Part 1 Flashcards
Radars have an important role in the field of meteorology. These devices
send out and receive signals providing valuable information about the location and intensity of precipitation
Advanced Doppler radar technology goes beyond simple detection to
providing high resolution reflectivity and estimated velocity data
Advanced Doppler radar technology goes beyond simple detection to providing high resolution reflectivity and estimated velocity data, which is vital to
short term forecasting and severe weather prediction.
The radar creates an
electromagnetic energy pulse which is focused by an antenna and transmitted through the atmosphere
The radar creates an electromagnetic energy pulse which is focused by an antenna and transmitted through the atmosphere. Objects in the path of this electromagnetic pulse, called
targets, scatter the electromagnetic energy. Some of that energy is scattered back toward the radar.
The receiving antenna (which is normally also
the transmitting antenna) gathers this back-scattered radiation and feeds it to a device called a receiver.
Wave Properties
wavelengths and phase shifts
The wavelength (lamada) of a wave is
the distance from one crest to the next.
The phase of a wave, measured in
degrees, where 360 degrees is one wavelength
The phase of a wave, measured in degrees, where 360 degrees is one wavelength, indicates
the current position of the wave relative to a reference position –>
phase shift
the wavelength did not change from T1 to T2, but the wave’s position relative to the vertical line changed 1/4 wavelength, or 90 degrees
Scattering of a Radar Pulse
by a target back to the receiver
When a pulse encounters a target
return signal
it is scattered in all directions. Of interest is the signal component received back at the radar. This signal is typically much weaker than the original sent from the transmitter
The larger the target, the
stronger the scattered signal.
The more targets there are to scatter the pulse, the
stronger the return will be because the return signals from each target combine to produce a stronger signal
The more targets there are to scatter the pulse, the stronger the return will be because the return signals from each target combine to produce a stronger signal. This means that
many large raindrops will produce a stronger return than a few small raindrops.
The quantity that a radar measures is the
returned power
reflectivity is AKA
reflectivity factor
reflectivity:
The quantity that a radar measures is the returned power which, with knowledge of other radar characteristics, is converted to a quantity
The magnitude of the reflectivity is related to
the number and size of the drops encountered by the electromagnetic pulse
The magnitude of the reflectivity is related to the number and size of the drops encountered by the electromagnetic pulse. For this reason,
high reflectivity generally implies heavy precipitation while low reflectivity implies lighter precipitation
Plots of the radar reflectivity, typically using colors to
depict its magnitude
. Plots of the radar reflectivity, typically using colors to depict its magnitude, show
both the location and intensity of precipitation
Extremely high reflectivities often indicate
hail
The factors which govern the choice of a wavelength to be used in a particular radar include
its sensitivity , which is its ability to detect weak targets at long range,
the radar’s ability to resolve small features,
the types of targets to be studied, and
the effects of the intervening atmosphere on the transmitted energy.
The factors which govern the choice of a wavelength to be used in a particular radar include its sensitivity, which is its ability to detect weak targets at long range, the radar’s ability to resolve small features, the types of targets to be studied, and the effects of the intervening atmosphere on the transmitted energy. Other factors also must be considered such as
the radar’s size, weight and cost
Most weather radars have wavelengths that range between
0.8 centimeters (cm) and 10.0 cm
Generally short wavelengths mean
smaller and less expensive equipment
Short wavelength radars are more effective in detecting
detecting small particles such as cloud droplets and drizzle drops.
Short wavelength radars are more effective in detecting small particles such as cloud droplets and drizzle drops. However, the short wavelength electromagnetic energy is
also partially absorbed by these same particles (a process called attenuation).
the short wavelength electromagnetic energy is also partially absorbed by these same particles (a process called attenuation). This makes it difficult to
accurately measure the intensity of back-scattered energy for more distant targets that lie beyond the range of closer targets.
The main advantage of using longer wavelengths is that
absorption by the intervening particles is drastically reduced. This means that a distant thunderstorm behind a closer thunderstorm will appear on the radar screen with its proper intensity.
Since detecting severe weather is one of the most important missions of operational radars, such as the National Weather Service’s WSR-88D Doppler radars, these radars typically use a
long wavelength
One must account for the curvature of the earth when determining
the altitude of a target
One must account for the curvature of the earth when determining the altitude of a target. Distant targets, which are close to the ground,
cannot be seen by a radar because they will be below the horizon.
The height of a distant target that is above the horizon will be
underestimated if the curvature of the earth is not taken into account. For example, the height of the target on the figure below would be underestimated as “h” rather than the actual height “H”.
