Creation of SUBS-M-Climate

The SUBS-M-Climate is derived from a set of sub-seasonal range re-forecasts.  These are created using the same calendar start dates over several years for data times either side of the time of the sub-seasonal ensemble run itself.  The re-forecast runs have the same resolution as the sub-seasonal medium range run (currently 36km) and run over the 46-day sub-seasonal range ensemble period. 

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 sub-seasonal forecasting system; they have two applications:

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

Selection of sub-seasonal range re-forecasts

Re-forecasts are made every two days during each month on 1/3/5/7/9/11/13/15/17/19/21/23/25/27/29/31 (excluding 29 February).  These are the re-forecast base-dates for sub-seasonal range.

The set of re-forecasts for the SUBS-M-climate is made up from:

  • five consecutive re-forecasts centred on the M-climate base date and covering a forecast period of 9 days.  The middle re-forecast corresponds to the closest base-date on or preceding the actual ensemble run date (e.g. if the actual ensemble run date is the 20th of the month, then the re-forecast base-date is the 19th and the re-forecasts used are for 15th, 17th, 19th, 21st, 23rd). 
  • re-forecasts are derived by re-running an 11-member ensemble (1 control and 10 perturbed members) using the same calendar start date for each of the last 20 years.

Thus there are 20 years x 5 re-forecasts x 11 ensemble members = 1100 re-forecast values.  These are available at forecast intervals of 6 hours for each forecast parameter, for each forecast lead-time and for each calendar start date and location.  They are used to define the SUBS-M-climate.  

Running on fixed days of the month allows direct comparison between re-forecasts produced at different resolutions and/or in different years.


The SUBS-M-climate is used in association with the sub-seasonal range ensemble forecast:

  • to present the day15 to day46 ensemble meteograms with the sub-seasonal range climate (SUBS-M-climate).
  • to highlight significant anomalies of forecast 2m temperature, wind speed, cloudiness and precipitation from the norm for a given location and time of year.  

Running on fixed days of the month allows direct comparison between re-forecasts produced at different resolutions and/or in different years.

Different priorities for M-Climate and SUBS-M-Climate

ECMWF uses different numbers of re-forecasts to build the M-Climate and the SUBS-M-Climate.

  • M-Climate uses a greater number of re-forecasts which are more widely spaced than for SUBS-M-Climate.
  • SUBS-M-Climate uses a smaller number of re-forecasts which are more closely spaced than for M-Climate.  

This means the "date range" spanned by the M-Climate is about 3 to 4 times larger than the "date range" spanned by the SUBS-M-Climate. 

Priorities with regard to model climate usage differ for different classes of lead time:  

  • For the medium range, the priority is the best possible capture of the climatological distribution of the tails (e.g. extreme forecast index (EFI) and shift of tails (SOT)).  
  • For sub-seasonal ranges, the priority is better representation of seasonal cycles (for example in the Indian monsoon onset time).


(FUG associated with Cy50r1)