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Creation of M-Climate

The 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 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 ensemble period.   

Medium range re-forecasts for verification

There is merit in examining the real-time performance of a forecasting system.   But the sample sizes created for one system are far too small to conclude anything about its true performance levels.  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 made up from:

  • 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 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.

In total, each set of re-forecasts consists of 20 years x 9 runs x 11 ensemble members = 1980 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 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 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 shift of tails (SOT) 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. 

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.

The same M-climate set is used for 00UTC and 12UTC ensemble runs in order to avoid inconsistencies between the validity period of the 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.


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.


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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