Transcript Document
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
• • • 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
Hodograph
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
-20 -15 -10 40 35 30 25 20 15 10 5 -5 0 0
V m w
5
V
obs 10 -5 -10 -15 -20
u (m s -1 )
15 20 25 30 35 40
Hodograph
• The hodograph line segments are the shear
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
-20 -15 -10 40 35 30 25 20 15 10 5 -5 0 0
V m w
5
V
obs 10 -5 -10 -15 -20
u (m s -1 )
15 20 25 30 35 40
Hodograph
• The hodograph line segments are the shear
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
-20 -15 -10 40 35 30 25 20 15 10 5 -5 0 0
V m w
5
V
obs 10 -5 -10 -15 -20
u (m s -1 )
15 20 25 30 35 40
• • Layer Shear magnitude The shear magnitude is the length of each hodograph line segment
Shear
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
-20 -15 -10 40 35 30 25 20 15 10 5 -5 0 0
V m w
5
V
obs 10 -5 -10 -15 -20
u (m s -1 )
15 20 25 30 35 40
Shear
• Total 0 – 6 km Shear magnitude – Equals the length of the hodograph line
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
40 35 30 25 20 15 1 km 10 2 km 5 -20 -15 1 km -10 2 -5 0 -5 0
V m w
5 3
V
obs 10 3 km 15 4 20 -10 -15 -20
u (m s -1 )
4 km 30 5 km 6 km 6 35 40
Shear
• Complications in the meaning of hodograph length with complicated hodographs – Large hodograph length but difficult to assess storm type.
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
40 35 30 25 -20 20
2 6
15
3
10 -15 5 1 km -10 -5 -5 0 0
V 4 m w
5
V
obs 10
5
15 -10 -15 -20
u (m s -1 )
20 25 30 35 40
Shear
• Mean shear – A simpler method of estimating shear magnitude – Subtract the 6 km wind from the mean of the lowest 500 m
XYZ 0000 UTC 6/15/2002 My Favorite 0-10 km Hodograph
40 35 30 25 -20 20
2 6
15
3 1
10 -15
0
km -10 5 -5 -5 0 0
V 4 m w
5
V
obs 10
5
15 -10 -15 -20
u (m s -1 )
20 25 30 35 40
• Shear curvature – Has as much impact on storm behavior as shear magnitude
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
Shear
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
Cross-wise vorticity
• Crosswise vorticity – Vorticity vector is perpendicular to velocity vector – Vertical ascent leaves vorticity outside the updraft.
vorticity
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 V r
C
= horizontal vorticity V r = storm-relative velocity
C = Storm motion 6
Streamwise vorticity
– Vorticity vector is parallel to velocity vector – Vertical ascent causes vertical vorticity to correlate with vertical velocity vorticity velocity
Streamwise vorticity
• Example of storm seeing streamwise vorticity – Storm motion is off the hodograph – Updraft is immediately correlated with vertical vorticity
0
km V r
C 6
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 V r
C 0
km Velocity can be ground relative or storm-relative
6
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
0
km
3
V r
C 6
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
0
km
3
V r
C 6
Storm Relative Helicity
• Limitations – Using storm-relative velocity, SRH depends on storm motion – Storm motion is difficult to forecast
0
km V r
C 6
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 V r
C 6
Summary
• • • • • We showed the creation of a hodograph Shear Storm-relative flow Stream-wise vs. Cross-wise vorticity Storm Relative Helicity