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What are reforecasts

  • Real-time ensemble forecasts can be calibrated using historical sets of reforecasts (also known as hindcasts)
  • Reforecasts are forecasts run using the same model version as the real-time forecast for a number of past dates

The way to produce reforecasts is not unique

  • Different centres or models use different configurations for the implementation of its reforecast system.  At the moment the following 2 configurations are used:
  1. fixed configuration:
    • In this case reforecasts for all past dates are produced once during the lifetime of a given model version.
    • For every new version of the model, a new set of reforecasts is produced.

  2. on-the-fly configuration:
    • In this case reforecasts are produced at the same time as the real-time forecasts.
    • Each reforecast refers to a real-time forecast.

Main Concepts

Realtime Date

  • It is the date of the real-time ensemble forecast to be calibrated.
  • If the purpose of the reforecasts is to create a model climatology, then the date of the real-time ensemble forecast is not relevant. 
  • In the MARS system this is expressed with the keyword "date".

Model Version Date

  • It is the version of the model  used to produce the reforecasts.
  • Because the version of the model is time-stamped with a date, the term "Model Version Date" is used instead.
  • It is very important that the version of the model used to produce the reforecasts AND the version of the model used to produce the real-time forecast are the same. One CANNOT calibrate a real-time forecast with reforecast that has been produced by a different version of the model.  
  • In the MARS system, this is expressed with the keyword "date".

Hindcast Date

  • It refers to a start date for any of the past dates of the reforecast.
  • In the MARS system, this is expressed with the keyword "hdate".

Examples

reforecast

configuration

centre

realtime

date

model version

date

hindcast dates

reforecast

length

reforecast

frequency

number of hindcast

dates available for a

model version date

on-the-flyECMWF20150406date=20150401hdate=19940401/19950401/../20130401/20140401past 20 years2/week21
fixedCMA20150406date=20140501hdate=19940101/19940102/.../20141230/201412311994-2014daily365*21=7665
fixedBoM20150406date=20140101

hdate=19810101/19810106/19810111/19810116/19810121/19810126/../

20131201/20131206/20131211/20131216/20131221/20131226

1981-20136/month6*12*33=2376
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