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Flashcards in Part 1 Deck (92):
1

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

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Advanced Doppler radar technology goes beyond simple detection to

providing high resolution reflectivity and estimated velocity data

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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.

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The radar creates an

electromagnetic energy pulse which is focused by an antenna and transmitted through the atmosphere

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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.

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The receiving antenna (which is normally also

the transmitting antenna) gathers this back-scattered radiation and feeds it to a device called a receiver.

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Wave Properties

wavelengths and phase shifts

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The wavelength (lamada) of a wave is

the distance from one crest to the next.

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The phase of a wave, measured in

degrees, where 360 degrees is one wavelength

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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 -->

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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

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Scattering of a Radar Pulse

by a target back to the receiver

When a pulse encounters a target

 

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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

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The larger the target, the 

stronger the scattered signal.

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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

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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.

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The quantity that a radar measures is the 

returned power

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reflectivity is AKA

reflectivity factor

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reflectivity:

The quantity that a radar measures is the returned power which, with knowledge of other radar characteristics, is converted to a quantity

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The magnitude of the reflectivity is related to

the number and size of the drops encountered by the electromagnetic pulse

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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

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Plots of the radar reflectivity, typically using colors to

depict its magnitude

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. Plots of the radar reflectivity, typically using colors to depict its magnitude, show

both the location and intensity of precipitation

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Extremely high reflectivities often indicate

hail

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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.

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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

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Most weather radars have wavelengths that range between

0.8 centimeters (cm) and 10.0 cm

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Generally short wavelengths mean

smaller and less expensive equipment

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Short wavelength radars are more effective in detecting

detecting small particles such as cloud droplets and drizzle drops.

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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).

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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.

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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.

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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

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One must account for the curvature of the earth when determining

the altitude of a target

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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.

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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".

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called refraction, also affects the path the electromagnetic energy will take as it propagates through the atmosphere. Normally, because

the atmosphere's density decreases rapidly with height, the radar beam will be deflected downward

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refraction, also affects the path the electromagnetic energy will take as it propagates through the atmosphere. Normally, because the atmosphere's density decreases rapidly with height, the radar beam will be deflected downward, much like

light passing through a glass prism.

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.............................................. the beam can bend down dramatically and even strike the ground.

In extreme cases, where temperature increases with height and dry air overlays warm air, (a condition often found along coastlines)

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In extreme cases, where temperature increases with height and dry air overlays warm air, (a condition often found along coastlines), the beam can bend down dramatically and even strike the ground. Meteorologists call this effect

"anamolous propagation".

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Both the curvature of the earth and normal atmospheric refraction must be accounted for when determining the

position of a target.

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Clear Air Returns (        )

insects and turbulence

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When a radar transmits energy, part of it may be

intercepted by targets on the ground, such as buildings, trees, cars, or other objects.

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ground clutter

When a radar transmits energy, part of it may be intercepted by targets on the ground, such as buildings, trees, cars, or other objects. The return signal from these objects is called "ground clutter".

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Ground clutter interferes with

the detection of meteorological targets, such as raindrops, because ground targets are large and typically produce high reflectivity.

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sidelobes

Ground clutter can result even if the main radar beam is above ground targets because part of the energy radiated from the antenna is emitted off the beam axis

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Back-scattered energy from the sidelobes is

interpreted by the radar processor to come from the main lobe, so ground targets hit by one of the sidelobes appear to a radar user in the same relative position in the main lobe.

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Ground clutter is usually the worst within about

20 kilometers of the radar site where the beam is still close to the earth's surface.

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Ground clutter is usually the worst within about 20 kilometers of the radar site where the beam is still close to the earth's surface. Farther from the site,

the beam is higher due to both its elevation angle and curvature of the earth away from the radar site.

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Ground clutter is easily identified with

a Doppler radar because the radial velocity measured by the Doppler radar will approximately be zero since none of the ground targets are moving with respect to the radar

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The radial velocity is not exactly zero because

moving targets within the beam, such as birds, bugs, or even raindrops, also contribute to the total power return to the radar

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Insects present

large targets to radar and they are always present during the warmer seasons

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Insects present large targets to radar and they are always present during the warmer seasons. This actually is beneficial to meteorologists. Doppler radars require

targets to determine the motion of the air.

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This actually is beneficial to meteorologists. Doppler radars require targets to determine the motion of the air. Outside of regions where precipitation is falling, there would be

no targets if there were no insects

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Outside of regions where precipitation is falling, there would be no targets if there were no insects. Airborne insects turn out to be very good

tracers of air motion since, on average, they blow along with the wind.

