Symposium on the Challenges of Severe Convective Storms


A numerical investigation of the synoptic environment associated with tornadic and nontornadic severe weather outbreaks

Hamish A. Ramsay, University of Oklahoma, Norman, OK; and C. A. Doswell and L. M. Leslie

Synoptic-scale signals typically play an elusive role in discriminating between ‘tornado outbreaks' and severe thunderstorm days without substantial tornadic activity. A series of numerical experiments is designed to examine the extent to which tornado outbreaks may be attributable to processes on the synoptic-scale, rather than the mesoscale and smaller. These numerical simulations use smoothed 'synoptic-scale' initial conditions, with no initial mesoscale information, for predictions ranging from one to three days. The MM5 and HIRES NWP models used here are initialized using the 2.5° by 2.5° NCEP/NCAR reanalysis dataset. Both models are multi-nested, with a final grid spacing of 1km. A range of meteorological covariates including CAPE, low-level wind shear, storm-relative helicity, and relative vertical vorticity, are used as proxy variables to diagnose the occurrence of tornadic supercells. The covariates are necessary, as the most sophisticated mesoscale models are presently incapable of explicitly predicting tornadoes. Our preliminary case studies suggest that synoptic-scale signals cannot be ruled out in discriminating between tornado outbreaks, and days with less substantial tornadic activity. Two particular cases, the May 3rd 1999 Oklahoma / Kansas tornado outbreak and the 26 April 1991 ‘Andover' tornado outbreak, reveal values for several meteorological parameters and covariates often associated with tornado outbreaks. We will investigate the spatial and temporal correlations between the simulated fields associated with tornado outbreak cases and cases involving primarily non-tornadic severe weather, using a family of composites developed from EOFs, which filter the data such that only the dominant synoptic-scale modes are retained.

Poster Session 1, The Observation, Modeling, Theory, and Prediction of Severe Convective Storms and Their Attendant Hazards
Wednesday, 1 February 2006, 2:30 PM-4:00 PM, Exhibit Hall A2

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