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The generation of the sub-seasonal and seasonal forecast signal relies on few major steps. The process is illustrated by a flowchart in Figure 1. The first ingredient is the actual forecast forecasts that is produced in real time.
Real time forecasts
This part The first major component is the hydrological forecasts produced in real time. This will give the actual predicted conditions for the sub-seasonal and seasonal products that will be compared to the climatologies to derive the forecast anomaly and uncertainty. In the following, we describe the The characteristics of these the real time forecast simulations are described below. Where appropriate, the difference between EFAS and GloFAS /GloFAS and sub-seasonal/seasonal is specified. If there is no EFAS/GloFAS or sub-seasonal/seasonal mentioned, then the method is identical between the two forecast systems:
- Hydrological model used: LISFLOOD (LINK!)
- Meteorological forcing used: the :
- Sub-seasonal:
- The combination of the 9km (horizontal resolution)
- ECMWF ensemble forecasts (LINK!) and the 36km ECMWF sub-seasonal forecasts (LINK!) from the previous
- run date
- The low-resolution sub-seasonal forcing is taken from the previous available forecast run, simply because of timing, as the sub-seasonal meteo forecasts for the same day are only available with several hours delay after the high-resolution forecasts.
- The high- and low-resolution forcing is currently combined by a simple mechanical blending by using the high-resolution meteorological forcing for days 1-15, while the low-resolution meteorological forcing for days 16-45, for each of the 51 ensemble members.
- The blending can create inconsistencies locally over complex orographical areas, due to the resolution change at day15, and also there could be some smaller inconsistencies between the ensemble because of the forcing change from 15 to 16 day, due to the mechanical blending which will mix different weather conditions from the low- and high-resolution meteorological data. However, overall as an ensemble of 51 scenarios, the blending not expected to lead to any larger discontinuities.
- Seasonal:
- 36 km seasonal forecasts (SEAS5, LINK)
- Number of ensemble members: 51 ensemble members (one unperturbed and 50 perturbed) to represent the equally likely forecast scenarios of the future in both sub-seasonal and seasonal.
- Sub-seasonal:
- River resolution:
- 1 arcmin (~1.5 km) in EFAS
- 3 arcmin (~5 km) in GloFAS
- Run frequency: Forecasts
- Sub-seasonal forecasts are generated daily at 00 UTC
- Seasonal forecasts are generated monthly on the 1st of each month at 00 UTC
- Lead time: 45 days
- Sub-seasonal has 45-day (1080 hours) lead time. Only 45 days, as although the low-resolution sub-seasonal meteo forecasts run out to 46 days, we can only rely on the 1-day-old (yesterday's) run, which means we can only run the simulations to 45 days ahead.
- Seasonal has 7-month lead time (strictly speaking 215 days for each run date)
- Forecast step:Forecast steps:
- 6-hourly in EFAS
- 24-hourly in GloFAS
- Forecast hydrological Hydrological forecast initialisation: From
- Sub-seasonal is initialised from a fillup simulation forced with the shortest-range ENS-Control (unperturbed member of the ECMWF ensemble forecast) meteorological conditions.
Component-2. Reforecasts
- Seasonal is initialised from hydrological monitoring simulation, which is forced with gridded meteorological observations in EFAS and ERA5 meteorological reanalysis data in GloFAS.
Reforecasts
The second major component is the reforecasts, that are used to generate the model climatologies. These climatologies are range-dependent, i.e. they change with the The sub-seasonal products rely on range-dependent climatologies that change with the forecast lead time. The climatologies are produced from a large set of hydrological reforecasts. In the following, we describe the characteristics of these reforecast simulations, which allows for robust estimate of the climatological behaviour of the systems. The characteristics of the reforecast simulaions are described below. Where appropriate, the difference between EFAS and GloFAS /GloFAS and sub-seasonal/seasonal is specified. If there is no EFAS/GloFAS or sub-seasonal/seasonal mentioned, then the method is identical between the two systems:
- Hydrological model used: LISFLOOD (LINK!)
- Meteorological forcing used: the :
- Sub-seasonal:
- The combination of the 9km (horizontal resolution) ECMWF ensemble forecasts (LINK!) and the 36km ECMWF sub-seasonal forecasts (LINK!) from the same run date
- The low-resolution sub-seasonal and high-resolution medium-range forcings are taken from the same forecast run date. The timing of the availability of the low-resolution forecasts is not an issue here, as these reforecasts are only produced retrospectively without any time constraint.
- The high- and low-resolution forcing is currently combined by a simple mechanical blending by using the high resolution meteorological forcing for days 1-15, while the low-resolution meteorological forcing for days 16-46, for each of the 11 ensemble members.
- The blending can create inconsistencies locally over very complex orographical areas, due to the resolution change at day15, and also there could be some smaller inconsistencies between the ensemble because of the forcing change from 15 to 16 day, due to the mechanical blending which will mix different weather conditions from the low- and high-resolution meteorological data. However, overall as an ensemble of 11 scenarios, this will not be expected to lead to any larger discontinuities.
- Seasonal:
- 36 km seasonal forecasts (SEAS5, LINK)
- Number of ensemble members:
- Sub-seasonal has 11 ensemble members (one unperturbed and 10 perturbed) to represent the equally likely forecast scenarios of the future.
- Seasonal has 25
- ensemble members (one unperturbed and 10 perturbed) to represent the equally likely forecast scenarios of the future.
- Sub-seasonal:
- River resolution:
- 1 arcmin (~1.5 km) in EFAS
- 3 arcmin (~5 km) in GloFAS.
- Run frequency: Forecasts
- Sub-seasonal reforecasts are generated always for the past 20 years. Before 12 Nov 2024, they were generated twice-weekly on Mondays and Thursdays (at 00 UTC), while from 12 Nov 2024 they are generated on given days of the months, i.e. 1, 5, 9, 13, 17, 21, 25 and 29 (excluding 29 Feb).
- Seasonal reforecasts are generated for each month of the past back to 1981, although we only use them from 2004-2023. The shorter period, in the same way as for the sub-seasonal, provides better better stationarity (see https://www.ecmwf.int/en/elibrary/81194-trends-glofas-era5-river-discharge-reanalysis), which can be very important for the reliability of the forecasts.
- Lead time: 46 days (1104 hours).
- Sub-seasonal reforecasts have 46-day (1104 hours) lead time. This is 1-day longer than the real time forecasts, as these for the reforecasts there is no time constraint and the blending can be done with the high-resolution and low-resolution meteorological forecasts of the same run date (no need to delay by 1 day the low-resolution).
- Seasonal reforecasts have the same 7-month (or 215-day) lead time.
- Forecast steps:Forecast steps:
- 6-hourly in EFAS
- 24-hourly in GloFAS
- Forecast hydrological initialisation: From the monitoring hydrological
- All reforecasts are initialised from the hydrological monitoring simulation, which is forced with gridded meteorological observations in EFAS and ERA5 meteorological reanalysis data in GloFAS. There is no reason to use fillup, since these are produced for dates well in the past (as for the real time forecasts).
Component-3. Climatologies
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