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Radar > ch5: Distributed targets > Flashcards

ch5: Distributed targets Flashcards

1
Q

What maybe within the sample volume?

A
  • Raindrops or cloud particles
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2
Q

What completely fills the radar beam?

A
  • Storms and clouds
    • Because they are usually so large
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3
Q

Place where storms and clouds don’t completely fill the radar beam

A
  • Along the boundary of a storm
    • Because the radar beam will be moving from no echo to echo and vice versa
  • Near the top or bottom of a storm
    • The beam will be partially in and partially out of echo
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4
Q

The power returned to the radar will come from

A
  • All the individual targets being illuminated by the radar beam wether the beam is completely filled or not
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5
Q

What may misinterpret the strength of the signal?

A
  • If the beam is only partially filled
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6
Q

Continental clouds contain as many as

A
  • 200 or more cloud droplets/cm3
  • That amounts to 2x108/m3
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7
Q

For a radar with a 1o antenna beam width the beam will be

A
  • 1 km in diameter at a range of 57 km
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8
Q

If the radar is using 1 us pulse length the effective sample volume in space will be

A
  • 150 m
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9
Q

The volume of the radar pulse is then illuminating (if pulse strength is 1 us)

A
  • More than 2x106 cloud droplets simultaneously
  • The number of precipitation sized particles is lower than this
  • Typical rain will have on the order of a few to a few hundred raindrops per cubic meter
  • Thus there might be something like 109 to 1012 raindrops in a single radar sample volume
  • This is still a very large number of particles
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10
Q

The return from meteorological targets is the combination of

A
  • Billions of returns being added together
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11
Q

The total backscattering cross sectional area of a meteorological target is

A
  • The sum of all of the individual backscattering cross sectional area
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12
Q

If we send a pulse of radar energy into a storm and get an echo back then send a second pulse into the storm immediately after the first

A
  • There would be little time fot the raindrops to change position relative to each other or relative to the radar
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13
Q

If pulses were sent nearly simultaneously

A
  • The returns measured by the radar would be virtually identical
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14
Q

If we waited reasonable length of time before sending a second pulse into the same point in space

A
  • The arrangement of particles bring sampled by the radar might be different
  • Between these two limits is a region of interest and importance for radar
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15
Q

When sampling raindrops or other hydrometeors with radar we need to

A
  • Wait long enough to allow the particles to reshuffle so a truly different arrangement can be reached
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16
Q

Why should we wait long enough to allow particles to shuffle

A
  • To get a good average of the true signal amplitude
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17
Q

Weather echoes are

A
  • Constantly changing
  • A single instantaneous measurement might not be a good measure of the true signal strength
  • By averaging several samples together we get a better measurement of a storm intensity
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18
Q

Time to independence:

A
  • AKA decorrelation time
  • Time it takes hydrometers to rearrange them selves so the measurements are independent of one another
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19
Q

Time of independence mathematical definition:

A
  • Time it takes for a sample of targets to decorrelate tto a value of 0.01 from perfect coefficient
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20
Q

Perfect correlation has a correlation coefficient of

A
  • 1.0 (or -1.0 if there is an inverse correlation)
21
Q

Correlation coefficient of 0 means that

A
  • There is no correlation at all
22
Q

Waiting for a signal to decorrelate to 0.01 means

A
  • We have waited long enough that the newest sample is almost completely different than the original sample
23
Q

The decorrelation time of a sample depends on:

A
  • Wavelength of the radar used
  • Hydrometeors
  • Turbulence within the sample volume
24
Q

Decorrelation times are shorter for

A
  • Shorter wavelength radars
    • When a short wavelength is used particles do not have to travel as far to change position significantly relative to the radar
25
Q

What might make the sample decorelate slowly?

A
  • Particles having the same size because that will make them fall together with the same terminal velocity
26
Q

What might make the sample decorrelate faster?

A
  • Storm containing a wife veriety of particle sizes
    • There will be many different particles falling in the same volume
27
Q

The shortest decorrelation times occur when

A
  • Hail and rain are in the same sample volume
28
Q

The time required for the autocorrection function to fall to a value of t= 0.01 is

A
  • Aprox.t= 2 lamda to 3 lamda
    • T is in milliseconds and
    • Lamda is in centimeters
29
Q

Measured decorrelation time have ranged from

A
  • 3.5 ms to nearly 30 ms
30
Q

Measured decorrelation time range depends on

A
  • Storm and radar
31
Q

If we want to sample as close together in time as possible but still have independent samples, we would want to

A
  • sample at a rate given approximately by 10t0.01.
    • this suggests we should sample at rates on the order of 3 to 30 times a second (3-30 Hz)
      • most radars sample at rates much higher than this
32
Q

modern Doppler radars often use PRFs

A
  • near 1000 Hz
33
Q

why did we use PRF near 1000 Hz and not 1000 Hz

A
  • because PRF of 1000 Hz give sampling time that is too close together to have truly independent samples
34
Q

why is it necessary to average many consecutive pulses together

A
  • in order to have the equivelant of just a few independent samples
35
Q

factors which can contribute to decreasing the time to independence of consecutive samples made with a radar

A
  • range averaging
  • moving the antenna in azimuth while collecting the data (almost always done)
  • wind shear within the sample volume
  • turbulence
36
Q

the horizontal and vertical beam widths must always

A
  • be in radians
37
Q

the pulse length h is:

A
  • the length in space corresponding to the duration t of the transmitted pulse
  • AKA pulse duration
38
Q

In the equation for sample volume we used h/2 because

A
  • We are interested in signals that return to the radar at precisely the same time
39
Q

How to find the total backscattering cross sectional area of targets within the radar sample volume?

A
  • Determine the backscattering cross sectional area of a unit volume and multiply this by the total sample volume
    • Where the summation is over all of the individual backscattering cross-sectional areas in a unit volume
40
Q

For sample volume we used the horizontal and vertical beam widths this assumed that

A
  • All of the energy in the radars transmitted pulse is contained within the half power beam widths
    • Real radars don’t have such nicely behaved beam patterns
41
Q

2 ln (2) in the dominator of the radar pulse volume equation accounts for:

A
  • The real beam shape better than the assumption
42
Q

Parameter that is related to the total backscattering cross sectional area

A
  • Radar reflectivity
43
Q

Radar reflectivity:

A
  • The summation is done over all individual targets in a unit volume in spave
  • Has units of 1/cm or cm^-1
  • Intensive parameter
44
Q

…………………… is related to the backscattering cross sectional area

A
  • Target size
45
Q

Reylight approximation applies if:

A
  • The particles are small compared to the wavelength
46
Q

If particles are large compared to the wavelength the targets

A
  • Will be in the optical region
47
Q

If particles are intermediate compared to the wavelength the targets

A
  • Will be mie scatterers
48
Q

Why is Rayleigh approximation applied in meteorology?

A
  • Because for most meteorological radars (wavelengths of 3 cm and larger) almost all raindrops are considered small compared to the wavelength

Radar (8 decks)

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