Discriminating between Tornadic and Non-Tornadic Supercells: A New Hodograph Technique

Thompson and Edwards (2000) first noted a prominent hodograph kink separating primarily speed shear from primarily directional shear in the environments of some supercells producing significant tornadoes. Responding to this observation, we compared similar thermodynamic and shear environments between the Moore, Oklahoma tornado of 3 May 1999 and non-tornadic supercell thunderstorms occurring in north Texas on 23 April 2003. The results suggest that certain characteristics of the kink could discriminate between tornadic and non-tornadic supercells. This combination of features consisted of a strong (> 10 m s -1 ) nearly straight-line hodograph below 500 m above ground level (AGL) and storm-relative inflow orthogonal to the base of this hodograph segment at 10 m, yielding almost purely streamwise storm-relative inflow. We evaluated this hypothesis by analyzing 67 severe convective events, 65 of which were supercells, in Oklahoma from 1997-2004, and dividing the events into non-tornadic, weak-tornadic (F0-­F1), and significant-tornadic (F2-­F5) storm classes. The results show improved discrimination between storm classes for 10-­500 m storm-relative helicity and bulk shear magnitude when compared to 10-­1000 m calculations of the same. Also, histograms of the critical angle (defined by the storm-relative inflow vector at 10 m and 10-­500 m shear vector) revealed that the tornadic storms, and in particular the significant tornadic storms, tended to be characterized by angles near 90°, whereas the non-tornadic storms were not. Although the results are based on a relatively small sample, they suggest that a careful consideration of the evolution of the low-level hodograph in both time and space in relation to the storm motion can potentially be a valuable aid in forecasting supercell tornadoes.

[1]  Daniel J. Miller.  Observations of Low Level Thermodynamic and Wind Shear Profiles on Significant Tornado Days , 2006 .

[2]  John D. Marwitz,et al.  The Structure and Motion of Severe Hailstorms. Part I: Supercell Storms , 1972 .

[3]  Morris L. Weisman,et al.  Low-Level Mesovortices within Squall Lines and Bow Echoes. Part I: Overview and Dependence on Environmental Shear , 2003 .

[4]  R. Woodruff,et al.  Confidence Intervals for Medians and Other Position Measures , 1952 .

[5]  Richard L. Thompson,et al.  Effective Storm-Relative Helicity and Bulk Shear in Supercell Thunderstorm Environments , 2007 .

[6]  Charles A. Doswell,et al.  On the Environments of Tornadic and Nontornadic Mesocyclones , 1994 .

[7]  Kevin M. McGrath,et al.  Association between NSSL Mesocyclone Detection Algorithm-Detected Vortices and Tornadoes , 2004 .

[8]  K. Browning.  Airflow and Precipitation Trajectories Within Severe Local Storms Which Travel to the Right of the Winds , 1964 .

[9]  Richard L. Thompson,et al.  Predicting Supercell Motion Using a New Hodograph Technique , 2000 .

[10]  Erik N. Rasmussen,et al.  Refined Supercell and Tornado Forecast Parameters , 2003 .

[11]  Charles A. Doswell,et al.  Some Noteworthy Aspects of the Hesston, Kansas, Tornado Family of 13 March 1990 , 1994 .

[12]  Erik N. Rasmussen,et al.  Variability of Storm-Relative Helicity during VORTEX , 1998 .

[13]  D. Dowell,et al.  Observations of Low-Level Baroclinity Generated by Anvil Shadows , 1998 .

[14]  K. A. Browning,et al.  Airflow and Structure of a Tornadic Storm , 1963 .

[15]  Richard L. Thompson,et al.  An Overview of Environmental Conditions and Forecast Implications of the 3 May 1999 Tornado Outbreak , 2000 .

[16]  Barry E. Schwartz,et al.  North American Rawinsonde Observations: Problems, Concerns, and a Call to Action , 1991 .

[17]  Robert B. Wilhelmson,et al.  Simulations of Right- and Left-Moving Storms Produced Through Storm Splitting , 1978 .

[18]  E. Rasmussen,et al.  A Baseline Climatology of Sounding-Derived Supercell and Tornado Forecast Parameters , 1998 .

[19]  Charles A. Doswell,et al.  Deaths in the 3 May 1999 Oklahoma City Tornado from a Historical Perspective , 2002 .

[20]  D. Stensrud,et al.  Operational Systems for Observing the Lower Atmosphere: Importance of Data Sampling and Archival Procedures , 1990 .

[21]  Harold E. Brooks,et al.  Tornadoes from squall lines and bow echoes. Part I: Climatological distribution , 2005 .

[22]  R. C. Miller,et al.  The Types of Airmasses in Which North American Tornadoes Form , 1954 .

[23]  Joseph B. Klemp,et al.  The Influence of the Shear-Induced Pressure Gradient on Thunderstorm Motion , 1982 .

[24]  Robert Davies-Jones,et al.  Streamwise Vorticity: The Origin of Updraft Rotation in Supercell Storms , 1984 .

[25]  Gregory J. Stumpf,et al.  A Reassessment of the Percentage of Tornadic Mesocyclones , 2005 .

[26]  Richard L. Thompson,et al.  Close Proximity Soundings within Supercell Environments Obtained from the Rapid Update Cycle , 2003 .

[27]  D. Wuertz,et al.  Comparison of Rawinsonde and Wind Profiler Radar Measurements , 1990 .

[28]  Charles A. Doswell,et al.  The Tornadoes of 3 May 1999: Event Verification in Central Oklahoma and Related Issues , 2002 .

[29]  Charles A. Doswell,et al.  A REVIEW FOR FORECASTERS ON THE APPLICATION OF HODOGRAPHS TO FORECASTING SEVERE THUNDERSTORMS , 1991 .

[30]  Harold E. Brooks,et al.  16.3 BASELINE CLIMATOLOGY OF SOUNDING DERIVED PARAMETERS ASSOCIATED WITH DEEP, MOIST CONVECTION , 2002 .

[31]  D. A. Merritt,et al.  The Precision and Relative Accuracy of Profiler Wind Measurements , 1987 .

[32]  Richard L. Thompson,et al.  Characteristics of Vertical Wind Profiles near Supercells Obtained from the Rapid Update Cycle , 2003 .