Radar Basics Strengths, Limitations and Myths

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Radar BasicsStrengths, Limitations and Myths

• 2009 DuPage CountyAdvanced Spotter
  TrainingWheaton College

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Radio Detection and Ranging
NEXRAD Next Generation Radar WSR-88D Weather
Surveillance Radar, 1988 Doppler
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Important Angles

• The azimuth angle is the compass angle from north
  (360)
• Targets are indicated by A/R (Azimuth/Range)
• Birds eye view is displayed on a PPI (Planned
  Position Indicator)
• A cross sectional display is an RHI (Range Height
  Indicator)
• Volume scan

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Reflectivity

• Reflectivity is a measure of how much power is
  returned from a radars pulse. The information
  is displayed in pixel-like units of area called
  gates
• The unit of reflectivity is a decibel of
  reflectivity or dBZ.

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Clear Air Mode
Range from -28 to 28 dBZ 16 4dBz increments
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Precipitation Mode
Range from 5 to 75 dBZ 16 5dBz increments
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Mode is Controlled by NWS and Can Be Problematic
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NIDS - Nexrad Information Dissemination Service
Base Reflectivity Various Elevation Angles
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NIDS - Nexrad Information Dissemination Service
Base (or Storm Relative) Velocity Various
Elevation Angles
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NIDS - Nexrad Information Dissemination Service
1 Hour Precipitation
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NIDS - Nexrad Information Dissemination Service
Storm Total Precipitation
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NIDS - Nexrad Information Dissemination Service
Composite Reflectivity
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Interpreting Doppler Radar
Radial Velocity is the velocity toward or away
from a radar
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Automated Detection of Meteorological Phenomena

• Algorithms help detect significant features
• MDA Mesocyclone Detection Algorithm
• Meso - donut
• Persistent, strong and detected over a large
  depth of the storm
• TDA Tornado Detection Algorithm
• TVS Tornado Vortex Signature
• Strong gate to gate shear
• HDA Hail Detection Algorithm
• SCIT - Storm Cell Identification and Tracking

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Composite Reflectivity with Storm Attribute Table
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Composite Reflectivity with Symbols
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Velocity Aliasing and Dealiasing
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AP and Ground Clutter
AP is enhanced by strong returns from buildings
(ground clutter)
Cooling in the evening hours results in a
low-level nocturnal inversion and AP
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Myth 1 All Storms are Supercells (Wouldnt that
be nice!)
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Myth 2 I Can Identify Supercells by Looking at
Reflectivity
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Which are supercells?
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Myth 3 Size Matters
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Myth 4 Supercells are Isolated Storms
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Truth 1 Always Use Velocity Data to Detect the
Mesocyclone
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Myth 5 Red Next to Green Means Rotation
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Truth 2 Azimuthal Shear Implies Rotation
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A TVS is Strong Gate-to-Gate Azimuthal Shear
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TVS
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Biggest Radar Myth Ever!Radar Will See All
Tornadoes
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Most Important TruthSpotters are Vital To
Success

• Radar is a very powerful tool
• It helps meteorologists understand what might be
  happening in the storm
• Only a good spotter can verify that info
• Radar is prone to a lot of errors and mistakes in
  interpretation

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Where to Get Radar Info

• http//weather.cod.edu/analysis
• http//radar.weather.gov/
• http//grlevelx.com/
• Other private vendors