88th Annual Meeting (20-24 January 2008)

Thursday, 24 January 2008: 9:15 AM
Forecast error dynamics in a coupled ocean-atmosphere prediction system
219 (Ernest N. Morial Convention Center)
Malaquias Pena, SAIC and EMC/NCEP/NOAA, Camp Springs, MD; and Z. Toth
Errors in retrospective ensemble forecasts of Nino 3.4 index from the NCEP Climate Forecast System (CFS) are analyzed. The forecast data used comprise 23 years of 15-members ensembles issued once a month. These ensembles are formed by collecting forecasts initialized within the previous month (referred to as “lagged” ensemble generation scheme). By design, the initial error of the ensemble mean in a given time is usually larger than if all members were initialized simultaneously. The study quantifies the skill loss in the lagged approach by partitioning forecast errors into initial-related errors and model-related errors. Results indicate a skill loss of about 10 days; about 5 days if ensembles are generated using the latest 8 members instead. Implications of these results when designing a sub-seasonal prediction system will be discussed.

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