GloFAS meteorological forcing data sets are coming from the latest ensemble of Numerical Weather Prediction (NWP) forecasts of the ECMWF IFS, with different model configurations used for the different GloFAS lead times and systems. Below, the main use of the meteorological forcing data sets is described.
GloFAS 30-day
The combination of the medium-range (ENS) and sub-seasonal (or extended-range; SUBS) ensemble forecasts is used to force the GloFAS 30-day system. ENS is the ensemble of possible forecast scenarios of ECMWF and consists of 51 members at ~9 km resolution up to 15 days, prodcued twice a day at 00 and 12 UTC. ENS is used to force the 30-day GloFAS in the first 15 days (from the 00 UTC forecast run). For the remaining period of days 16 to 30, the sub-seasonal forecasts (SUBS) are taken as meteorological forcing, which are on a much lower 36 km resolution. The sub-seasonal meteorological forecasts are produced once a day at 00 UTC only and consists of 101 ensemble members, of which the first 51 are used in GloFAS. As the sub-seasonal forecasts are available considerably later than the 15-day medium-range ones, the real-time generation of the GloFAS 30-day forecasts can only use the 1-day old sub-seasonal meteorological forecasts.
The combination of the two different forecasts is done in a simple manner, in which the 1st ensemble member of the medium-range forecast (in the first 15 days) will be followed by the 1st ensemble member from the (1-day old) sub-seasonal forecasts (in the 16-30-day range), and so on the same with all of the 51 members. Although, the combination of the two different meteorological forecasts at two different resolutions from different run dates can create some local discontinuities at day15, especially at complex orographical areas, on average the GloFAS ensemble forecasts are not expected to suffer any major disturbance after day15 and rather just benefit from the added value of the higher resolution meteorology used in first 15 days of the forecasts.
In addition to the general ensemble use, there are aspects of the GloFAS system that uses specifically the ENS-CNTL, which is a special member of the ensemble, as it uses the most accurate estimate of the current initial conditions, while the other 50 ensemble members start from slightly perturbed conditions.
GloFAS sub-seasonal
The combination of the medium-range and sub-seasonal (or extended-range) ensemble forecasts (with 51 members) is used for the sub-seasonal GloFAS forecasts and also for the GloFAS reforecasts. The sub-seasonal GloFAS forecasts use the same methodology as the 30-day forecasts by combining the medium-range and sub-seasonal forecast, with the only difference that there is no need to use the 1-day old sub-seasonal, so instead both the medium-range and sub-seasonal forecasts are from the same run date.
GloFAS seasonal
The seasonal-range NWP forecasts of ECMWF, SEAS5 (SEAS5) is used to force the GloFAS seasonal forecast. SEAS5 is the ensemble seasonal forecast of ECMWF (operational since 5 November 2017), and one of the seasonal forecast models of the Copernicus Climate Change Service. It consists of 51 members at ~36 km horizontal resolution, generated each month (from the 1st of the month) with 7 months lead time (extended to 13 months once a season).