3DVAR analysis and forecast experiments for the 27 April 2011 tornado outbreak

In this study, we will present preliminary analysis and forecast results from assimilating WSR-88D observations of reflectivity and radial velocity for the 27 April 2011 tornado outbreak case using the Advanced Regional Prediction System (ARPS) 3DVAR as the data assimilation tool and Weather Research and Forecasting (WRF) model as the forecast tool. The 27 April 2011 tornado outbreak was the costliest and one of the deadliest tornado outbreaks in history. The outbreak produced destructive tornadoes in Alabama, Arkansas, Georgia, Mississippi, Tennessee, Virginia, and extending into the northeastern United States. Four of the tornadoes were rated EF5, which is the highest possible ranking, and 238 tornado-related deaths in Alabama alone were reported. For this particular case, the ARPS 3DVAR with a 200km x 200km domain size was running in near real-time during the tornado outbreak. Several supercells with very strong mesocyclones were automatically detected from a floating domain analysis whose location was determined either using an automatic domain position system or from a user specified domain center point. After the outbreak, we also created a series of offline analyses several hours long using a bigger domain of 1200km x 1000km with 5-minute analysis intervals and assimilating data from nearly 30 WSR-88D radars. The storm tracks derived from these analyses can be used to explore the value of the 3DVAR to identify supercells and to verify the accuracy of the NWP forecasts produced using the WRF model. Preliminary forecast experiments with the WRF model using initial conditions produced by ARPS 3DVAR analyses will be examined. Different from the above analyses, a cycling approach in which the ARPS model is used to advance the 3DVAR analysis forward in time between the available observation times will be used to prepare the initial condition for the WRF model. The cycling period will cover a 30-minute time window beginning at 1830 UTC and ending at 1900 UTC 27 April 2011. The WRF forecast will be initialized at 1900 UTC and integrated for 5 hours, encompassing the primary time period of the tornado outbreak. The forecast supercell tracks will be computed from the forecast fields and compared to the supercell tracks derived from the above analyses.