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The returns from insects allow meteorologists to see

 air motions outside the storm circulation which in many cases is important for predicting where new storms are likely to occur.

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Turbulence provides another way in which

electromagnetic energy from a radar can be back-scattered

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Turbulence provides another way in which electromagnetic energy from a radar can be back-scattered. Turbulence is associated with

variations in density in the atmosphere.

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Turbulence is associated with variations in density in the atmosphere. When variations in density occur on a scale of

half the wavelength of the radar, energy is scattered through a process called diffraction

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Radar echoes in a clear atmosphere will be more common on days when

the lower atmosphere is unstable, as when there are thermals present, or when the wind increases rapidly with height just above the ground, so that there is mechanical turbulence.

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 Radar returns are due to

insects, turbulent motions, and ground targets

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 Radar returns are due to insects, turbulent motions, and ground targets. These echoes, which extend out to about

100 kilometers from the radar, allow the radar operator to see the air motion.

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radial velocity field. The distribution of inbound (green) and outbound (yellow) velocities indicate

that the low level winds are out of the south-southeast.

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The radar needs 3 pieces of information to determine the location of a target

azimuth angle 

elevation angle

distance from radar to target

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azimuth angle

the angle of the radar beam with respect to north

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elevation angle

the angle of the beam with respect to the ground

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Distance is determined by 

measuring the time it takes for the pulse to make a round trip from the radar to the target and back using the relation distance = (time) * (velocity).

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The velocity is

the speed of light

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Since the pulse has to travel to

the target and back, the total distance is 2D

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If t is the time it takes, then

2D = ct or D = ct/2.

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The two main types of scans used in meteorology are

the Plan Position Indicator (PPI) and the Range Height Indicator (RHI) scans.

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Plan Position Indicator (PPI): 

When scanning in PPI mode, the radar holds its elevation angle constant but varies its azimuth angle. The returns can then be mapped on a horizontal plane. If the radar rotates through 360 degrees, the scan is called a "surveillance scan". If the radar rotates through less than 360 degrees, the scan is called a "sector scan".

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Range Height Indicator (RHI):

When scanning in RHI mode, the radar holds its azimuth angle constant but varies its elevation angle. The returns can then be mapped on a vertical plane. The elevation angle normally is rotated from near the horizon to near the zenith (the point in the sky directly overhead).

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Constant-Altitude Plan Position Indicator (CAPPI):

It is a composite radar display constructed by assembling radar data from many PPIs at successive elevation angles to obtain the pattern of the data at a specified constant altitude.

*CAPPIs are the common radar images displayed by Environment Canada (at 1.5 km above the surface from the radar site).

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radial velocity

Doppler radars can measure the component of the velocity of targets toward or away from the radar. This component is called the "radial velocity". 

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phase shift

The distance to target has changed from times T1 to T2, resulting in a phase shift between the two return signals, which Doppler radars are capable of measuring

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By knowing the phase shift, the ................................ can be computed 

wavelength and the time interval from T1 to T2, the velocity the target has moved toward or away from the radar can be computed

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If the target is moving sideways so that its distance relative to the radar

does not change, the radar will record zero radial velocity for that target.

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WSR-88D Radar Imagery

detects ........

precipitaiton

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The word radar is an acronym from

"Radio Detection and Ranging"

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The word radar is an acronym from "Radio Detection and Ranging". Radar images are useful for

locating precipitation

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As a Magnetic Resonance Imaging (MRI) scan examines the inside of a human body, a radar examines

the inside of a cloud

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A radar sends 

a pulse of energy into the atmosphere and if any precipitation is intercepted by the energy, part of the energy is scattered back to the radar. These returned signals, called "radar echoes", are assembled to produce radar images.

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The location of the colored radar echoes indicate where

precipitation is falling and the various colors indicate the intensity of the precipitation through the color code in the lower left corner of the image.

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. Regions of light and dark blue indicate regions of

lighter precipitation

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areas of red and pink indicate

strong, to occasionally severe thunderstorms

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Normally, it is difficult to distinguish precipitation type on the basis of the

radar reflectivity alone.

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Snow and light drizzle both produce radar reflectivity with

with about the same value

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. Melting snow and moderate rain

also have similar values

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Very high reflectivities

(the grays on the scale on the image above) are always associated with hail.

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Heavier precipitation reflects more

microwave energy back to a radar than lighter rain

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Heavier precipitation reflects more microwave energy back to a radar than lighter rain. However, more distant rain

also gives a weaker return signal