Creation of M-Climate

The M-Climate is derived from a set of medium 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 ensemble run itself.  The re-forecasts have the same resolution (currently 9km) and period (15 days) as the medium range ensemble.   

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:

• medium range forecast verification metrics are based on the re-forecasts
  
• re-forecasts allow computation of the M-climate.

Selection of medium range re-forecasts

The set of re-forecasts is made up from:

• a set of re-forecasts using the same calendar start dates for each of the last 20 years.  Re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February).
• nine consecutive re-forecasts (covering a 5-week period).  The middle one corresponds to the preceding date that is closest to the actual ensemble run date.
• each re-forecast is from an 11-member ensemble (1 control and 10 perturbed members) run over the 15-day ensemble forecast period.

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 for each forecast parameter, forecast lead-time, calendar start date, location, at forecast intervals of 6 hours. variable

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

• to present the 15-day ensemble meteograms with the medium range climate (M-climate)
• to deliver the extreme forecast index (EFI) and shift of tails (SOT) products 
• to highlight significant forecast departures of variables from the norm for a given location and time of year.

Values evaluated in M-climate

• 2m temperature.
• soil temperature.
• sea-surface temperature.
• mean sea level pressure.
• precipitation.
• cloudiness. 

The same M-climate set is used for 00UTC and 12UTC ensemble runs.  This is to avoid inconsistency between the validity period of the ensemble and M-climate.  So, for example:

• Day1 M-climate is used with T+12h to 36h forecasts from the 12UTC run and T+0h to 24h forecasts from the 00UTC run.
• Day2 M-climate is used with T+36h to 60h forecasts from the 12UTC run and T+24h to 48h forecasts from the 00UTC run.

Updates to M-climate

In Cy49 and later, re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February).  Using fixed days of the month allows direct comparison between re-forecasts produced with different resolutions and/or in different years.

Mean temperatures change most rapidly day by day during Spring and Autumn.  This can cause problems using newly updated M-climate, particularly in these seasons.

In Cy48 and earlier re-forecasts were run twice per week with M-climate updates on Mondays and Thursdays. The interval between creation of re-forecasts remained similar to that in Cy49 bringing similar problems in Spring and Autumn. 

Cy41r1 in spring 2015, the M-climate was constructed from only 500 re-forecasts was more prone to sampling errors and as a result.

Different reference periods 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)