Doppler Meteorological Radar

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Doppler Meteorological Radar
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Radar Basics

• RADAR stands for Radio Detection And Ranging.
• A radar system consists of transmitter which
  emits pulses of electromagnetic radiation and a
  receiver that measures the characteristics of the
  radiation scattered back to the radar site.

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Radar Basics (Cont.)
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Radar Basics (Cont.)

• Weather radars typically transmit radiation with
  wavelengths of 3 cm or 10 cm.
• A radar transmission pulse usually lasts about
  0.0000016 seconds. The receiver then listens for
  about 0.00019 seconds. Pulses are typically
  transmitted about 1300 times per second.

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Radar Basics (Cont.)

• Weather radars typically measure
• the intensity of the returned (backscattered)
  signal
• the frequency of the returned signal and
• the elapsed time from the transmission of the
  pulse.

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Radar Basics (Cont.)

• The intensity of the returned signal is used to
  determine the type and intensity of the
  precipitation that is backscattering the radiant
  energy to the receiver.

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Radar Basics (Cont.)

• The radars software uses a reflectivity/rainfall
  relationship (sometimes also called the Z-R
  equation) to relate the magnitude of the
  backscattered energy to the rainfall rate.

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Radar Basics (Cont.)

• These equations usually take the form
• Z aRb
• where
• Z is the magnitude of the backscattered energy
• R is the rainfall rate and
• a and b empirically determined coefficients.

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Radar Basics (Cont.)

• Two factors can affect the amount of energy
  scattered back to the radar.
• Beam Spreading
• Refraction

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Radar Basics (Cont.)

• Beam spreading the energy transmitted by a
  radar pulse spreads out in a conical shape as it
  moves away from the radar. The width of the beam
  expands at a rate of roughly 300 m for every 16
  km it travels. Thus, the beam is approximately
  1000 m wide by the time it has travelled 50 km.

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Radar Basics (Cont.)
Beam Spreading
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Radar Basics (Cont.)

• The effect of beam spreading is to reduce the
  spatial resolution as distance from the radar
  site increases. The practical effect of beam
  spreading is to make it more difficult for the
  radar to detect smaller features as their
  distance from the radar increases.

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Radar Basics (Cont.)

• Refraction refers to the bending of the radar
  beam toward the surface of the Earth due to
  differences in atmospheric density. Density
  differences are caused by variations in
  temperature, moisture, and pressure.

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Radar Basics (Cont.)

• Density differences affect the propagation speed
  and direction of the radar beam. The changes in
  speed may result in either acceleration or
  deceleration of the electromagnetic waves. The
  wave bends in the direction of the deceleration
  and that causes refraction.

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Radar Basics (Cont.)

• Greater density slows the waves more.
• Less dense air does not slow the waves as much.
• Since density normally decreases with height, the
  radar beam is refracted toward the surface of the
  Earth.

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Refraction
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Subrefraction

• If the decrease in density with height is more
  than normal, then the beam bends less than normal
  and this is called subrefraction.
• In this case the beam might shoot over the target
  and miss the precipitation.

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Subrefraction (Cont.)
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Superrefraction

• If the decrease in density with height is less
  than normal, then the beam bends more than normal
  and this is called superrefraction.
• In this case the beam bends more toward the
  surface of the Earth, and it may undershoot the
  target.

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Superrefraction (Cont.)
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Ducting

• If the decrease in density with height is much
  less than normal, the beam may bend down to the
  surface of the Earth in a process called ducting.
• If the beam is backscattered to the receiver, it
  may result in Anomalous Propagation (AP) or
  false echoes.

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Ducting (Cont.)
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WSR-88D

• The radar currently in operational use by the
  National Weather Service is the Weather
  Surveillance Radar 1988 Doppler or WSR-88D.
• The WSR-88D transmits electronic pulses with a
  wavelength of 10 cm.

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WSR-88D (Cont.)

• The WSR-88D measures
• the intensity of the returned (backscattered)
  signal
• the frequency of the returned signal and
• the elapsed time from the transmission of the
  pulse.

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WSR-88D (Cont.)

• The intensity of the backscattered radiation
  provides information about the type and intensity
  of precipitation.

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WSR-88D (Cont.)

• The frequency of the returned signal allows for
  the determination of the Doppler shift.
• The Doppler shift is the shift in frequency of
  the backscattered radiation caused by the
  movement of an object (e.g. a raindrop) toward or
  away from the radar.

