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Table of Contents

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.

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• 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.  

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• 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 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 using a span of 1 week (660 re-forecast values).  The tails should not be so prone to having a reduced sample size.

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