On the ability of global Ensemble Prediction Systems to predict tropical cyclone track probabilities

The ability of ensemble prediction systems to predict the probability that a tropical cyclone will fall within a certain area is evaluated.  Ensemble forecasts issued by the European Centre for Medium-Range Weather Forecasts (ECMWF) and the United Kingdom Meteorological Office (UKMET) of up to 5 days were first evaluated for the 2008 Atlantic and western North Pacific seasons. In the Atlantic, the ECMWF ensemble mean was comparable in skill to a consensus of deterministic models.  Dynamic "probability circles" that contained 67% of the ECMWF ensemble captured the best track in ~67% of all cases for 24-84 hour forecasts, and were slightly underdispersive beyond 96 hours. In contrast, the Goerss Predicted Consensus Error (GPCE) was overdispersive for this sample. The addition of the UKMET ensemble yielded improvements in the short-range, and degradations for longer-range forecasts. The ECMWF ensemble performed similarly when the size was reduced from 50 to 20, and it possessed more effective degrees of freedom than an ensemble comprising different deterministic models. The 67% circles normally captured the best track during straight-line motion, but less so for sharply turning tracks. In contrast to the Atlantic, the ECMWF ensemble (and GPCE) was unable to capture sufficient verifications within the 67% probability circles in the western North Pacific, due in part to a less skilful ensemble mean (and consensus). Evaluations for the 2009 Atlantic and western North Pacific seasons will also be presented, including the construction of "probability ellipses" that account for along- and cross-track errors, based on similar principles to the new GPCE-AX method.