Eta Model storm-relative winds associated with tornadic and nontornadic supercells

Abstract A conceptual model for sustained low-level mesocyclones is tested as a tornado forecast tool with observations and forecasts from the operational Eta Model. In the conceptual model, a balance between low-level storm inflow and outflow allows the development of a persistent low-level mesocyclone along the rear flank of a supercell thunderstorm, owing largely to the strength of the midlevel storm-relative winds. The present work draws on this conceptual model to identify preferred ranges of low- (model surface level), middle- (500 mb), and upper-level (250-mb) storm-relative wind speeds for 131 supercells, from gridded Eta Model fields. The observations reveal that the 500-mb storm-relative wind speed has a distinct lower bound of approximately 8 m s−1 for the tornadic supercells, while differences between surface-level and 250-mb storm-relative wind speeds for tornadic and nontornadic supercells are much less pronounced. The storm-relative wind speeds are also compared to the bulk Richardson numbe...

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