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The ER-M-Climate is derived from a set of medium range re-forecasts created using the same calendar start dates over several years for data times either side of the time of the extended ENS ensemble run itself.  The re-forecast runs are at the same resolution as the medium range run itself (currently 9km?) and run over the 15-day medium range ENS range ensemble period.   

Medium range re-forecasts for verification

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The set of re-forecasts is based on using the made up from:

• a set of re-forecasts using the same calendar start dates for each of the last 20 years.
• nine consecutive re-

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• forecasts (covering a 5-week period)

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• , the middle one of which corresponds to the preceding Monday or Thursday that is closest to the actual

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• ensemble run date.

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• each re-forecast is from an 11-member ensemble (1 control and 10 perturbed members) run over the 15-day

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• ensemble forecast period.

In total, each set of re-forecasts consists of  Therefore altogether 20 years x 9 runs x 11 ENS ensemble members = 1980 re-forecast values.  These are available to define the M-climate for each forecast parameter, forecast lead-time, calendar start date and , location, at forecast intervals of 6 hours.  These are used to define the M-climate.

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The M-climate is used in association with ENS 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  and shift of tails (SOT) products and the 15-day ENS meteograms with a model climate (M-climate) 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.  

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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. This is because they are used in different ways.  :  

• For shorter ranges, the priority is the best possible capture of the climatological distribution of the tails .  It has been shown that using 1980 realisations (spanning 4 weeks) achieves this much better than using 660 (spanning 1 week(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 spanning 1 week rather than 4.  The extended range uses week-long averaging and the using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

M-climate is updated twice a week, every Monday and Thursday, and is based on 00UTC runs only (there are no 12UTC re-forecast sets).  The new files start to be used from the 00UTC run the next day.

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.  The same M-climate set is used for 00UTC and 12UTC ENS ensemble runs in order to avoid inconsistencies between the validity period of the ENS ensemble and M-climate (e.g. day1 M-climate and day2 M-climate distributions are used with respectively, the T+0 to 24h and T+24 to 48h forecasts from 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.

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

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