Copy of S-M-climate, the Seasonal Model Climate

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) as the seasonal run in question.  Re-forecasts are created using the same calendar start dates during the 36 year period: Jan 1981 to Dec 2016.  Currently, this is not updated. 

The set of re-forecasts is made up from:

• a set of re-forecasts using the same calendar start dates for each of the 36 year period: Jan 1981 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 36 years x 1 runs x 25 ensemble members = 900 re-forecast values.  These are available for each forecast parameter, forecast lead-time, month, location, at forecast intervals of 6 hours.  This is used to define the S-M-climate.

The number of re-forecasts, and overall all near 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.