Doppler Radar Interpretation Module 2'4b

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Doppler Radar InterpretationModule 2.4b

• 1. Introduction
• 2. Basic Principles
• 3. Inherent Limitations
• 4. Velocity Azimuth Display
• 5. Doppler Velocity Interpretation
• Synoptic
• Mesoscale

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1. Introduction(1 of 1)

• Wave phenomena subject to Doppler effect
• Doppler effect associates target motion with
  frequency shift in returned radiation
• In Canada .. first Doppler installed in 1984 at
  King City, near Toronto
• Doppler introduces velocity information of
  weather targets .. dynamics

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2. Basic Principles(1 of 10)

• Most of discussion that applied to conventional
  radar applies to Doppler radar

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2. Basic Principles(2 of 10)

• We want to measure doppler frequency shift of
  moving targets
• But we have a technical problem .. measuring
  small frequency shift (say 400 hz) on a 6 Ghz
  signal .. within a 2 microsecond pulse ..
• So .. instead .. we measure the change in phase
  from pulse to pulse ..

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2. Basic Principles(3 of 10)
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wind
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2 us pulse 5 cm wavelength 11,200 cycles
drop movement
phase shift
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2. Basic Principles(4 of 10)

• the problem with radial velocity ..

.. is one of ambiguity ..
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2. Basic Principles(5 of 10)

• The interpretation of Doppler data is an attempt
  to infer a wind field from the measured radial
  wind field.
• Generally two fields of interpretation ..
• Synoptic ( uniform over larger area)
• Mesoscale (small scale variation)

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2. Basic Principles(6 of 10)

• targets have a range of velocities inherent to
  turbulence in the sample volume
• we obtain a histogram (usually 64 samples) and
  perform a FFT to obtain a continuous power
  spectrum and the average radial velocity

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2. Basic Principles(7 of 10)

• Notation ..
• RED-AWAY
• BLUE-TOWARDS

RABT
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2. Basic Principles(8 of 10)
A first example ..

• TECHNIQUE
• draw line to zero Vr
• V is perpendicular

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2. Basic Principles(9 of 10)
A second example ..
30-33 m/s
winds uniform from the NE at 30-33 m/s
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2. Basic Principles(10 of 10)
A third example ..
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3. Inherent Limitations(1 of 4)

• A pulse travelling to a target at range rmax and
  back will cover a distance 2rmax
• The pulse will make it back to the radar before
  the next pulse is emitted if

We saw this in the conventional radar module as
well..
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3. Inherent Limitations(2 of 4)

• A maximum unambiguous radial velocity arises in
  Doppler sampling.
• This is not inherent to the Doppler effect, but
  rather a side effect of the method we use to
  determine radial velocity (pulse to pulse phase
  shift)

Referred to as the Nyquist Velocity
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3. Inherent Limitations(3 of 4)

• For NRP Doppler ..
• PRF1190 hz, Vmax16 m/s, Rmax126 km
• PRF892 hz, Vmax12 m/s, Rmax168 km

The Doppler Dilemma
velocity
range
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3. Inherent Limitations(4 of 4)

• fortunately .. we can use a trick that allows
  us to extend the range of unambiguous velocity
• the DUAL-PRF TECHNIQUE is used to extend the
  range of the velocity

• For NRP Doppler ..
• PRF1190 hz, Vmax16 m/s, Rmax126 km
• PRF892 hz, Vmax12 m/s, Rmax168 km

Using the Dual-PRF (1190/892) technique
.. Vmax48 m/s, Rmax126 km
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4. Velocity Azimuth Display(1 of 3)

• Doppler velocity scan defined by PPI .. a cone

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4. Velocity Azimuth Display(2 of 3)
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4. Velocity Azimuth Display(3 of 3)
direction
speed
look at one height at a time .. example 1.8 km
_at_ 1.8 km 20 m/s from 60 deg
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Example of VAD Product in URP2
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5. Synoptic Velocity Interp.(1 of 10)

• Assume that wind field is uniform within range
  of the radar .. but that it IS generally height
  dependent
• Wind field can be characterized by a direction
  and speed, as a function of height
• Basically, this amounts to practical application
  of the VAD technique (section 4)

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5. Synoptic Velocity Interp.(2 of 10)

• Wind veering with height .. warm air advection
• Wind backing with height .. cold air advection
• Frontal surfaces ..
• Maximum of wind speed at a given height .. jet

.. how about the southern hemisphere ? ..
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5. Synoptic Velocity Interp.(2 of 10)

• TECHNIQUE
• draw line to zero Vr
• direction of V is perpendicular to the line
• speed of V obtained from point at 90 deg and same
  range

32m/s
NE
32m/s
NE
winds uniform from the NE at 30-33 m/s
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5. Synoptic Velocity Interp.(4 of 10)
32m/s
22m/s
winds from the NE .. increasing with height
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5. Synoptic Velocity Interp.(5 of 10)
43m/s
36m/s
22m/s
winds veering and increasing with height
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5. Synoptic Velocity Interp.(6 of 10)
winds veering, with a jet maximum _at_ 2 km
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constant winds backing with height
5. Synoptic Velocity Interp.(7 of 10)
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5. Synoptic Velocity Interp.(8 of 10)
low lvl sely frontal surface high level swly
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5. Synoptic Velocity Interp.(9 of 10)
diffluent winds constant with height
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5. Synoptic Velocity Interp(10 of 10)
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5. Mesoscale Velocity Interp.(1 of 16)

• Assume that wind field is locally uniform
• Due to limited size .. can more or less ignore
  effect of height variation (PPI)
• Use pattern recognition of typical weather
  entities
• mesocyclone (rotation)
• downburst (divergence)
• combinations..

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Mesocyclones

• On the order of 1 to 10 km
• Combination of convergence and pure rotation
• Occur with varying depth within storm cell as
  well as varying duration
• 90 of supercells produce severe weather
• 5 produce tornadoes used to believe 30

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Pure Rotation
Convergence and Rotation
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divergence
converging meso
rotation
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Mesocyclones

• Burgess (SELs 98) for sig tornadoes, lowest level
  PPI - convergence (below cld level) , next PPI -
  rotation
• Funnel Clouds

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Exeter 23 May 2000

• Low Topped Supercell
• F2 Tornado
• track fm 290o

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Rimbey AB18 May 992140z
Log Z
0.5 Rad Vel

• Persistent Meso ( 150 min)
• RM 90o to right
• No tornado reported
• Severe Hail

1.5 Rad Vel
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Meso Matching 0.5PPI RadVel
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Downbursts

• tend towards pure divergence
• Pulse storms, supercell RFD, FFD, microbursts

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Pure Divergence
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Downburst Sample

• Damaging winds just southwest of Lake Simcoe
  (Angus, Thornton, Fennell)

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Straight Line Winds

• Convective lines
• Squall Lines
• Bow Echoes
• Derechoes

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Outflow Boundaries

• Provides line of low level vorticity
• wind shift, meso/synoptic airmass change
• colliding boundaries
• kicker to capped environment

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Outflow Boundary

• Best viewed on lowest level PPI
• Animation vastly improves human detection
• F0 tornado 90 min later