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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

2

Advanced Doppler radar technology goes beyond simple detection to

providing high resolution reflectivity and estimated velocity data

3

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.

4

The radar creates an

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

5

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.

6

The receiving antenna (which is normally also

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

7

Wave Properties

wavelengths and phase shifts

8

The wavelength (lamada) of a wave is

the distance from one crest to the next.

9

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 -->

11

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

12

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.

15

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

16

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.

17

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

22

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

25

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.

26

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

27

Most weather radars have wavelengths that range between

0.8 centimeters (cm) and 10.0 cm

28

Generally short wavelengths mean

smaller and less expensive equipment

29

Short wavelength radars are more effective in detecting

detecting small particles such as cloud droplets and drizzle drops.

30

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).