Matthew J. Bunkers

WFO Rapid City, SD

Last Updated 2/4/2002

Predicting SupercellMotion Using Hodograph

Techniques

Matthew J. Bunkers, UNR

Brian A. Klimowski, UNR

Jon W. Zeitler, HGX

Richard L. Thompson, SPC

Morris L. Weisman, NCAR

by

Based on:Based on:

Predicting Supercell Motion Predicting Supercell Motion Using A New Hodograph Using A New Hodograph

TechniqueTechnique

Develop a dynamically based method that consistently predicts the motion of both right- and left-moving supercells (using only a hodograph)

Compare the new method with existing methods of predicting supercell motion

Recommend a preferred method for predicting supercell motion

Objectives of StudyObjectives of Study

All supercells move to the right of the mean wind (not true—can move to the left of the mean wind!)

If a storm is moving to the right of the mean wind, it is a supercell (not true—could just be a multicell storm)

Supercell Motion MythsSupercell Motion Myths

Some currently used methods fail under certain situations (because they are not Galilean invariant)

Most supercells (> 90%) produce severe weather (i.e., hail, flooding, winds, tornadoes)

Nearly all strong or violent tornadoes are produced by supercells

Justification forJustification forthis Studythis Study

Supercell motion is needed to evaluate storm-relative helicity—helping to discern tornadic potential

Anvil-level storm-relative flow may be important in distinguishing among HP, CL, and LP supercells

Most methods do not address the motion of left-moving supercells

Justification Justification (Continued)(Continued)

Next, idealized hodographs are used to illustrate how Galilean invariance applies to predicting supercell motion; methods based on the mean wind are not Galilean invariant

1st slide: cyclonic supercell moves slower and to the right of the mean wind (typical)

2nd slide: cyclonic supercell moves faster and to the right of the mean wind (northwest flow)

3rd slide: cyclonic supercell moves slower and to the left of the mean wind (rare)

Importance ofImportance of Galilean Invariance Galilean Invariance

SFC 6 kmM

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Upper-Right Quadrant

6 kmMSFC

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Lower-Right Quadrant

SFC 6 kmM

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Upper-Left Quadrant

Maddox (1976)…30R75 Colquhoun (1980)…inflow == outflow Davies and Johns (1993)…30R75 and 20R85—

the “JDL” method Weisman (1996)…COMET Program module Davies (1998)…modification of DJ93 above Rasmussen and Blanchard (1998)…offset from 0-

4 km AGL shear Bunkers et al. (1998, 2000)…this study

Supercell Motion Supercell Motion Prediction MethodsPrediction Methods

A modification of Weisman (1996) and Weisman and Klemp (1986)

Based on the internal dynamics of the supercell—called the ID method

Galilean invariant and shear-relative

Observationally, dynamically, and theoretically based on studies from the 1940s to present (consistent pattern to supercell motion)

Our MethodOur Method

Uses the following physical concepts:

Advection of the storm by the mean wind

Interaction of the convective updraft with the sheared environment to promote rotation and propagation

Other external factors, including atmospheric boundaries and orography, are not accounted for

The ID MethodThe ID Method

Following is a graphical depiction

Plot the hodograph

Plot the mean wind

Draw the vertical wind shear

Draw a line perpendicular to the vertical wind shear that passes through the mean wind

Locate storm motion

The ID MethodThe ID Method

SFC

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SFC

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VMean

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SFC

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SFC

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V V D *

V kVRM m eanshear

shear

Equation for theEquation for theCyclonic SupercellCyclonic Supercell

260 right-moving (cyclonic) supercells

30 left-moving (anticyclonic) supercells

Data gathered from previous studies and the northern High Plains—primary sources include: Davies and Johns (1993) Brown (1993) Thompson (1998)

Data Used in StudyData Used in Study

Most data gathered from ± 3 hours from 0000 UTC using radiosondes

Some cases utilized WSR-88D, profilers, and averaged soundings

Atypical hodographs were defined as those with:

0-6 km AGL mean wind < 10 m/s, or

a surface wind with a northerly component and > 5 m/s

Data Used Data Used (Continued)(Continued)

Several iterations were performed to minimize the error in predicting supercell motion, with the final results being:

0-6 km AGL non-pressure weighted mean wind

7.5 m/s deviation from the mean wind

0-0.5 km to 5.5-6 km mean shear vector

Optimizing the ID MethodOptimizing the ID Method

Results (260 hodographs):Results (260 hodographs):

Mean Error(m/s)

BetterPredicted*

ID 4.1 65 to 77%

RB98 5.1 29%

D98 5.3 33%

DJ93 5.1 35%

C80 6.7 23%

M76 5.2 35%

*ID Method compared individually to others*ID Method compared individually to others

