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Hodograph analysis

Hodograph analysis . James LaDue FMI Severe Storms Workshop June 2005. Outline. The hodograph Shear Hodograph length Bulk shear Shear curvature Shear orientation Storm relative flow Vorticity Storm Relative Helicity. Hodograph.

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Hodograph analysis

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  1. Hodograph analysis James LaDue FMI Severe Storms Workshop June 2005

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

  3. 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

  4. Hodograph

  5. Hodograph • The hodograph line segments are the shear

  6. Hodograph • The hodograph line segments are the shear

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

  8. 3 km 2 km 1 km 4 km 5 km 6 km Shear • Total 0 – 6 km Shear magnitude • Equals the length of the hodograph line 5 4 6 3 1 km 2

  9. 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

  10. 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

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

  12. 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

  13. 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

  14. Cross-wise vorticity • Crosswise vorticity • Vorticity vector is perpendicular to velocity vector • Vertical ascent leaves vorticity outside the updraft. vorticity velocity shear

  15. Cross-wise vorticity  • Example of storm seeing cross-wise vorticity • Storm motion is on the hodograph • Updraft not initially correlated with vertical velocity 0 km 6 Vr C = horizontal vorticity Vr = storm-relative velocity C = Storm motion

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

  17. 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

  18. 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

  19. 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

  20. 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

  21. C Storm Relative Helicity  • Limitations • Using storm-relative velocity, SRH depends on storm motion • Storm motion is difficult to forecast 0 km 6 Vr

  22. C 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 0 km 6 Vr

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

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