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Radar > Ch6: doppler velocity, radar product, dual-polarimetric radars > Flashcards

Ch6: doppler velocity, radar product, dual-polarimetric radars Flashcards

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1
Q

radar bands:

A
  • L
  • S
  • C
  • X
  • ku
  • K
  • Ka
  • W
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2
Q

frequency and wavelength of radar bands

A
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3
Q

higher frequency means

A

smaller antenna

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4
Q

Cloud radars

A

W and K

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5
Q

Precipitation radars

A

X

C

S

L

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6
Q

choice of wavelength affects

A

–Ability to detect weak targets at long range

–Resolution of small features

–Types of targets to study

–Effects of propagating through atmosphere

–Radar size, weight, and cost

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7
Q

•Short wavelength

A

–Smaller, less expensive

–More effective in detecting small particles such as cloud droplets and drizzle drops

–Also partially absorbed by these particles (attenuation)

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8
Q

•Long wavelength

A

–Absorption by intervening particles drastically reduced

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9
Q

Period:

A

•time it takes for the wave to propagate from one crest to the next (about 1 millisecond, from the beginning of one pulse to the next)‏

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10
Q

Pulse duration:

A

about 1 microsecond (from beginning of pulse to end of same pulse)

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11
Q

Pulse Repetition Frequency (PRF):

A

•rate at which radar transmits (cycles per second)‏

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12
Q

Backscatter

A
  • amount of energy that is scattered and travels back to the receiver
  • Very small amount
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13
Q

Backscatter depend on

A

–Size

(relative to wavelength)

–Shape

–State

–Temperature

–Dielectric property

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14
Q

Reflectivity (transmitted power)

A

•105 – 106 W

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15
Q

Reflectivity (recieved power)

A

•10-13 – 10-14 W

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16
Q

Reflectivity factor (Z)

A

•Σ(N*D6)

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17
Q

dBZ stands for

A

decibel relative to Z.

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18
Q

dBZ =

A

•10log10(Z)/volume

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19
Q

•dBZ = 10log10(Z)/volume is

A

•a logarithmic dimensionless technical unit used in radar, mostly in weather radar, to compare the equivalent reflectivity factor (Z) of a radar signal reflected off a remote object to the return of a droplet of rain with a diameter of 1 mm.

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20
Q

•dBZ = 10log10(Z)/volume

helps to determine

A

type of precipitation

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21
Q

………………………………………. are commonly referred to as reflectivity when the context is clear

A

Both the radar reflectivity factor and its logarithmic version

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22
Q

Z-R relationship

calculate

A

•Calculate rain rates (R) from reflectivity

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23
Q

Z-R relationship

equation

A

•Z = aRb

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24
Q

•Z = aRb

the a and b depend on

A

•on size of drops and type of system

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25
Z-R relationship source of error
–Attenuation –Large drops –Beam Blockage –Air motions
26
azimuth angle
The angle of the radar beam with respect to north
27
elevation angle
Angle of the beam with respect to the ground
28
PPI
* Plan Position Indicator * Range, azimuth display * Constant elevation angle * Rotates 360°: surveillance scan * Less than 360°: sector scan
29
RHI
* Range Height Indicator * Range, elevation display * Constant azimuth * Vertical structure * Typically rotated from near horizon to near zenith
30
Scanning Mode
•How fast the radar makes sweeps
31
•Clear air mode
–Much slower –Scattering by differences in refractivity –Scattering from insects and birds –Lower reflectivity scale
32
•Precipitation Mode
–Much faster than clean air mode –Detects smaller particles
33
As beam moves away from radar
•moves higher above surface of Earth
34
Radar returns from targets near radar represent
low-level wind field
35
returns from distant targets represent
•wind field at higher levels
36
distance away from radar in center of display represents
•changes in horizontal and vertical distance
37
Attenuation
* Due to scattering and absorption that weakens the power * shorter the wavelength, more attenuation
38
Attenuation by:
•By gases –Very small, neglected •By clouds –Significant for x-band –Small over long distances •By hydrometeors –Affects radars with wavelengths less than 10cm
39
Maximum ambiguous range
–Range to which a transmitted pulse can travel and return before the next pulse is transmitted
40
Bright band
* Area of high reflectivity * Generated as snow falls through melting layer * Below melting layer, snow acquires a coat of water, so looks like a big rain drop
41
Bow Echo
•A bow echo is a term describing the characteristic radar return from a mesoscale convective system that is shaped like an archer’s bow.
42
Bow echo ## Footnote •A bow echo is a term describing the characteristic radar return from a mesoscale convective system that is shaped like an archer’s bow. these systems can produce
•severe straight-line winds and occasionally tornadoes, causing major damage.
43
V-Notch
* The v-notch, also called the “flying eagle,” * is a v-like pattern seen in the upper part of the precipitation shield (usually northeast of the hook echo area of the storm).
44
V notch occur when
* the updraft of the storm is so strong, and the cloud itself is so tall, * that upper level winds are forced to be deflected around the core of the storm effectively spreading the precipitation outward.
45
Supercells and Hook Echoes
The most recognized and well-known radar signature for tornadic supercells.
46
Supercells and Hook Echoes occure when
the strong counter-clockwise winds circling the mesocyclone (rotating updraft) are strong enough to wrap precipitation around the rain-free updraft area of the storm. ! Not all tornadic storms display a hook echo and not all hook echoes produce tornadoes!
47
Doppler Effect
•a change in the frequency with which waves (as of sound or light) from a given source reach an observer when the source and the observer are in motion with respect to each other so that the frequency increases or decreases according to the speed at which the distance is decreasing or increasing
48
Doppler
•Determines direction of movement based on shift in frequency (Doppler shift)
49
the closer the doppler the
higher the wave frequency (higher pitch)
50
Doppler f=
•2V/wavelength where V is the velocity of the target (dr/dt)
51
Inbound:
negative and cool colors
52
Outbound
: positive and warm colors
53
Zero Isodop
: zero velocity (perpendicular to the beam)
54
Nyquist velocity.
The maximum unambiguous velocity that can be detected at a given PRF
55
Vmax =
PRF\*wavelength/4 (Nyquist velocity)
56
Rmax=
•Rmax=c/2PRF
57
As PRF increases,
•Rmax decreases and Vmax increases –Can not maximize both Rmax and Vmax
58
Polarimetric Radar
•Alternating or simultaneous transmission of both horizontally and vertically polarized waves
59
Polarimetric Variables
•Comparing power returned in the horizontal and vertical provides additional information on the size, shape, and ice density of cloud and precipitation particles
60
Polarimetric Variables example:
•Differential Reflectivity (ZDR) –Ratio of horizontal to vertical power return –Good indicator of size shape (0dB for tumbling hail, 2-3dB for rain)
61
dual-polarization
•weather radars send and receive microwaves at one polarization, usually horizontal. By transmitting and/or receiving radar waves at more than one polarization, additional information can be obtained on the nature of the targets
62
The most common dual-polarization scheme is
•the transmission and reception of horizontally and vertically polarized waves.
63
Applications of polarimetric radar
* Improved estimation of rain and snow rates * Discrimination of hail from rain * Gauging hail size * Precipitation type in winter storms * Electrically active storms * Aircraft icing conditions * Identification of birds, insects, etc. * Elimination of “bad” echo
64
Doppler on Wheels
* 3 cm X-band mobile Doppler radars * Mounted on back of truck
65
Cloudsat
* Cloud Profiling Radar * 94 GHZ * Measures the power backscattered by clouds as a function of distance from the radar

Radar (8 decks)

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