Numerical Prediction of 8 May 2003 Oklahoma City Tornadic Supercell and Embedded Tornado using ARPS with the Assimilation of WSR-88D Radar Data

The 8 May 2003 Oklahoma City tornadic supercell is predicted with the ARPS model using four nested grids with 9-km, 1-km, 100-m, and 50-m grid spacings. The Oklahoma City WSR-88D radar radial velocity and reflectivity data are assimilated through the ARPS 3DVAR and cloud analysis on the 1-km grid to generate an initial condition that includes a well-analyzed supercell and associated low-level mesocyclone. Additional 1-km experiments show that the use of radial velocity and the proper use of a divergence constraint in the 3DVAR play an important role in the establishment of the low-level mesocyclone during the assimilation and forecast. Assimilating reflectivity data alone failed to predict the mesocyclone intensification. The 100-m grid starts from the interpolated 1-km control initial condition while the further nested 50-m grid starts from the 20 minute forecast on the 100-m grid. The forecasts on both grids cover the entire period of observed tornado outbreak, and successfully capture the development of tornadic vortices. A tornado on the 50-m grid reaches high-end F-3 intensity while the corresponding simulated tornado on the 100-m grid reaches F-2 intensity. The timing of tornadogenesis on both grids agrees with observations very well, although the predicted tornado was slightly weaker and somewhat shorter lived. The predicted tornado track parallels the observed damage track although is displaced northward by about 8 km. The predicted tornado vortices have realistic structures similar to those documented in previous theoretical, idealized modeling and some observational studies. The prediction of an observed tornado in a supercell with a similar degree of realism has not been achieved before. 1. Introduction For the goal of increasing the lead time of warnings issued on severe weather hazards, and in particular on tornadoes, Stensrud et al. (2009; 2013) discuss the necessity of shifting from warn-on-detection to the warn-on-forecast paradigm where advanced warnings are issued based on high-resolution numerical prediction of severe convective storms and associated severe weather rather than relying on detections in current observations and extrapolation-based nowcasting. Although the average warning lead time for tornadoes has increased from 3 min in 1978 to 14 min in 2011 in the United States, this improvement has been mostly due to improved detection capability, especially from the deployment of the operational Doppler radar network. Despite the progresses, the average lead time for all tornadoes with positive warning lead time was stagnant at around 20 min during the period of 1986-2006 (Stensrud et al. …