Ensemble prediction of a severe weather event: a study of the 2009 Southern Ontario storm

The tornadic storm of August 20, 2009 of Southern Ontario is studied using a numerical prediction model. It is found that a 3km resolution simulation works as well as a 1km resolution model to model the storms underlying physical processes relevant to supercell mesocyclone formation and storm propagation, although both models showed a significant phase bias in the storm system's squall line position. A 3m resolution ensemble of 20 members is used to model the storm system further, and it is found that the ensemble mean shows the same bias that the 1km and 3km resolution models exhibited. Investigation of ensemble perturbation growth rates from ensemble mean values reveals differing growth rates for baroclinic and convective modes. Ensemble-based sensitivity analyses reveal that there are strong correlations of squall line position with model variables up to 12 hours previously.