Real-Time Storm-Scale Ensemble Forecast 2008 Spring Experiment

Funded partly by NOAA CSTAR program and in its second project year, a real-time storm-scale ensemble forecasting experiment has been conducted as part of the NOAA Hazardous Weather Testbed (HWT) 2008 Spring Experiments. At 4-km horizontal grid spacing, the WRF-ARW-based ensemble system, developed at the Center for Analysis and Prediction of Storms (CAPS), runs daily for 30 hours from 14 April through 6 June, for a domain covering most of the continental United States. The ensemble system has ten hybrid perturbation members that consist of a combination of perturbed initial conditions and various microphysics and PBL physics parameterization schemes. Several major changes from the 2007 experiment are made for the 2008 season. The model domain is enlarged; Daily 30 h forecasts are initiated at 0000 UTC; Eight members are hybrid with both initial perturbations and physics variations; The most drastic change for the 2008 season is the inclusion of WSR-88D data (both reflectivity and radial wind) through ARPS 3DVAR and cloud analysis package into all but one members.

Systematically high QPF biases observed in 2007 season are found to be caused by inappropriate IC background and LBC data set used. The change to initiate ensemble forecasts at 0000 UTC using 00Z NAM analysis and forecast as IC background and LBCs for the 2008 season helps significantly bringing down the QPF biases. The inclusion of radar dada helps boost the ETS scores for the initial hours and the impact can last for up to 12 h. The analysis of statistical characteristics of the ensemble system and various QPF scores, and the comparison with the 2007 result will be presented in the conference.