Stochastic forcing , ensemble prediction systems , and TIGGE

Many operational NWP centres now produce global medium-range (≤ 14 day) and higher-resolution, limited-area, shorter-range (≤ 3 day) ensemble forecasts. These provide probabilistic guidance and early warning of the likelihood of high-impact weather. There are two main challenges in the design of ensemble prediction systems: (1) properly simulating the initial condition uncertainty, including the definition of the initial ocean, land, and sea-ice states, and (2) properly simulating the uncertainty due to inadequate representations of physical processes, especially parameterizations. Post-processing the output from the ensemble prediction systems using past forecasts and observations/analyses can dramatically reduce systematic errors in forecast products and improve skill and reliability. The generation of products from multi-model ensembles (facilitated by the TIGGE database, sharing global operational ensemble forecasts) has also been shown to frequently improve the skill and reliability of ensemble predictions.