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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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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 week on Mondays and Thursdays.  So M-climate quantile plots for the same lead-time from two forecast runs on e.g. 00UTC Thursday and 00UTC Friday 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 limitation of twice weekly updates to the M-climate can be significant.  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:  

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