You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 15 Next »

Creation of ER-M-Climate

The ER-M-Climate is derived from a set of extended 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 extended medium range run itself and run over the 46-day extended range ENS period. 

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:

  • extended range forecast verification metrics are based on the re-forecasts.
  • re-forecasts allow computation of the ER-M-climate which allows actual forecasts to be converted into an anomaly format.   Forecasts in terms of anomalies relative to a model climate (rather than relative to the observed climatology) mean that some calibration for model bias and drift into the products is incorporated.

Selection of extended range re-forecasts

The set of re-forecasts is based on using the three consecutive dates surrounding the day and month of the extended ENS run in question.  Re-forecasts are created using the same calendar start dates for each of the last 20 years.  

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.
  • three consecutive re-forecasts (covering a 1 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 46-day ensemble forecast period.

In total, each set of re-forecasts consists of 20 years x 3 runs x 11 ensemble members = 660 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 ER-M-climate.

A lower number of re-forecasts than for evaluating M-climate is justified because the tails are less important and should not be so prone to having a reduced sample size.

The ER-M-climate is used in association with the extended range ensemble forecast:

  • to present the day15 to day46 ensemble meteograms with the extended range climate (ER-M-climate).
  • to highlight significant anomalies of forecast 2m temperature, wind speed, cloudiness and precipitation from the norm for a given location and time of year.  

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.

  • No labels