Preliminary Analysis on the Real-Time Storm-Scale Ensemble Forecasts Produced as a Part of the NOAA Hazardous Weather Testbed 2007 Spring Experiment

A real-time storm-scale WRF-ARW-based ensemble forecast system at 4-km resolution is being developed at CAPS and will be run daily for 33 hours as part of the NOAA Hazardous Weather Testbed (HWT) 2007 Spring Experiment, for a domain covering the eastern 2/3 of the continental U.S. This pilot system consists of ten hybrid perturbation members that consist of a combination of perturbed initial conditions and various microphysics and PBL physics parameterization schemes. The design considerations and the scientific questions that the system intends to address will be presented and discussed.

In addition to traditional ensemble products widely used in large-scale and mesoscale ensemble forecasting systems, such as the mean, spread, and probability of selected forecast fields, emphases are given to the generation and assessment of products specific to storm-scale, cloud-resolving ensemble forecasts. Such products include but are not limited to: probability of storm type (e.g., linear vs. cellular), large hail probability, icing potential (high super-cooled water content probability), damaging wind gusts at surface, reflectivity exceedance, updraft rotation, and supercell thunderstorm detection in the form of probability or joint probability for Supercell Composite Parameter, Significant Tornado Parameter, Supercell Detection Index, and Updraft Helicity. Many of these products are created in real time through existing capabilities in the SPC version of the N-AWIPS system for the use and evaluation by researchers and operational forecasters during the experiment. The statistical consistency of the ensemble system, in terms of spread-error relation, is assessed using the two-months of data after the experiment. The performance of the ensemble forecasts, in terms of quantitative skill scores, is compared with the NCEP operational SREF and 12 km NAM forecasts, and a CAPS 2-km WRF forecast over the same domain and period. Skill scores for sub-groups of the ensemble will be examined to assess the effectiveness of initial condition and physics perturbations.