Operational Weather Radar Featuring: WSR-88D Doppler Radar

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Operational Weather Radar Featuring WSR-88D
Doppler Radar
Transmission Power 750,000 W Gain 35,481 Beam
Width lt2º _at_ 125 nmi Pulse Length - 0.47 or 1.35
km Transmission Time 1.57 of 4.5 µs
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DOWs measure record high wind speeds of 301 /-
20 mph (135 /- 10 m/s) in 03 May 1999 Oklahoma
City tornado. 
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Abstract

• Sensors
• WSR88 standard weather service radar
• 158 Operational Sites Across the Country
• Common Radar Bands K,Ku, C,X, Ka
• ?1-10cm, 1-10x size of particles (rain ice) of
  interest
• Makes use of Rayleigh Scattering
• Phased Array Radar
• Algorithms
• Radar Equation
• Attenuation

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Brief Weather Radar History

• 1950 RADAR discovered
• RADAR RAdio Detection And Ranging
• In early use, WWII military operators picked up
  on what they thought was noise
• Really environmental returns, only noise if you
  want to see airplanes
• 1950-1980 Continued development of reflectivity
  radar
• 1980-2000 Invention and implementation of
  Doppler radar
• As of 1988 the US started using 10 cm wavelength
  radars (WSR-88D) as their standard radar device
  for radar networks
• 2000-present Dual Polarization Radar and Phased
  Array Radar

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source http//www.cocorahs.org/media/docs/radar_b
asics.pdf
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source http//www.cocorahs.org/media/docs/radar_b
asics.pdf
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Introduction

• Key concepts
• Doppler effect
• Rayleigh scattering
• Sensing the return
• Reflectivity, dbz
• Vertical cross section
• Range Height Indication Scan
• Constant Azimuth
• Improvement with WSR-88D
• Volume Scanning
• Vary azimuth along with elevation
• Optimally interpolate 3d volume or reflectance

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Radar, Doppler and Raleigh

• Radar Equation
• Whn transmiter and reciever are in the same
  place, signal decreases in power to the fourth
  power
• Doppler Equation
• Where F observed frequency, F emitted
  frequency, v velocity in medium, vs velocity
  of source
• Raleigh Scattering
• EM radiation returned from a field with average
  particle size smaller than the wavelength of the
  radiation

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Reflectivity off a target

• Power of the return signal - reflectivity (z)?
• z has the units mm6/m3 -
• density of water droplets which would return the
  reflectivity
• emissivity is implicit
• To cover a large range it is often described in
  decibels
• Intensity of the return echo helps determine the
  precipitation rate
• significant precipitation
  usually is above 15 dBz
• Reported reflectivity is an average of
  at least 25 pulses from a given azimuth
  and elevation

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Applications

• Nowcast/ short term forecast
• useful for extreme weather
• Easily deployed in remote locations
• Lee Rotors
• Precipitation Estimation
• Links to related topics
• RUCOOL Codar Maps
• Radar Altimetry
• Police Radar
• Small Aircraft Detection
• Benefits from radial
  movement of parts

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Radar Bands and Uses

• 2-4 S GHz, 7.5-15cm. Sirius and XM radio. Long
  range weather, marine weather, ATC
• 4-8 Ghz - C band satellite transponders,
  satellite TV, raw satellite communications,
  weather.
• 18-27 Ghz - K - police, small drizzle/fog
  research ?
• 27-40 Ghz Ka police
• 8-12 Ghz - X airport radar very long range,
  missile guidance

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Making a Signal

• The listen time (millisecond) is 1000x the
  pulse duration
• This allows the radar an opportunity to be able
  to receive the signal again without interference
  from previous signals
• Must wait until signal has reached maximum range
  and returned.
• Knowing exact pulse travel time allows for
  calculation of the horizontal distance to the
  target
• Intensity of the return, or backscatter -gt target
  size
• Radial Target Motion -gt Doppler shift

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What happens to the returned signal?

• Must orient the returns (power at certain
  frequencies) into an accurate measure of
  precipitation
• Measurements over a spherical volume are sliced
  up into horizontally and vertically to produce
  2,3 and 4d visualizations
• Account for ambient background noise

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Source http//cimms.ou.edu/rvamb/Documents/Report
_7.pdf
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Algorithms

• VIL
• Vertically Integrated Liquid
• HAIL
• Estimates presence and size of
  hailstones
• Potential Wind Gusts
• Uses VIL, cloud top height
• Estimates the winds under the cloud (ie
  downdraft)?
• Doppler measurement readily reveals wind shear
  storm relative velocity
• Mesocyclone Detection Algorithm
• Tornado Detection Algorithm
• Wind Shear Detection (low level)?

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Products

• Base reflectivity
• how much precipitation is falling
• precipitation type
• assess a storm's structure and dimension
• Composite Reflectivity
• Scans from all elevations, imaging precipitation
  intensity and storm structure
• Base Velocity
• radial wind field, speed of fronts/strong wind
• range of 140 mi
• Storm relative motion
• Track a circulation (show up well in doppler
  return) over time to determine storm motion.
• Removing the storm relative motion from base
  radials gives an estimate of the flow with
  respect to the storm.

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Complications

• Migrating Birds
• Insects
• Aircraft, solid buildings,
• large aerosols
• for air traffic controller rain
• for meteorologist airplanes.
• Identification of snow, and snow type,
• modification of observed matter before hitting
  the ground (fallstreaks)?
• big enough sample of precip for identification?
• In precip estimates Z-R relationship

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Phased Array Radar
Proven NAVY technology

• Multiple beams with variable dwell times scan
  continuously
• perform full volume scan every 20-30s
• track cooperative aircraft
• track non-cooperative aircraft
• perform medium dwell scan on heavy cumuliform
  convection
• perform long dwell scan on area of suspected
  tornadic development.
• Expected that tornado warning could in to to 45
  min

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In Closing

• Take precipitation rates with caution
• Highly accurate under most conditions
• Misleading in cases of extreme precipitation
• The ultimate in nowcast and high resolution
  spatial time series for precipitation.
• Extremely useful in adaptive forecasting of
  extreme weather events