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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 medium range ensemble run itself.  The re-forecast runs are at the same resolution as the medium range ensemble (currently 9km) and run over the 15-day medium 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 medium range forecasting systems; 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

Re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February).  These are the re-forecast dates for medium range.

The set of re-forecasts for the M-climate s 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 (covering a period of 15 days).  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 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 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 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 2m temperature, wind speed, cloudiness and precipitation 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 inconsistencies 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 (introduced in autumn 2024) and later, re-forecasts are made every four days during each month on 1/5/9/13/17/21/25/29 (excluding 29 February).  There remain problems using each newly updated M-climate, particularly in spring and autumn when mean temperatures change most rapidly day by day. 

In Cy48 and earlier (prior to autumn 2024) the re-forecasts were run twice per week with M-climate updates on Mondays and Thursdays.  There were problems using each newly updated M-climate, particularly in spring and autumn when mean temperatures change most rapidly day by day. 

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

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)

 

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