Creation of S-M-Climate

The S-M-Climate is derived from a set of seasonal re-forecasts are created using using the same calendar start dates for each of the last 20 years for the same data time of the seasonal run itself.  The re-forecast runs are at the same resolution as the seasonal run itself (currently 36km) and run over the 7 month or 13 month period of a seasonal forecast 

Seasonal re-forecasts for verification

There is merit in examining the real-time performance of a forecasting system.   But the sample sizes created for one system are far too small to conclude anything about its true performance levels.  Re-forecasts are used to increase the available data to produce a model climate.   The results of forecast system may be compared with this model climate.

Re-forecasts are a fundamental component of all seasonal forecasting system; they have two applications:

• seasonal forecast verification metrics are based on the re-forecasts.
  
• re-forecasts allow computation of the S-M-climate which allows actual forecasts to be converted into an anomaly format.   Forecasts in terms of anomalies relative to a model climate (rather than relative to the observed climatology) means that some calibration for model bias and drift into the products is incorporated.

Selection of seasonal re-forecasts

The set of re-forecasts is based on using the same calendar start date (the 1st of each month) during a fixed period (36 years: Jan 1981 to Dec 2016).   Currently, this is not updated.

Each re-forecast consists of a 25-member ensemble (all members are perturbed) run over a 7-month forecast period for monthly runs, and a 13-month forecast period for quarterly runs.  Therefore altogether 36 years x 25 ENS members = 900 re-forecast values are available to define the S-M-climate for each forecast parameter, forecast lead-time, calendar start date (the 1st of each month) and location.  These are used in their entirety for skill and reliability assessment, and verification maps are accordingly provided on the web.

Seasonal re-forecasts for the S-M-climate

The set of re-forecasts is based on using the same calendar start date (the 1st of each month) as the seasonal run in question.  Re-forecasts are created using the same calendar start dates during the 24 year period: Jan 1993 to Dec 2016.  

The set of re-forecasts is made up from:

• a set of re-forecasts using the same calendar start dates for each of the 24 year period: Jan 1993 to Dec 2016.
• each re-forecast is from an 25-member ensemble (all perturbed members) run over a 7-month forecast period for monthly runs, and a 13-month forecast period for quarterly runs.

In total, each set of re-forecasts consists of 24 years x 1 runs x 25 ensemble members = 600 re-forecast values.  These are available for each forecast parameter, forecast lead-time, month, location, at forecast intervals of 6 hours.  These are used to define the S-M-climate.

A lower number of re-forecasts, and overall nearer to the current date, is justified:

• to be compatible with other seasonal forecast products provided through the auspices of Copernicus (this is their standard re-forecast period).
• to be more compatible with "current climate" (which is changing primarily due to global warming).

The S-M-climate is used in association with the seasonal ensemble forecast:

• to highlight significant monthly anomalies of forecast parameters (2m temperature, 500 hPa height, 850 hPa temperature, mean sea level pressure, precipitation) from the norm for a given location and time of year.