Creation of SUBS-M-Climate

The SUBS-M-Climate is derived from a set of sub-seasonal range re-forecasts 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 are at 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 in terms of anomalies relative to a model climate (rather than relative to the observed climatology) mean that 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 dates for sub-seasonal range.

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

• a set of re-forecasts using the same calendar start dates for each of the last 20 years.
• nine consecutive re-forecasts (each covering a period of 45 days).  The middle one corresponds to the re-forecast date that is the same as, or immediately preceding, the actual ensemble run date.
• each re-forecast is formed from an 11-member ensemble (1 control and 10 perturbed members).

In total, each set of re-forecasts consists of 20 years x 9 runs x 11 ensemble members = 1980 re-forecast values.  

These are available at forecast intervals of 6 hours for each forecast parameter, 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 reference periods but essentially the same re-forecast runs to build the M-Climate and the SUBS-M-Climate.   The key difference is that those runs are grouped and used in different ways:  

• For shorter ranges, 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 longer ranges, the priority is the correct representation of seasonal cycles.

(FUG Associated with Cy49r1)

(FUG Associated with Cy49r1)