Absorption
The atmosphere absorbs some amount of EM
energy, mainly by oxygen and water vapor. It becomes
trapped within these parcels long enough to become
unrecognizable to the radar. Absorption, like any form
of attenuation, results in lost power and decreased
radar performance. Like scattering, the degree of
absorption is dependent on particle size, particle
composition, and wavelength. The longer the
wavelength, the smaller the attenuation due to
absorption.
Solar Effects
Due to the high sensitivity of the WSR-88D,
anomalous returns near sunrise or sunset may occur.
These false returns are generated because the sun
radiates energy in the same region of the
electromagnetic spectrum as the WSR-88D. These
echoes are recognized by their continuous, narrow
"baseball bat" appearance.
Q25.
Q26.
Q27.
Q28.
Q29.
Q30.
Q31.
Q32.
REVIEW QUESTIONS
What is refraction?
What atmospheric parameter has the greatest
impact on refraction and refractivity?
What normally happens to N-units with
increasing altitude?
If a radar beam is consistently overshooting
targets, which refractive condition might be
present ?
The phenomenon of ducting is most likely to
occur under what atmospheric condition?
What is the cause of ground clutter?
What type of scattering is NOT attenuation?
What are the main absorbers of EM energy in
the atmosphere?
PRINCIPLES OF DOPPLER
RADAR
LEARNING OBJECTIVES: Define Doppler
and Doppler shift. Define phase shift and radial
velocity. Recognize the effects of target motion
on radial velocity. Define velocity aliasing.
Recognize the effects of velocity aliasing on
Doppler radar. Compute Nyquist velocity.
Identify Doppler dilemma.
So far, we have discussed basic principles of
electromagnetic energy common to all radar systems.
The following text expands on the theory and
principles already discussed and introduces concepts
unique to Doppler radar.
Doppler is a means to measure motion. Doppler
radars not only detect and measure the power received
from a target, they also measure the motion of the
target toward or away from the radar. Although
Doppler radar enjoyed widespread use for many years,
cost made it an impractical tool for weather detection.
Only recently has expense been offset by the
technological breakthroughs of the Doppler
meteorological radar, WSR-88D. This shore-based
radar has capabilities that far exceed those of older
Doppler systems. These capabilities include a
complementary mix of velocity detection, increased
power and sensitivity, and the integration of high tech
computers. This automation provides forecasters with
a wealth of information. The WSR-88D not only can
detect target motion and velocity, but can also examine
internal storm circulations as well as detect
atmospheric motions in clear air.
The WSR-88D excels in detecting severe weather
events, and more important, increases advance
warning time. In addition, the increased sensitivity of
the WSR-88D allows various meteorological
boundaries to be identified. These boundaries include
synoptic fronts, gust fronts, drylines, land and sea
breeze fronts, and thunderstorm outflows.
DOPPLER SHIFT
In 1842, the Austrian physicist Johann Christian
Doppler first related motion to frequency changes in
light and sound. Doppler discovered that the shift in
frequency caused by moving sources of sound was
directly proportional to the speed of the source. He
then developed mathematical formulas to describe this
effect called the Doppler Shift. While not given much
thought, you experience Doppler shifts many times
each day. The change in pitch of a passing train whistle
and a speeding automobile horn demonstrate its
effects. When you hear a train or automobile, you can
determine its approximate location and movement.
Doppler radar accomplishes much the same thing,
but to a higher degree of accuracy. As a target moves
toward a radar, frequency is increased; if the target is
moving away from a radar, the frequency is reduced.
The radar then compares the received signal with the
frequency of the transmitted signal and measures the
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