Major upgrade for global flood forecasts

Ervin Zsótér
Christel Prudhomme
Shaun Harrigan

 

On 14 November 2018, the Global Flood Awareness System (GloFAS) of the Copernicus Emergency Management Service (CEMS), for which ECMWF is the computational centre, was upgraded to version 2.0. The upgrade implemented a number of major system developments for improved global early warnings.

GloFAS aims to provide transboundary early flood guidance over all medium- to large-scale rivers of the world, helping national hydro-meteorological services, humanitarian agencies and commercial companies to improve their response to flood-related hazards. GloFAS has been developed in close cooperation between the European Commission’s Joint Research Centre (JRC) and ECMWF, with support from national authorities and research institutions such as the University of Reading. GloFAS has a 30‑day component (GloFAS 30‑day), which has been producing probabilistic flood forecasts up to 30-days in a semi-operational fashion since 2011, and a seasonal hydrological outlook system (GloFAS Seasonal), which has been providing high- and low-flow anomalies up to 16 weeks ahead since November 2017. GloFAS became part of CEMS in April 2018, offering a fully operational, 24/7 service.

Main elements of the upgrade

The upgrade included a new calibration of the hydrological routing component to improve the river flood simulation worldwide. It also involved changes in the configuration to produce an improved hydrological reanalysis for more realistic climatological thresholds and improved real-time forecast initialisation. In response to requests from users, the upgrade also introduced a version numbering system for full traceability of GloFAS flood forecasts, significantly enhanced user documentation (now available on the web) and the publication of over 20 years of discharge forecast reruns.

Calibration: GloFAS version 2.0 implements a major calibration exercise for river routing parameters to better account for regional variability in hydrological processes. The calibration was conducted for 1,287 stations worldwide where daily discharge observations were available. As shown in the figure, the calibration has improved discharge simulations for the majority of stations. The GloFAS team is grateful to all data providers who have given us access to their data.

New calibration. GloFAS provides improved river discharge simulations after calibration over most of the catchments with available observations. Green/blue colours show higher skill, while orange/red colours indicate lower skill in simulations using the newly calibrated model. (Chart from an article by F.A. Hirpa et al. (2018) published in the Journal of Hydrology, doi:10.1016/j.jhydrol.2018.09.052, reproduced under the CC BY 4.0 licence.)

Reanalysis: The hydrological reanalysis provides atmosphere, land-surface and river state variables (e.g. runoff and river discharge) as close as possible to observed conditions. It is used in evaluation and to define the GloFAS discharge thresholds, which help to assess the severity of predicted flood events. In GloFAS version 2.0, the hydrological reanalysis uses high-resolution (31 km grid spacing) input data from ERA5, ECMWF’s new atmospheric reanalysis.

Initialisation: To initialise the real-time ensemble discharge forecasts, the GloFAS state variables need to be updated every day. In GloFAS version 2.0, the simulations that produce the state variables also use high-resolution input data from ERA5. GloFAS version 2.0 ensures the highest possible consistency between the hydrological reanalysis and the real-time forecasts, addressing a known weakness of earlier GloFAS versions.

Versioning: A 2-digit version numbering system was introduced. The first number relates to major upgrades that affect the hydrological modelling chain, while the second number relates to other changes impacting the discharge. GloFAS 30-day and GloFAS Seasonal are considered as two independent systems, but after this upgrade they both share the version number 2.0.

Forecast reruns: The new discharge forecast reruns cover the period 1997 to 2018 for GloFAS 30-day and 1981 to 2017 for GloFAS Seasonal. They are simulations performed for a set of past dates with the same model version that is used for the real-time forecasts. These discharge forecast reruns provide a long, homogeneous forecast time series, ideal for analysing GloFAS performance.

Documentation: The GloFAS documentation has been expanded and updated with information on the modelling and different forecast configurations, as well as other user information. Details on the upgraded system are available in the General and Technical Information sections on the GloFAS website: www.globalfloods.eu