Seasonal Forecasting System 3

• Description of System 3
• Re-forecasts and pre-operational data
• Seasonal forecast dissemination products
• Seasonal Forecasting System 3 dissemination requirements
• Seasonal forecast monthly means and monthly mean anomaly dissemination products
• GRIB section 1 definition for Seasonal Forecast System 3 data
• Transmission of Seasonal Forecasting System products

Please note that Seasonal Forecast System 3 is at this stage still in development. The description of the System 3 can be found on http://www.ecmwf.int/products/changes/system3.

A seasonal forecasting system has been run at ECMWF since 1997. Initially it was run as an experimental system, with one ensemble member run each day, and ensemble forecast products created each month using one month's worth of ensemble members. Model resolution was T63, with 31 levels.

Output from the system was initially only available in the form of graphical products made available via the INTERNET. The forecast data were later restructured and re-archived in MARS, under the designation SYSTEM=1.

A substantially revised seasonal forecasting system was introduced in January 2002. The ensemble changed to use 40 members, all run from the same initial date, namely the 1st day of each calendar month. Model resolution was TL95, with 40 levels. An extensive range of forecast data are archived in MARS as SYSTEM=2, and it was from this system that dissemination of products first became available in May 2004.

The seasonal forecasts are made using coupled ocean-atmosphere model integrations. Because of the long lead time, model errors are significant and must be corrected for. This is done by using a set of 'back integrations', that is forecasts for previous dates, to estimate model systematic error. In the case of the 2002 system (SYSTEM=2), these back integrations cover the period 1987-2001. Interpretation of a real time forecast requires the data from these back integrations, unless the fields are obtained as forecast anomalies, in which case the comparison with the back integrations has already been made. Documentation of the seasonal forecast system is available on the seasonal forecast section of the ECMWF web-site (see http://www.ecmwf.int/products/forecasts/seasonal/documentation/index.html).

The seasonal forecast system GRIB products have to distinguish between forecasts made with different systems for the same date, as well as between different ensemble members. Use is made of ECMWF GRIB local extensions - see Meteorological Bulletin 1.9/3 for the definitions. By convention, ECMWF always uses octet 41 to define which set of local definitions follow.