Hodograph%20analysis

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Hodograph analysis

• James LaDue
• FMI Severe Storms Workshop
• June 2005

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Outline

• The hodograph
• Shear
• Hodograph length
• Bulk shear
• Shear curvature
• Shear orientation
• Storm relative flow
• Vorticity
• Storm Relative Helicity

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Hodograph

• Storm type is critically dependent on vertical
  wind shear and storm-relative winds
• Vertical wind shear difficult to analyze by
  visualizing wind barbs
• Best way to visualize vertical wind structure

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Hodograph
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Hodograph

• The hodograph line segments are the shear

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Hodograph

• The hodograph line segments are the shear

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Shear

• Layer Shear magnitude
• The shear magnitude is the length of each
  hodograph line segment

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Shear

• Total 0 6 km Shear magnitude
• Equals the length of the hodograph line

5
4
6
1 km
2
3
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Shear

• Complications in the meaning of hodograph length
  with complicated hodographs
• Large hodograph length but difficult to assess
  storm type.

2
6
3
5
1 km
4
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Shear

• Mean shear
• A simpler method of estimating shear magnitude
• Subtract the 6 km wind from the mean of the
  lowest 500 m

2
6
3
1
5
0 km
4
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Shear

• Shear curvature
• Has as much impact on storm behavior as shear
  magnitude

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Shear

• Shear orientation
• Does not affect supercell behavior
• May be an indication of large scale synoptic
  conditions
• Profile A indicates cold air advection and
  subsidence

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Storm-relative flow

• An observer sees the winds in this hodograph
  marked by the red vectors
• The storm sees the winds in this hodograph marked
  by the blue vectors

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Cross-wise vorticity

• Crosswise vorticity
• Vorticity vector is perpendicular to velocity
  vector
• Vertical ascent leaves vorticity outside the
  updraft.

vorticity
velocity
shear
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Cross-wise vorticity
?

• Example of storm seeing cross-wise vorticity
• Storm motion is on the hodograph
• Updraft not initially correlated with vertical
  velocity

6
0 km
Vr
C
? horizontal vorticity
Vr storm-relative velocity
C Storm motion
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Streamwise vorticity

• Vorticity vector is parallel to velocity vector
• Vertical ascent causes vertical vorticity to
  correlate with vertical velocity

vorticity
velocity
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Streamwise vorticity
?

• Example of storm seeing streamwise vorticity
• Storm motion is off the hodograph
• Updraft is immediately correlated with vertical
  vorticity

Vr
C
6
0 km
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Storm-Relative Helicity
?

• Definition
• Dot product of velocity and horizontal vorticity
• SRH ? V?? dZ
• Integrate over a vertical layer
• 0-1 km, 0-3 km

Vr
6
C
0 km
Velocity can be ground-relative or storm-relative
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Storm-Relative Helicity
?

• SRH
• Is proportional to the area swept out between the
  hodograph and C between two levels
• The 0 3 km SRH is swept out

3
Vr
6
C
0 km
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Storm-Relative Helicity applications
?

• SRH 0-3 km The 0 3 km SRH is a good indicator
  of supercell potential
• SRH 0-1 km The 0 1 km SRH is a good indicator
  of supercell tornado potential

3
Vr
6
C
0 km
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Storm Relative Helicity
?

• Limitations
• Using storm-relative velocity, SRH depends on
  storm motion
• Storm motion is difficult to forecast

6
0 km
Vr
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SRH vs shear as a supercell forecasting tool

• Shear can be used without knowing storm motion
• Once storm motion is known, use SRH to estimate
  supercell strength

6
0 km
Vr
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Summary

• We showed the creation of a hodograph
• Shear
• Storm-relative flow
• Stream-wise vs. Cross-wise vorticity
• Storm Relative Helicity