1
Q
What maybe within the sample volume?
A
- Raindrops or cloud particles
2
Q
What completely fills the radar beam?
A
- Storms and clouds
- Because they are usually so large
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
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
5
Q
What may misinterpret the strength of the signal?
A
- If the beam is only partially filled
6
Q
Continental clouds contain as many as
A
- 200 or more cloud droplets/cm3
- That amounts to 2x108/m3
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
8
Q
If the radar is using 1 us pulse length the effective sample volume in space will be
A
- 150 m
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
10
Q
The return from meteorological targets is the combination of
A
- Billions of returns being added together
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
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
13
Q
If pulses were sent nearly simultaneously
A
- The returns measured by the radar would be virtually identical
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
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
16
Q
Why should we wait long enough to allow particles to shuffle
A
- To get a good average of the true signal amplitude
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
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
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
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
What might make the sample decorelate slowly?
* Particles having the same size because that will make them fall together with the same terminal velocity
26
What might make the sample decorrelate faster?
* Storm containing a wife veriety of particle sizes
* There will be many different particles falling in the same volume
27
The shortest decorrelation times occur when
* Hail and rain are in the same sample volume
28
The time required for the autocorrection function to fall to a value of t= 0.01 is
* Aprox.t= 2 lamda to 3 lamda
* T is in milliseconds and
* Lamda is in centimeters
29
Measured decorrelation time have ranged from
* 3.5 ms to nearly 30 ms
30
Measured decorrelation time range depends on
* Storm and radar
31
If we want to sample as close together in time as possible but still have independent samples, we would want to
* 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
modern Doppler radars often use PRFs
* near 1000 Hz
33
why did we use PRF near 1000 Hz and not 1000 Hz
* because PRF of 1000 Hz give sampling time that is too close together to have truly independent samples
34
why is it necessary to average many consecutive pulses together
* in order to have the equivelant of just a few independent samples
35
factors which can contribute to decreasing the time to independence of consecutive samples made with a radar
* range averaging
* moving the antenna in azimuth while collecting the data (almost always done)
* wind shear within the sample volume
* turbulence
36
the horizontal and vertical beam widths must always
* be in radians
37
the pulse length h is:
* the length in space corresponding to the duration t of the transmitted pulse
* AKA pulse duration
38
In the equation for sample volume we used h/2 because
* We are interested in signals that return to the radar at precisely the same time
39
How to find the total backscattering cross sectional area of targets within the radar sample volume?
* 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
For sample volume we used the horizontal and vertical beam widths this assumed that
* 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
2 ln (2) in the dominator of the radar pulse volume equation accounts for:
* The real beam shape better than the assumption
42
Parameter that is related to the total backscattering cross sectional area
* Radar reflectivity
43
Radar reflectivity:
* 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
…………………… is related to the backscattering cross sectional area
* Target size
45
Reylight approximation applies if:
* The particles are small compared to the wavelength
46
If particles are large compared to the wavelength the targets
* Will be in the optical region
47
If particles are intermediate compared to the wavelength the targets
* Will be mie scatterers
48
Why is Rayleigh approximation applied in meteorology?
* Because for most meteorological radars (wavelengths of 3 cm and larger) almost all raindrops are considered small compared to the wavelength
