Matthew J. Bunkers WFO Rapid City, SD Last Updated 2/4/2002 Predicting Supercell Motion 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: Predicting Supercell Motion Using A New Hodograph Technique

 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 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 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 for this 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 (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  1 st slide: cyclonic supercell moves slower and to the right of the mean wind (typical)  2 nd slide: cyclonic supercell moves faster and to the right of the mean wind (northwest flow)  3 rd slide: cyclonic supercell moves slower and to the left of the mean wind (rare) Importance of Galilean Invariance

Upper-Right Quadrant

Lower-Right Quadrant

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 Prediction 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 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 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 Method

Equation for the Cyclonic 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 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 (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  km to km mean shear vector Optimizing the ID Method

Results (260 hodographs): *ID Method compared individually to others

Typical Hodograph:

Results (148 Typical Hodographs): *ID Method compared individually to others

Atypical Hodograph:

Results (77 Atypical Hodographs): *ID Method compared individually to others

Australian Hodograph:

Importance 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 Relationship

Supercell-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) Highly Deviant Supercells, 20 th SLS 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) Highly Deviant Supercells, 20 th SLS 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) Highly Deviant Supercells, 20 th SLS 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!Recommendations