Individual forecast ensemble

The design and configuration of each ensemble forecast system depend on specific requirements, priority, and resources in each contributing centre. The full description of each system configuration is available here. For example, each system comes with a set of reforecasts covering different periods and with different ensemble sizes. This variety of system configuration presents a big challenge.

To create consistent individual forecast products and multi-model products we need to use model climate estimates based on a set of reforecasts covering a fixed period of 10 to 20 years common to all the models. Unfortunately, reforecasts for a common and sufficiently long set of years is not always available. As a compromise we use the longest common period currently available covering 1999-2010. However, for some systems, we are forced to use even a shorter climate period. This approach might be revisited in due course. (see Table).

Individual and multi-model combination are constructed using the forecast initiated around each Thursday. For systems with ensemble size smaller than 8 members we consider forecast initiated 1 or 2 days earlier depending on the system configuration. Although not ideal, for Bejing system we combine the forecast of Thursday and previous Monday. It is worth noting that for Bejing system only 3 years of common reforecast period are available (see Table). The method used to create the forecast for Bejing is going to be revised in due course and a different climate period will be used. The entries in parenthesis indicate that, for some systems, the method to sample the model climate has been recently revised.

It is important to note that for the systems with reforecast produced on the fly and covering just the most recent years, the use of a fixed common period is not always possible. This is the case of Bejing system which (at the time of writing) has only 3 years of reforecast for the common period 1999-2010 (see here). Because of the above, the Beijing forecast is defined as the departure from the model climate estimated by the whole reforecast set covering the most recent 15 years.

The table below shows, for each model configuration, the forecast and re-forecast data used to construct the forecast charts.