6 KM

SFC

VRM

V20R85

V0-6km

V30R75

Vobs

VRB98

-5 0 5 10 15 20 25 30

Typical Hodograph:Typical Hodograph:

Results (148 Typical Results (148 Typical Hodographs):Hodographs):

Mean Error(m/s)

BetterPredicted*

ID 4.3 56 to 73%

RB98 5.1 31%

D98 5.4 35%

DJ93 4.6 44%

C80 6.6 27%

M76 4.9 44%

*ID Method compared individually to others*ID Method compared individually to others

6 KM

SFC

VRM

V20R85

V0-6km

V30R75

Vobs

VRB98

-15 -10 -5 0 5 10

Atypical Hodograph:Atypical Hodograph:

Results (77 Atypical Results (77 Atypical Hodographs):Hodographs):

Mean Error(m/s)

BetterPredicted*

SRHError

SRHBias

ID 3.8 70 to 84% 31 -5

RB98 5.0 25% 38 11

D98 5.6 30% 41 -4

DJ93 5.9 18% 59 -57

C80 6.8 16% 77 -74

M76 6.0 18% 59 -56

*ID Method compared individually to others*ID Method compared individually to others

6 KM

SFC

VLM

V0-6km

Vobs

-10 -5 0 5 10 15 20 25

Australian Hodograph:Australian Hodograph:

Importance of Storm MotionImportance of Storm Motion Research and operational studies have

focused on Storm Relative Helicity (SRH) as a measure of supercell rotation and tornadic potential

To determine SRH, Storm Motion must be known, or estimated (by definition)

Following are some examples illustrating the variability of SRH, and the stability of the 0-6–km vertical wind shear

Supercell-Helicity RelationshipSupercell-Helicity Relationship

Supercell-Shear RelationshipSupercell-Shear Relationship

The ID method, which is based on the theory for supercell propagation, is superior to the other proposed methods evaluated for all hodographs in this study (by ~ 1 m/s)

This method offers even more improvement in anticipating supercell motion and storm-relative parameters for atypical hodographs

Summary

The ID method allows for the prediction of left-moving supercells (unlike most other methods)

When the 0–6-km vertical wind shear exceeds 30 m/s, supercells become more likely (assuming convective initiation)

The Eta model changed on April 21, 2000 to use the Bunkers et al. (2000) method for supercell motion input to SRH calculations

Summary (continued)

Cold pool/shear interactions (internal) storm acceleration with time

Boundaries, merging storms (external) e.g, drylines, fronts, outflows

Orographic influences (external) Deeper or shallower storms (internal)

e.g, mini-supercells, supercells over higher terrain, elevated supercells

Complications in Predicting Storm Motion

If the shear is confined to the low levels, the supercell may become outflow-dominated stronger gust-front lifting; less ventilation aloft

If the shear is marginal and the CAPE is large, erratic movement may occur watch for boundaries/convergence zones new cell growth can dominate storm motion

Complications in Predicting Storm Motion (continued)

If the shear is exceptionally large, significant deviations from the mean wind may occur

Complications in Predicting Storm Motion

Bunkers and Zeitler (2000)Bunkers and Zeitler (2000)Highly Deviant Supercells, 20Highly Deviant Supercells, 20ththSLSSLS

• Even the ID Method fails to accurately predict the motion of some supercells (i.e., error > 5 m/s)

• A number of factors could account for these “highly deviant” supercells* unrepresentative wind profile* inappropriate mean wind layer* exceptionally strong vertical wind shear* weak mid-level vertical wind shear

Bunkers and Zeitler (2000)Bunkers and Zeitler (2000)Highly Deviant Supercells, 20Highly Deviant Supercells, 20ththSLSSLS

• Focused on exceptionally strong vertical wind shear and weak mid-level vertical wind shear

• Expanded the dataset to 339 cases

• 245 (72%) predictions had a mean absolute error of 2.7 m/s (Dataset #1)

• 94 (28%) predictions had a mean absolute error of 7.3 m/s (Dataset #2)

Bunkers and Zeitler (2000)Bunkers and Zeitler (2000)Highly Deviant Supercells, 20Highly Deviant Supercells, 20ththSLSSLS

• Dataset #2 was split into 3 partitions:1)Weak 0–8-km vertical wind shear

• Stronger gust front lifting (outflow dominated)

2)Strong 0–8-km vertical wind shear• Updraft–shear interactions more important

(supercell processes dominated)

3)Strong 0–3-km shear/Weak 4–8-km shear• Combination of gust front lifting and updraft–

shear interactions

Use the ID method as a starting point to predict supercell motion

Determine if a shallower or deeper mean wind than 0-6 km is warranted

Identify boundaries and orography that may influence supercell motion

Understand that the supercell motion will change with time

Examine the distribution of the vertical wind shear

Be aware of your environment!

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