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Radars

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Why Radar Can't (Usually) See Tornadoes The network of WSR-88D Doppler radars across the US has certainly proven itself for the ability to detect severe weather. – PowerPoint PPT presentation

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Title: Radars


1
Radars
  • Sandra Cruz-Pol
  • Professor
  • Electrical and Computer Engineering Department
  • University of Puerto Rico at Mayagüez
  • CASA- Collaborative Adaptive Sensing of the
    Atmosphere
  • 2006

2
What is a Radar?Radio detection and ranging
  • How does a radar work?
  • Radar Concepts
  • Games

3
The frequency of the em wave used depends on the
application. Some frequencies travel through
clouds with virtually no attenuation. ALL em
waves move at the speed of light
4
  • How does a radar work?

5
(No Transcript)
6
Compare to Acoustic Echo-location
hello
7
Acoustic Echo-location
hello
8
Acoustic Echo-location
hello
distance
9
Hi !!
t 2 x range / speed of sound
Example range 150 m Speed of sound 340
meters/second t 2 X 150 / 340 1 second
10
RADAR Echolocation(RADAR RAdio Detection And
Ranging)Microwave Echo-Location
Tx
Rx
Microwave Transmitter
Receiver
11
Target Range
Tx
Rx
time
t 2 x range / speed of light measure t, then
determine Range
Example t .001 sec Speed of light c 3x108
meters/second Range .001 x 3x108 / 2
150,000 m 150 km
12
Thresholding
Threshold Voltage
time
  • Measure time elapsed between transmit pulse
  • and target crossing a threshold voltage.
  • Then calculate range.
  • Dont report back any information from targets
    that
  • dont cross the threshold

13
Range-Gating
Range Gates
time
14
  • We will see that Radars work by
  • Transmitting microwave pulses.
  • and measuring the
  • Time delay (range)
  • Amplitude
  • Frequency
  • Polarization
  • of the microwave echo in each range gate

15
Target Size
Scattered wave amplitude conveys size of the
scattering objects. Measure amplitude, determine
size.
time
16
Target Radial Velocity
Frequency ft
Frequency ft fd
17
Target Radial Velocity
Frequency ft
Frequency ft fd
18
Zero Velocity for Crossing Targets
Frequency ft
Doppler Frequency
Frequency ft fd
19
Target Spatial Orientation
Large Drops
Polarization Pt
Small Drops
Polarization Ps
20
Example Weather Echoes
Microwave Transmitter
Receiver
21
Echo versus Range(range profile)
Transmitted Pulse 1
Cloud Echo
time
22
In summary, radars work by Transmitting
microwave pulses. and measuring the
of the microwave echo in each range gate
Time delay (range)
Amplitude (size)
Frequency (radial velocity)
Polarization (spatial orientation oblateness)
23
Other concepts of Radars
24
Colors in radar images
  • The colors in radar images indicate the amount of
    rain falling in a given area.
  • Each raindrop reflects the energy from the radar.
    Therefore, the more raindrops in a certain area,
    the brighter the color in the radar image of that
    area.
  • The bright red color around the eye of a
    hurricane radar image indicates the area of
    heaviest rainfall. The green colored area has a
    moderate amount of rain, while the blue areas
    represent the least amount of rain.

Hurricane Andrew, 1992
25
QPE Quantitative Precipitation Estimation
0.1 mm/hr
1 mm/hr
15 mm/hr
100 mm/hr
gt150 mm/hr
26
Why Radar Can't (Usually) See Tornadoes
  • The network of WSR-88D Doppler radars across the
    US has certainly proven itself for the ability to
    detect severe weather. Tornado warnings, in
    particular, are much better now that National
    Weather Service forecasters have this fantastic
    new (new as of the early 1990s) tool.
  • But did you know that Doppler radar (usually)
    can't see an actual tornado? When Doppler radar
    is cited in a tornado warning it is generally
    because meteorologists see evidence the storm
    itself is rotating. It is a supercell
    thunderstorm or at least contains an area of
    rotation called a mesocyclone.
  • When can and when can't Doppler radar see a
    tornado? It's math! Let's figure it out. We'll be
    looking into two factors
  • 1) the first is something you learned in school a
    loooong time ago - the earth is curved, and
  • 2) the radar "beam" is 1 degree wide.

27
NEXRAD System Today
Gap
28
May 3, 1999 Tornado Outbreak in Oklahoma
29
NWS has 150 NEXrad radars in US1 in Cayey, PR
30
Proposed CASA radar network
31
CASA radars will complement NWS radars
Water spout at Mayaguez Beach, PR- Sept 2005
unseen by NEXRAD
32
Radar "Beamwidth"
  • The geometry of the dish and a few other factors
    help determine the pulse volume, which can be
    specified in degrees.
  • NEXRAD radar sends discrete pulses (and spends
    99.57 of the time listening for return echoes)
  • Meteorologists like to use the convenient terms
    "beam" and "beamwidth" to describe where the
    radar is pointing and the effective resolution of
    the air being sampled.

33
Antennas
  • Antenna is a transition passive device between
    the air and a transmission line that is used to
    transmit or receive electromagnetic waves.

34
Antenna Beamwidth
radians D is the antenna
diameter ? is the wavelength of signal in air
Tradeoff Small wavelengths (high frequencies)
small antennas But small wavelengths attenuate
more
35
Beamwidth Size vs. Object Size
  • Beamwidth
  • What can a radar see? Beamwidth is one
    consideration. Earth curvature and height of the
    feature is another (addressed on the next page).
  • For the moment, we'll keep the problem in two
    dimensions and ignore height above ground.
  • The geometry is an isosceles triangle. Be sure to
    note which beamwidth you are calculating for
    (i.e. 1 degree).

36
Beamwidth
0.7 mi 1.4 mi 2.1 mi 2.8 mi
37
Object Size
  • How wide and tall are various things we want to
    see?

Width of Meteorological Objects (i.e. Storms,
Tornadoes)
38
Earth Curvature
Fill in the table with values you calculate
 
 
 
0.17 mi 0.35 mi 0.52 mi 0.70 mi 7 mi 16 mi 23
mi 31 mi
39
Play related games
40
Play the games to learn the basics
  • http//whyfiles.org
  • http//meted.ucar.edu/hurrican/strike/index.htm
  • http//meted.ucar.edu/hurrican/strike/
  • http//meted.ucar.edu/hurrican/strike/info_3.htm
  • http//www.nws.noaa.gov/om/hurricane/index.shtml
  • http//www.nws.noaa.gov/om/edures.htm

41
More Games for Kids 4-104
  • http//www.nws.noaa.gov/om/reachout/kidspage.shtml

42
References
  • The COMET project http//www.comet.ucar.edu/
  • NASA TRMM
  • NCAR (National Center for Atmospheric Research) -
    University Corporation for Atmospheric Research
    (UCAR)
  • NOAA Educational Page http//www.nssl.noaa.gov/ed
    u/ideas/radar.html
  • Dave McLaughlin Basics of Radars presentation
  • NWS http//www.crh.noaa.gov/fsd/soo/doppler/doppl
    er.htm
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