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

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• a set of re-forecasts using the same calendar start dates for each of the last 20 years.
• nine consecutive re-forecasts (covering a 5-week period), the .  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 15-day ensemble forecast period.

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The same M-climate set is used for 00UTC and 12UTC ensemble runs in order .  This is to avoid inconsistencies between the validity period of the ensemble and M-climate (e.g. day1 climate.  So, for example:

• Day1 M-climate

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• is used with T+

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• 12h to 36h forecasts from the 12UTC run and T+

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• 0h to

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• 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.

Limitation of twice weekly updates to M-climate

M-climate is updated twice per weaken Mondays and Thursdays.  So M-climate quantile plots for the same verifying  00UTC runs, and the T+12 to 36h and T+36 to 60h forecasts from 12UTC runs). If for the same lead-time, one compares, the M-climate quantile plots (e.g. for a Thursday 00UTC run), and a run 24hrs later, they will be slightly different.  This time from two forecast runs straddling the M-climate update will be slightly different.  So, for example, the quantile plots for forecast runs on 00UTC Thursday and 00UTC Friday will not be exactly the same.  However, this limitation of twice weekly updates to the M-climate can be significant.   It  It can be particularly evident in spring and autumn when mean temperatures are changing most rapidly day by day.

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 climate and the ER-M-Climateclimate.   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 extreme forecast index (EFI) and shift of tails (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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