Impact of Radar Data Assimilation on the Analysis and Prediction of the 8-9 May 2007 Oklahoma Tornadic Mesoscale Convective System, Part II: Sub-storm-scale Mesovortices on a 400 m Grid

In this two-part paper, the impact of assimilating data from the WSR-88D and the Engineering Research Center for Collaborative Adaptive Sensing of the Atmosphere's (CASA) IP-1 radar network on the prediction of a tornadic mesoscale convective system is examined. The Advanced Regional Prediction System (ARPS) prediction model is employed to perform high-resolution numerical simulations of a mesoscale convective system (MCS) and the associated cyclonic line-end vortex (LEV) that spawned several tornadoes in central Oklahoma on 8-9 May 2007. In part I of this study, the assimilation procedure, and the results of analysis and prediction of the MCS and LEV scales on a 2-km-resolution grid were discussed. In this part II, nested grid simulations (400-m resolution grid within the 2-km grid) are used to examine the impact of radar data on the prediction of meso-γ-scale vortices (mesovortices). An 80-min assimilation window is used in experiments that examine the impact of radar data on this high-resolution grid. A second set of experiments examines the impact of decreasing assimilation window length on the prediction of these mesovortices. Results show that highly accurate forecasts of mesovortices up to 80 min in advance of their genesis are possible when the low-level wind and temperatures fields are effectively analyzed. Accurate analysis of low-level wind and temperature fields relies on assimilating high-resolution low-level wind information. The most accurate analysis (and resulting prediction) is obtained in experiments that assimilate low-level radial velocity data from the CASA radars. Experiments that do not assimilate low-level wind data are unable to resolve the gust front structure, precluding accurate prediction of mesovortex development. It is found that a fairly accurate prediction is still possible with an assimilation window of as short as 20 min.