beam will hit the surface of the Earth, causing some of the beam ’s energy
to backscatter. This often leads to false echoes, also known as anomalous propagation (AP), to appear in
the radar display.
Chapter 15, Weather Radar 15-10
Figure 15-11. Ducting
15.2.8 Radar Beam Overshooting and Undershooting
Radar beam overshooting may occur because the lowest radar beam can be higher than the top of
precipitation. This will most likely occur with stratiform precipitation and low -topped convection. For
example, at a distance of 124 NM from the radar, the lowest radar beam is at an altitude of approximately
18,000 ft; at 248 NM the beam height is approximately 54,000 ft. Any precipitation with tops below these
altitudes and distances will not be displayed on a single-site radar image. Therefore, it is quite possible that
precipitation may be occurring where none appears on the single-site radar image.
Radar overshooting occurs more often in the mountainous western U nited States where some radars are
located on a mountaintop (e.g., the WSR-88D near Cedar City, UT).
Radar undershooting occurs when precipitation occurs above the lowest radar beam, usually with
high-cloud-based precipitation near the radar site. This often occurs in the western United States during the
summer months.
Undershooting may occur at and near the radar site even in mosaic products when the precipitation is above
the highest elevation angle. This region above the radar is known as the “cone of silence ” (see Figure
15-12).
Figure 15-12. Cone of Silence
Chapter 15, Weather Radar 15-11
15.2.9 Beam Blockage
Beam blockage (see Figure 15-13) occurs when the radar beam is blocked by terrain and is particularly
predominant in mountainous terrain. See Section 24.6.1 for more information on the WSR-88D.
Beam blockage is most easily seen on the lowest radar beam (also known as “Base Reflectivity,” “Lowest
Tilt,” and “Reflectivity at Lowest Altitude ”) images where it appears as a pie -shaped area (or areas)
perpetually void of echoes. When animating the imagery, the beam blockage area will remain clear of
echoes even as precipitation and other targets pass through. In many cases, the beam blockage effect seen
on a single-site radar can be minimized by viewing mosaic images.
Figure 15-13. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example
15.2.10 Ground Clutter
Ground clutter (see Figure 15-14) is radar echoes ’ returns from trees, buildings, or other objects on the
ground. It appears as a roughly circular region of high reflectivity at ranges close to the radar. Ground
clutter appears stationary when animating images and can mask precipitation located near the r adar. Most
ground clutter is automatically removed from WSR-88D imagery, so typically it is does not interfere with
image interpretation.
Chapter 15, Weather Radar 15-12
Figure 15-14. WSR-88D Weather Radar Ground Clutter Example
15.2.11 Ghost
A ghost (see Figure 15-15) is a diffused echo in apparently clear air caused by a “cloud” of point targets,
such as insects, or by refraction returns of the radar beam in truly clear air.
The latter case commonly develops at sunset due to superrefraction during the warm season. The ghost
develops as an area of low reflectivity echoes (typically less than 15 decibels of Z ( dBZ)) near the radar
site and quickly expands. When animating the imagery, the ghost echo shows little movement.
Chapter 15, Weather Radar 15-13
Figure 15-15. WSR-88D Weather Radar Ghost Example
15.2.12 Angels
Angels are echoes caused by a physical phenomenon not discernible by the eye at the radar site. They are
usually caused by bats, birds, or insects. Angels typically appear as a donut -shaped echo with low
reflectivity values (see Figure 15-16). When animated, the echo expands and becomes more diffuse
with time.
Angels typically only appear when the WSR-88D radar is in Clear Air Mode because of their weak
reflectivity (see Section 24.6.1.3 for additional information on modes). Echoes caused by birds are typically
detected in the morning when they take flight for the day. Echoes caused by bats are typically detected in
the evening when they are departing from caves.