Ensemble forecast and verification of low level wind shear by the NCEP SREF system (Formerly Poster P1.64)

Model uncertainty and predictability have been concerns of the numerical weather prediction community for a long time. To deal with these issues, ensemble forecasting, a technique based on probabilistic forecasts and statistical theory, was developed in the early 1990's, and has since become the focus of much effort by various researchers, weather centers and institutes. Following the global ensemble forecast system implementation at the National Centers for Environmental Prediction (NCEP), a multiple-initial-condition- and multiple-model-based Short Range Ensemble Forecast (SREF) system was implemented operationally at NCEP in 2001. In 2002, the SREF system was extended to produce aviation weather forecasts, such as the probabilistic prediction of in-flight icing, clear-air turbulence (CAT), surface visibility, low level ceiling, low level wind shear (LLWS), jet stream, convection, etc. We have finished the product generation and are just beginning the product verification/evaluation phase.

The verification for an ensemble forecast system has three aspects: (1) system evaluation, evaluating the design quality of ensemble system; (2) deterministic verification, measuring the forecast skill, particularly the ensemble mean skill score and in comparison with a single model; (3) probabilistic verification, examining the probabilistic prediction performance of an ensemble system through measures like reliability, resolution, etc. The verification of LLWS is our initial attempt to verify/evaluate a SREF aviation ensemble product. In aviation weather, LLWS is defined as the wind vector vertical change between the ground and the 2000-foot level. LLWS > 20 knots/2000feet is the threshold where LLWS will be hazardous to an airplane's landing operation, however, the 2000-foot level is not defined in the SREF member models (Eta or RSM). In this paper, the computation method for LLWS in the SREF system is introduced, and then the verification/evaluation for all three aspects will be examined. We are making effort to add the verification of LLWS and other products to the NCEP Forecast Verification System (FVS). If this turns out to be feasible, the verification of aviation ensemble products can be performed routinely at NCEP in the future.