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

2. OutlineOutline The hodographThe hodograph ShearShear –Hodograph length –Bulk shear –Shear curvature –Shear orientation Storm relative flowStorm relative flow VorticityVorticity Storm Relative HelicityStorm Relative Helicity

3. HodographHodograph Storm type is critically dependent on vertical wind shear and storm- relative windsStorm type is critically dependent on vertical wind shear and storm- relative winds Vertical wind shear difficult to analyze by visualizing wind barbsVertical wind shear difficult to analyze by visualizing wind barbs Best way to visualize vertical wind structureBest way to visualize vertical wind structure

4. HodographHodograph

5. HodographHodograph The hodograph line segments are the shearThe hodograph line segments are the shear

6. HodographHodograph

7. ShearShear Layer Shear magnitudeLayer Shear magnitude The shear magnitude is the length of each hodograph line segmentThe shear magnitude is the length of each hodograph line segment

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

9. ShearShear Complications in the meaning of hodograph length with complicated hodographsComplications in the meaning of hodograph length with complicated hodographs –Large hodograph length but difficult to assess storm type. 1 km 2 3 4 5 6

10. ShearShear Mean shearMean shear –A simpler method of estimating shear magnitude –Subtract the 6 km wind from the mean of the lowest 500 m 1 2 3 4 5 6 0 km

11. ShearShear Shear curvatureShear curvature –Has as much impact on storm behavior as shear magnitude

12. ShearShear Shear orientationShear 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 vectorsAn observer sees the winds in this hodograph marked by the red vectors The storm sees the winds in this hodograph marked by the blue vectorsThe storm sees the winds in this hodograph marked by the blue vectors

14. Cross-wise vorticity Crosswise vorticityCrosswise 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 vorticityExample of storm seeing cross-wise vorticity –Storm motion is on the hodograph –Updraft not initially correlated with vertical velocity 60 km  VrVr  = horizontal vorticity V r = storm-relative velocity C = Storm motion C

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 vorticityExample of storm seeing streamwise vorticity –Storm motion is off the hodograph –Updraft is immediately correlated with vertical vorticity 6 0 km  VrVr C

18. Storm-Relative Helicity DefinitionDefinition –Dot product of velocity and horizontal vorticity –SRH =  V  dZ –Integrate over a vertical layer –0-1 km, 0-3 km 6 0 km  VrVr C Velocity can be ground- relative or storm-relative

19. Storm-Relative Helicity SRHSRH –Is proportional to the area swept out between the hodograph and C between two levels –The 0 – 3 km SRH is swept out 6 0 km  VrVr C 3

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 6 0 km  VrVr C 3

21. Storm Relative Helicity LimitationsLimitations –Using storm-relative velocity, SRH depends on storm motion –Storm motion is difficult to forecast 60 km  VrVr C

22. SRH vs shear as a supercell forecasting tool Shear can be used without knowing storm motionShear can be used without knowing storm motion Once storm motion is known, use SRH to estimate supercell strengthOnce storm motion is known, use SRH to estimate supercell strength 60 km VrVr C

23. SummarySummary We showed the creation of a hodographWe showed the creation of a hodograph ShearShear Storm-relative flowStorm-relative flow Stream-wise vs. Cross-wise vorticityStream-wise vs. Cross-wise vorticity Storm Relative HelicityStorm Relative Helicity