Probabilistic prediction of low-level vortices associated with mesoscale vortex tornadoes using EnKF data assimilation and implications for warn-on-forecast
The Advanced Regional Prediction System (ARPS) EnKF data assimilation system is used to assimilate CASA and WSR-88D radar data. The ensemble used contains 40 members, and radar data are assimilated every 5 minutes for 1 hour. Three experiments are conducted, examining the impact of CASA radar data and use of a mixed-microphysics ensemble. Results from these 3 experiments will also be compared to those of ongoing experiments assimilating mesonet, surface, and aircraft observations, and to results obtained using a 3DVAR assimilation method and an identical grid setup.
During the forecast period, the experiment using both CASA radar data and a mixed-microphysics ensemble produces the best forecast of low-level reflectivity structure and mesocyclone location. The ensemble system is successful in predicting with high probability the mesoscale circulation associated with the first of three reported tornadoes; the control experiment indicates a maximum probability of 0.65 of a significant low-level vortex being present, located within 10 km of the reported tornado and its corresponding mesoscale circulation as observed by WSR-88D. The results of these experiments suggest that both the inclusion of additional low-level radar data and the use of a mixture of microphysics schemes within an ensemble can markedly improve the quality of short-term storm-scale numerical forecasts, and that an operational EnKF-based ensemble analysis and probabilistic forecast system to support a convective-scale warn-on-forecast paradigm may soon become plausible, limited only by the availability of sufficient computational resources.