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The set of re-forecasts is based on using the three consecutive dates surrounding the day and month of the extended ENS run in question.  Re-forecasts are created using the same calendar start dates for each of the last 20 years.  

Each re-forecast consists of an 11-member ensemble (1 control and 10 perturbed members).  Therefore altogether 20 years x 3 runs x 11 ENS members = 660 re-forecast values are available to define the ER-M-climate for each forecast parameter, forecast lead-time, calendar start date and location.  These are used to define the ER-M-climate.

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.
• three consecutive re-forecasts (covering a 1 week period), the middle one of which corresponds to the preceding Monday or Thursday 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 46-day ensemble forecast period.

In total, each set of re-forecasts consists of 20 years x 3 runs x 11 ensemble members = 660 re-forecast values.  These are available for each forecast parameter, forecast lead-time, calendar start date, location, at forecast intervals of 6 hours.  These are used to define the ER-M-climate.

A lower number of re-forecasts than for evaluating M-climate is justified because the tails are less important and should not be so prone to having a reduced sample size.

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

• to present the day15 to day46 ensemble meteograms with the extended range climate (ER-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.  

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Different reference periods for M-Climate and ER-M-Climate

ECMWF uses different reference periods but essentially the same re-forecast runs to build the M-Climate and the ER-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. for EFI and SOT).  This can be better achieved using a re-forecast span of 5 weeks (1980 re-forecast values).  
• For longer ranges, the priority is the correct representation of seasonal cycles.  This can be better achieved by using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

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