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:
- 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.
- 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-fly | ECMWF | 20150406 | date=20150406 | hdate=19940406/19950406/../20130406/20140406 | past 20 years | 2/week | 20 |
| fixed | CMA | 20150406 | date=20140501 | hdate=19940101/19940102/.../20141230/20141231 | 1994-2014 | daily | 365*21=7665 |
| fixed | BoM | 20150406 | date=20140101 |
| 1981-2013 | 6/month | 6*12*33=2376 |