Overview
Reforecasts (also sometimes called hindcasts) are sets of past forecasts that are computed retrospectively using the same (or as close as possible) model as the real-time forecast for a number of past dates. In CEMS-Flood, the hydrological reforecasts are generated for medium, extended and seasonal ranges using the same hydrological modelling chain as for the operational forecasts and reanalysis datasets.
Compared with operational forecasts, reforecasts have two main advantages:
- they span a much longer time period (usually 20+ years), hence can describe better the interannual variability better and contain larger sample of possible realisations useful to derive robust statistics
- they are generated retrospectively, using the same (or as close as possible) NWP model as the real-time forecast, hence containing the latest model development representative of the operational model
Hydrological reforecasts can be used for two main activities:
- to evaluate the skill of the forecast system, by comparing (proxy) observations observations (or proxi observations such as the reanalysis) with reforecasts for a set of past dates as long as possible
- to define climatological statistics consistent with the forecast modelling chainforecasts (with varying lead times), which may have different biases than a reference run based on the historical simulations run with reanalysis or observationsobservation forcing.
The CEMS-Flood reforecast configuration
CEMS-Flood reforecasts are produced using reforecasts from ECMWF ENS medium-range and seasonal range systems as meteorological forcing.
Medium to extended range
ECMWF-ENS medium and extended range reforecasts are currently generated every Monday and Thursday, for the same date in the past 20 years
forwith 11 ensemble members out to a lead time of 46 days. From EFAS v5 and GloFAS v4, they are generated everyday out of a lead time of 10 days (9-km resolution from EFAS v5 and GloFAS v4, 18-km resolution for earlier versions) and 46 days (36-km resolution). These are then forced through CEMS-Flood hydrological modelling chain to produce 20 years of river discharge reforecasts, for all start date available, for 11 ensemble members. A schematic of the ECMWF-ENS reforecast configuration until June 2023 is given in Figure 1 for the reference period January to December 2019.
In total, there are 2080 start dates in an one-year CEMS-Flood medium-range reforecast set (52 weeks x 2 per week x 20 years). For EFAS, medium-range river discharge reforecasts are run for each river cell with an upstream area > 350 km2 at a 6 hr time-step out to a lead time of 46 days with 11-ensemble members each. For GloFAS, river discharge reforecasts are run for each river cell with an upstream area > 1000 km2 at a 24 hr time-step out to a lead time of 30 days with 11-ensemble members each.
Figure 1: ECMWF-ENS reforecast configuration schematic for the reference period January to December 2019.
From GloFAS v4 and EFAS v5, CEMS-Flood medium/extended-range reforecasts generated from ECMWF 48r1 for start dates across April to July 2023 were available. As ECMWF IFS 48r1 continues to be generated operationally each Monday and Thursday for the corresponding dates in the last 20 years (i.e. 2003-2022), the CEMS-Flood medium/extended range reforecasts continues to be produced.
Seasonal range
ECMWF SEAS5 reforecasts are generated on the first of each month for the 36-year period 1981-2016 for 25 ensemble members and used out to a lead time of 16 weeks. These are then forced through the CEMS-Flood Seasonal hydrological modelling chain to produce 36 years of river discharge reforecasts, once per month, for 25 ensemble members at a weekly time-step.