Evaluation of the NSSL Mesoscale Ensemble during the High-Impact Severe Weather Events of May 2013

The NSSL Mesoscale Ensemble (NME) was developed as a means to provide realistic mesoscale backgrounds for downscaling to high-resolution (grid spacing of order 1-km or less) radar data assimilation experiments, as part of the NOAA Warn-on-Forecast (Stensrud et al. 2009) project. A variety of three-dimensional model fields from the NME were examined by participants during the 2013 NOAA Hazardous Weather Testbed Spring Forecast Experiment to assess the ability of the NME to reproduce convective environments on severe weather days.

The NME is a 36-member, ensemble-based data assimilation system using the Weather Research and Forecasting (WRF) model that was run daily during the period 06 May – 10 June 2013 over a CONUS domain with a horizontal grid spacing of 18 km. Initial and boundary conditions were provided by the Earth System Research Laboratory-Rapid Refresh (ESRL-RAP, i.e. RAPv2) 1200 UTC forecast cycle. Uncertainties in the ESRL-RAP analysis were accounted for through random perturbations applied to the initial and boundary conditions using the fixed-covariance perturbation method of Torn et al. 2006. The WRF physics options were also varied amongst the ensemble members to address deficiencies in the physical parameterizations schemes. Routinely available observations of altimeter setting, temperature, dewpoint, and horizontal wind components from land and marine surface stations, rawinsondes, and aircraft – as well as satellite winds – were assimilated using an ensemble Kalman filter (DART software) at hourly intervals from 1300 UTC to 0300 UTC the following day.

The evaluation of NME behavior will focus on the period 15-31 May 2013 due to the occurrence of several high-impact severe weather events during this period, including the Moore and El Reno, OK EF5 tornadoes. NME ensemble-mean 1-h forecasts and analyses of commonly used severe weather parameters (i.e. CAPE, 0-6 km shear) will be compared to the same fields from the ESRL-RAP in order to benchmark the performance of the NME during intense convective episodes.