Weather Radar

1
Weather Radar

• Notes for Lab 6

2
RADAR

• RAdio Detection And Ranging
• Transmits a microwave into the atmosphere and
  measures the return power
• 10, 5, 3 cm typical
• Size chosen depends on use

3
Radar Parts
Antenna
Transmitter
Transmit/Receive Switch
Receiver
Display
4
How does radar work

• Pulse of microwave energy sent out (emitted from
  antenna to parabolic dish reflector), dish
  focuses energy into beam
• Beam travels through atmosphere
• If the beam hits an object, then some of the
  energy is reflected back to the radar
• Return power measured
• Data processed to a visual display

5
Energy Return

• Radar measures the intensity of the returned
  signal, the frequency of the returned signal, and
  the elapsed time from the transmission of the
  pulse
• Energy beam travels at the speed of light
• Knowledge of the elapsed time allows the
  computation of the distance from the radar site
• Frequency uses doppler shift to determine movement

6
Return Power

• Only a fraction of the emitted energy gets
  returned from reflection
• amplified and measured in decibels (dbz),
  Reflectivity
• 1dbz 10 log(p2/p1)
• p2 power received at radar, varies
• Reflectivity is dependent upon the size of the
  object
• In meteorology, the objects are precipitation
  particles
• A plotted region of high reflectivity is called
  an echo

7
Reflectivity Dependence

• The return power (reflected beam) is dependent on
  the number of particles present, and the size of
  the particles
• Particle diameter6 dependence
• Number1 dependence
• Larger drops lead to larger reflectivities echoes
• Reflectivity mostly based on particle size

8
General Reflectivities

• Drizzle 20 - 25 dBz
• Light rain 25 35 dBz
• Heavy Rain 35 50 dBz
• Thunderstorm Heaviest Rainfall gt50 dBz
• Light Snowfall 15 25 dBz
• Heavy Snowfall 25 35 dBz

9
Rainfall Rates

• How much rain falls to the surface in a given
  hour
• Rinches/hour
• a and b are constants
• Higher reflectivity generally corresponds to
  higher rainfall rates