Matthew J. Bunkers WFO Rapid City, SD Last Updated 2/4/2002 Predicting Supercell Motion Using...
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Transcript of Matthew J. Bunkers WFO Rapid City, SD Last Updated 2/4/2002 Predicting Supercell Motion Using...
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
R
L
0
10
20
30
40
0 10 20 30 40
Upper-Right Quadrant
6 kmMSFC
L
R
-40
-30
-20
-10
0
0 10 20 30 40
Lower-Right Quadrant
SFC 6 kmM
R
L
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-40 -30 -20 -10 0
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
8 km
-15
-5
5
15
-5 5 15 25
SFC
8 km
VMean
-15
-5
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-5 5 15 25
SFC
8 km
VMean
-15
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-5 5 15 25
SFC
8 km
VMean
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SFC
8 km
VMean
VRM
VLM
-15
-5
5
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-5 5 15 25
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!
RecommendationsRecommendations