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WSR-88D

• The Doppler shift is then used to determine the
  radial velocity of the object that scattered the
  radiation back to the radar.

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WSR-88D (Cont.)

• The elapsed time enables the radar to determine
  the distance of the object from the radar.
• distance (elapsed time) X 3x108 m s-1 2

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WSR-88D (Cont.)

• The distance along with the azimuth and the
  elevation angle of the antenna allow for the
  determination of the location of the
  backscattering element.

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Radar Displays

• Plan Position Indicator (PPI) mode displays data
  horizontally giving the location with respect to
  the radar site.
• Range Height Indicator (RHI) mode displays data
  vertically giving the location with respect to
  the surface of the Earth.

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WSR-88D Components

• Radar Data Acquisition Unit
• Radar Product Generator Unit
• AWIPS (Automated Weather Information Processing
  System) workstation

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WSR-88D Components (Cont.)

• The Radar Data Acquisition (RDA) Unit is
  comprised of the antenna, pedestal, radome,
  transmitter, receiver, signal processor, and
  status and control processor. More than
  200,000,000 pulses are processed every five
  minutes.

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WSR-88D Components (Cont.)

• The Radar Product Generator (RPG) Unit is
  comprised of computers and communications
  equipment. It converts the base data from the
  RDA into meteorological and hydrological products.

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WSR-88D Components (Cont.)

• The AWIPS workstation is able to display the
  meteorological and hydrological products
  generated by the RPG. Users can request products
  from the RPG, display, edit, annotate, distribute
  and locally store products, monitor the status of
  the RDA and RPG and perform local archival
  functions.

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WSR-88D Base Data

• The RDA generates three types of base data
• reflectivity (i.e. the amplitude of the
  backscattered signal
• radial velocity (i.e. the rate of movement toward
  or away from the radar site)
• spectrum width (i.e. the range of the Doppler
  shifts).

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WSR-88D Scanning Technique

• An Elevation Scan is completed when the radar
  scans 360 degrees at a single elevation.
• The WSR-88D can complete an elevation scan in 20
  to 120 seconds depending on the scanning mode.

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WSR-88D Scanning Techniques (Cont.)

• A Volume Scan occurs when a series of Elevation
  Scans conducted at different elevations are
  combined into a single scanning product.

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WSR-88D Scanning Modes

• The WSR-88D has three basic scanning modes
• Clear air mode
• Precipitation detection mode
• Severe weather detection mode.

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Clear air mode

• In clear air mode the WSR-88D makes five
  Elevation Scans at elevation angles of 0.5, 1.5,
  2.4, 3.4 and 4.3 degrees in ten minutes. This
  mode is also called VCP (Volume Coverage Pattern)
  31.
• In this mode the radar antenna has the slowest
  rate of rotation.

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Precipitation Detection Mode

• In precipitation detection mode the WSR-88D makes
  nine Elevation Scans at elevation angles of 0.5,
  1.5, 2.4, 3.4, 4.3, 6.2, 10, 14, and 19.5 degrees
  in six minutes. This mode is also called VCP 21.
• This mode is used to detect light to moderate and
  stratiform precipitation.

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Severe Weather Detection Mode

• In Severe Weather Detection Mode the WSR-88D
  makes fourteen Elevation Scans at elevation
  angles of 0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5,
  8.7, 10, 12, 14, 16.7, and 19.5 degrees in five
  minutes. This mode is also called VCP 11.
• The antenna is rotating fastest in this mode in
  an attempt to monitor severe convective systems.

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WSR-88D Reflectivity Products

• Base reflectivity is a PPI plot of the energy
  backscattered to the radar during a single
  Elevation Scan.
• Composite reflectivity is a PPI plot of the
  maximum energy backscattered to the radar for
  each location during a complete Volume Scan

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WSR-88D Reflectivity Products (Cont.)

• Storm series products includes algorithms able to
  identify, track and project the future movement
  of individual storms.
• Hail index is a product produced by an algorithm
  designed to indicate whether or not a storms
  structure is conducive to hail formation.

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WSR-88D Reflectivity Products (Cont.)

• Vertically Integrated Liquid (VIL) displays the
  amount of liquid water contained in a vertical
  column over each radar grid area.
• Cross-section displays a vertical cross-section
  (i.e. a RHI mode display) of a complete Volume
  Scan over a line denoted by beginning and end
  points chosen by the operator.

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WSR-88D Reflectivity Products (Cont.)

