Page History

Creation of M-Climate

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

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There is some merit in examining the real-time performance of a seasonal forecasting system.  But   But the sample sizes created for one system are far too small to conclude anything about the its true performance levels of that system. Hence we use the re-forecasts.  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 all seasonal forecasting system; 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

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

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• 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.The large number of re-forecasts is justified because it is essential to minimize sampling noise in the M-climate tails for EFI and SOT computations. 

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.  

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.

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 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 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. 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 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 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.

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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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