Data and Methods
The ECMWF Ensemble Prediction System (ENS) is the operational ensemble forecasting product of the ECMWF and consists of 51- member ensemble global forecasts. The weather forecast component has a horizontal grid resolution of about 32 km for 10 days, increasing to 65 km from day 11 to 15. In the GloFAS system, ENS weather forecasts of the 00:00 UTC forecast are processed by the land surface module
The ERA-Interim archive, a global atmospheric reanalysis produced by the ECMWF, contains gridded estimates of meteorological variables and surface parameters. It has horizontal resolution of about 80 km and covers the period from 1 January 1979 onwards. For the reference climatology used in GloFAS the ERA-Interim precipitation dataset has been bias-corrected using the Global Precipitation Climatology Project (GPCP).
HTESSEL is the land surface scheme used by ECMWF in its operational weather forecast system. HTESSEL computes the land surface response to atmospheric forcing, and estimates the surface water and energy fluxes and the temporal evolution of soil temperature, moisture content and snowpack conditions. Operational ensemble forecasts of surface and sub-surface runoff (soil to groundwater percolation) are extracted from the daily output of the ECMWF forecasts and then resampled to 0.1° resolution to be used as input by the river routing model. Further, an offline simulation of HTESSEL forced by ERA-Interim near-surface fields and bias-corrected ERA-Interim precipitation was performed to derive a 21 yr climatology starting in 1990, including surface and sub-surface.
Lisflood is a GIS-based spatially distributed hydrological model, which includes a one-dimensional channel routing model. The Lisflood model is currently running within the European Flood Awareness System (EFAS) on an operational basis covering the whole of Europe on a 5 km grid. In the context of global flood modeling, the transformation from precipitation to surface and sub-surface runoff is done by HTESSEL. For routing, Lisflood global is set up to simulate the groundwater and routing processes. Surface runoff is routed via overland flow to the outlet of each cell. Subsurface storage and transport are modeled using two linear reservoirs.
Ensemble streamflow predictions (ESPs) are run operationally on global scale by feeding surface and subsurface runoff into the Lisflood hydrological model.
Although the precipitation input spans 15 days, hydrological simulations are computed for a 45-day time horizon, to account for the delayed routing of flood waves
in large river basins, with time of concentration of the order of one month.
From day 16 to day 45, input maps of surface and subsurface runoff are set to zero; therefore, the hydrological model (i.e., Lisflood) will simply
convey towards the outlet water already within each river basin.
- Initial condition maps to start up the model are first taken from the last available day of ERA-Interim dataset
- Initial conditions for subsequent simulations are then extracted from the results of the model run with the ENS control run, after the first day of simulation
- Resulting ESP maps for each daily time step and ensemble member are compared with reference threshold maps derived from the streamflow climatology, corresponding to return periods of 2, 5 and 20 yr
- Summary threshold exceedance maps are calculated accordingly, which show the maximum probability of exceeding the 5 and 20 yr return period within the forecast horizon
- In addition, reporting points are chosen at fixed and dynamic locations in the river network where upcoming flood hazard is detected
Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F. GloFAS – global ensemble streamflow forecasting and flood early warningHydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.