Evaluation and Interpretation of the Supercell Composite and Significant Tornado Parameters at the Storm Prediction Center
Richard L. Thompson, NOAA/NSSL/SPC, Norman, OK; and R. Edwards and J. A. Hart
A sample of 458 close proximity supercell soundings, derived from RUC-2 model analyses, is used to examine the utility of composite parameters in diagnosing environments conducive to the formation of supercells and significant tornadoes, respectively. Our investigation begins with the "supercell composite parameter" (SCP) which consists of three components: CAPE, storm-relative helicity (SRH), and the Bulk Richardson Number denominator (BRN shear). Each component is normalized to "threshold" values for supercells, per these published studies, and the product of the normalized components results in the SCP. Our proximity soundings were separated into significant tornadic (F2+ damage), weak tornadic (F0-F1 damage), and nontornadic groups, and the SCP was computed for each sounding via the SHARP sounding analysis software. Based on our sample of RUC-2 model close proximity soundings, an SCP value of 1 appears to be a good discriminator between supercells and a sample of discrete non-supercell storms.
The SCP concept is then extended to include measures of low-level wind shear and moisture, which are all combined in the "significant tornado parameter" (STP). Our analyses supports that of several recent studies which suggest that 0-1 km storm-relative helicity and LCL height are two important factors in discriminating between environments supportive of nontornadic supercells, and those environments that favor significant tornadoes. As with the SCP, the components of the STP [0-6 km shear magnitude, 0-1 km storm-relative helicity, lifted condensation level (LCL) height, and CAPE] are each normalized to reasonable "threshold" values for significant tornadic supercells. The samples of significant tornadic and nontornadic supercells were offset by roughly two quartiles through the middle 80% of each distribution, and an STP value of 1 serves as an apparent "threshold" between the two samples of supercells. This formulation of the STP has been applied to hourly RUC-2 model grid analyses for several severe storm cases, as part of a larger objective analysis scheme at the Storm Prediction Center, and it appears to have value in identifying environments favorable for significant supercell tornadoes.
Extended Abstract (172K)
Joint Session 3, Severe Local Storms Forecasting (Joint with 21SLS and 19WAF/15NWP)
Tuesday, 13 August 2002, 8:00 AM-9:00 AM
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