• Weak Echo Region displays graphically the
  three-dimensional structure of the reflectivity
  structure of thunderstorms.
• Echo Tops represents the altitude of the
  reflectivity that occurs at the greatest height
  above the Earths surface over each grid area.

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WSR-88D Reflectivity Products (Cont.)

• One hour, three hour, 24 hour and storm total
  rainfall accumulations are available and are
  based on the summed reflectivity data over the
  desired time period.

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Radial Velocity Products

• Base velocity displays the radial velocity data
  (i.e. the rate of motion toward or away from the
  radar site).
• Storm relative velocity subtracts the movement of
  the storm from the base velocity in order to
  determine the motion relative to the storm.

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Radial Velocity Products (Cont.)

• Mesocyclone detection is an algorithm that
  identifies vertically correlated
  three-dimensional wind shear regions within a
  storm.

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Radial Velocity Products (Cont.)

• Tornado Vortex Signature (TVS) is an algorithm
  which identifies embedded smaller-scale
  circulations and is designed to alert the user to
  areas that indicate a very high threat of
  tornadic circulations.

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Detecting Rotation

• A velocity couplet may indicate rotation.

(radar site)
outound radial velocities
inbound radial velocities
X
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Detection Rotation (Cont.)

• Not all velocity couplets indicate rotation.

inbound radial velocities
(radar site)
outbound radial velocities
Linear (straight line) flow over the radar site.
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Radial Velocity Products (Cont.)

• Velocity Azimuth Display (VAD) products depict a
  vertical profile of the true wind speed and
  direction derived from radial velocity
  measurements.

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Automated Radar Summary Chart

• Shaded areas depict the composite reflectivities.
  Contours are drawn for levels 1,3, and 5.

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Contour Levels

• Level 1 0 dBz
• Level 2 30 dBz
• Level 3 41 dBz
• Level 4 46 dBz
• Level 5 60 dBz

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Echo Heights

• HHH indicates the height of the echo tops in
  hundreds of feet.
• For example, 340 would indicate 34,000 ft.
• ___
• BBB indicates the height of the echo base in
  hundreds of feet.
• ___
• 170 would indicate 17,000 feet.

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Echo Movements

• Arrows point in the direction of movements of
  areas and lines. The speed of the movement of
  areas and lines is denoted by tail symbols just
  like with wind arrows
• Arrows also give the movement of individuals
  cells, but the speed in knots is given at the tip
  of the arrowhead.
• LM indicates little movement.

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Precipitation Types

• TRW Thunderstorm with rain shower
• TSW Thunderstorm with snow shower
• TZRW Thunderstorm with freezing rain shower
• TIPW Thunderstorm with ice pellet shower
• R Rain
• RW Rain shower

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Precipitation Types (Cont.)

• ZR Freezing rain
• ZRW Freezing rain shower
• S Snow
• SW Snow shower
• L Drizzle
• ZL Freezing drizzle

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Precipitation Types (Cont.)

• IP Ice pellets
• IPW Ice pellet shower
• indicates new cells and increasing intensity
• - indicates decreasing intensity

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Remarks

• BWER Bounded Weak Echo Region or Echo Free
  Vault
• HAIL Radar-Indicated Hail
• HOOK Hook Echo
• LEWP Line Echo Wave Pattern
• PCLL Persistent Cell

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Remarks (Cont.)

• SLD indicates a solid line
• FNLN indicates a fine line
• NE indicates No Echoes
• NA indicates Not Available
• OM indicates Operational Malfunction

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Weather Watch Boxes

• WT - indicates a Tornado Watch
• WS - indicates a Severe Thunderstorm Watch

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Additional Sources

• Federal Meteorological Handbook No. 11 Doppler
  Radar Meteorological Observations (WSR-88D)
  contains the official documentation. It can be
  found at
• http//www.ofcm.gov/fmh11/fmh11.htm

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Additional Sources (Cont.)

• http//www.crh/noaa.gov/fsd/science/doppler/index.
  php
• http//www.srh.noaa.gov/jetstream/doppler/doppler_
  intro.htm
• http//ihop.norman.noaa.gov/papers/dopplerguide

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Additional Sources (Cont.)

• http//www.crh/noaa.gov/fsd/science/doppler/index.
  php
• http//www.srh.noaa.gov/jetstream/doppler/doppler_
  intro.htm
• http//ihop.norman.noaa.gov/papers/dopplerguide

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Additional Sources (Cont.)

• Real time radar images can be found at
• http//radar.weather.gov