Severe and Unusual Weather ESAS 1115

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Severe and Unusual Weather ESAS 1115

• Spotter Training and Radar Meteorology
• Part 2 Introduction to Radar Meteorology

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Meteorological Sensors

• Two types of two types of sensors
• Remote vs. In-situ
• Active vs. Passive
• Our passive eyes can only see features of the
  storm
• In order to see the inner workings of a
  thunderstorm, and to understand it better, we
  need an active remote sensor weather radar
• Radar will allow us to interrogate information
  about the storm by detecting precipitation and
  wind information within

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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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Gate Size

• Reflectivity Information is displayed in
  pixel-like units of area called gates
• Gate size is determined by pulse length and
  angular beam width

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Reflectivity

• dBm 10log(power returned/1milliwatt)
• Z is reflectivity
• dBZ is a logarithmic scale similar to dBm
• 1dBZ is the power returned by a sphere of 1mm6/m3
• Doubling of power results in a linear increase of
  3dBZ
• Z is proportional to D6

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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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The Significance of Clear Air Mode During Snowfall
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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
VIL Vertically Integrated Liquid
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NIDS - Nexrad Information Dissemination Service
Echo Tops
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NIDS - Nexrad Information Dissemination Service
VAD Velocity Azimuth Display
VWP VAD Wind Profile
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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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Using the Zero Isodop
When the radial is perpendicular to the the wind,
the radar displays zero velocity - This zero
zone is called the Zero Isodop.
What percentage of actual wind will the radar
detect? 00 100 - Parallel 150 97 300
87 450 71 600 50 750 26 900 0 -
Perpendicular
When the wind velocity is parallel to the radial,
the full component of the wind is measured
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Interpreting Doppler Radar

• Winds will flow perpendicular to the zero isodop
  from green to red
• Veering wind profile is denoted by an S on the
  overall winds display
• Veering winds with height indicate warm air
  advection and hence rising air

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Interpreting Doppler Radar

• Backing wind profile is denoted by a backwards
  S on the overall winds display
• Backing winds with height indicate cold air
  advection and hence sinking air (subsidence)

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What Does this Represent?
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Veering Winds on VWPVelocity Azimuth Display
(VAD) Wind Profile
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Hurricane Katrina
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Divergence
Divergence is indicated by radial shear (along
the radius)
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Rotation
Cyclonic rotation is indicated by azimuthal shear
(from one azimuth to another)
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Low-level Rotation and Storm Top Divergence
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Rotation or Convergence?
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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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Gate to Gate Shear (TVS)
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TVS
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TVS
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Azimuth Resolution Considerations
Rotational couplet identification can be affected
by azimuth resolution. As the diagram shows, the
closer a rotation is to the radar the more likely
it will be identified correctly. If the rotation
is smaller than the 10 beam width (possible at
long ranges) then the rotation will be diluted or
averaged by all the velocities in that sample
volume. This may cause the couplet to go
unidentified until it gets closer to the radar.
Azimuth 3
Weak inbound, weak outbound
Rotation too small to be resolved
Azimuth 2
Strong inbound, strong outbound
Azimuth 1
Stronger inbound than outbound
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Strong TVS
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SRV vs. Base Velocity with Subtle Rotation
Base Velocity
Storm Relative
When diagnosing straight line winds use base
velocity. The strength of an advancing line of
storms producing straight line winds is the sum
of the winds produced by the storms, plus the
movement of the storms.
When diagnosing rotation, use storm relative
velocity. SRV subtracts out the motion of a storm
to display pure rotational characteristics of
that storm.
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FAR vs. POD

• FAR (False Alarm Ratio) An event is warned for
  but does not occur results in a false alarm
• POD (Probability of Detection) An event that
  occurs and has been warned for results in a 100
  POD
• A high POD is achieved at the expense of an
  increased FAR

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Doppler Dilemma

• Radial velocity and range is limited by PRF
• High PRFs result in short unambiguous ranges and
  vice versa
• Low PRFs result in velocity aliasing and vice
  versa
• Given a PRF, the radar can determine the radial
  velocity within some range, or some velocity,
  called the velocity interval
• Any velocity beyond that range will fold over
  into the incorrect value
• Bad data must be dealiased
• The Doppler Dilemma There is no single PRF that
  maximizes both Rmax and Vmax

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Velocity Aliasing and Dealiasing
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Three-Body Scatter Spike (TBSS)

• The flare appears further than the main core
  along the same radial as the highest core
• The added distance the beam makes from stone to
  ground to stone (3 bodies) results in a display
  further than reality

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TBSS Examples
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Same Storm, Different Radar
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TBSS in Velocity Data
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Anomalous Propagation (AP)
In the wake of this line of thunderstorms, a
low-level inversion created by the cold pool
results in superrefraction and thus AP
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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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Sunset Spike
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Sunset Spikes
Sunset spikes
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Sunset Spike
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What the Heck Is This?