Since 21 August 2023, a prototype of the global component of the Weather-Induced Extremes Digital Twin, developed at ECMWF as part of the Destination Earth initiative of the European Commission, has been producing daily forecasts on ECMWF’s Atos supercomputer at a resolution of 4.4 km. By comparing these forecasts with ECMWF’s operational forecasts at 9 km resolution, we can assess the additional benefits of the kilometre-scale resolution to predict extreme weather events. Here, we describe some successes and challenges we have seen over the last four months. The example of tropical cyclones will provide some evidence of the capabilities of forecasts at a resolution of 4.4 km.
The global Extremes Digital Twin
Destination Earth (DestinE) aims to create a digital twin of the Earth to enhance Europe’s ability to respond and adapt to extreme weather and climate change. The initiative is implemented by ECMWF, ESA and EUMETSAT under the leadership of the Directorate-General Communications Networks, Content and Technology (DG CNECT) of the European Commission. ECMWF oversees the development of the first two high-priority digital twins, on Climate Change Adaptation and Weather-Induced Extremes, and of the Digital Twin Engine.
The Weather-Induced Extremes DT (or the Extremes DT) will support decision-making for a rapid response to meteorological, hydrological and air quality extremes, on a timescale of a few days ahead. It will have both a global component developed at ECMWF, run regularly to produce simulations at resolutions of a few kilometres for four to five days ahead, and a regional component, implemented in an activity procured by ECMWF from a large partnership led by Météo-France. It will be possible to configure and activate this regional component on demand, allowing users to zoom‑in on extreme events happening across Europe with simulations at even higher resolutions (a few hundred metres).
Since the beginning of DestinE in December 2021, ECMWF has worked on developing a prototype of the global component of the Extremes DT. The prototype developed so far builds on the operational version of ECMWF’s Integrated Forecasting System (IFS) (Cycle 48r1). However, it advances that version by increasing the horizontal resolution from 9 km to 4.4 km; by tailoring the representation of physical and dynamical processes to km-scale resolutions; and by adding novel simulated satellite imagery capabilities, specifically for visible channels.
Currently, the prototype global Extremes DT is initialised from the ECMWF operational analysis also used for ECMWF operational forecasts at 9 km resolution. The simulations are coupled, but for now the resolution of the NEMO ocean model is the same as in the operational forecasts (0.25 degrees). The resolution of the ECWAM wave model is 0.05 degrees compared to 0.125 degrees in the operational forecasts.
Work is also ongoing to integrate impact-sector elements in the global Extremes DT, notably for river discharge and inundated areas and a prognostic treatment of selected aerosol species. In addition, we are working on improving the DT initial conditions by using more continuous and higher-resolution data assimilation and through a better use of high-resolution observations.
First results
Initial simulations performed at 4.4 km, with the operational configuration of the IFS apart from the resolution, showed an improvement in the prediction of tropical cyclone intensity and of orographic precipitation maxima. However, the large-scale circulation scores were degraded, and work was needed to improve the model configuration used for kilometre-scale simulations. Notably, extra-filtering of the model mean orography was needed at small scales to reduce gravity waves that the hydrostatic model cannot simulate well. The model time-step was also decreased to increase model accuracy. This optimised configuration of the IFS was used to demonstrate the capability to produce daily forecasts with the prototype global Extremes DT.
Five-day deterministic forecasts at 4.4 km resolution have been run daily (at 00 UTC) with this prototype since August 2023. An offline suite is also run with the hydrological CaMa-Flood model, forced with the global Extremes DT to produce river discharge and inundated areas at 3 arcmin resolution (~5 km). ECMWF analysts can now compare the DestinE simulations at 4.4 km with ECMWF operational forecasts at 9 km on a daily basis.
Successes include the prediction of the intensification of tropical cyclone Idalia. This happened just a few days after the daily simulations with the global Extremes DT were initiated. Idalia made landfall on 30 August 2023 in Florida, with hurricane-force wind. While the control of ECMWF’s operational ensemble forecasts did not even manage to predict that the cyclone would reach category 1, the prototype DT predicted a category 2 when the forecast was initialised on the 28 August 00 UTC (see the forecast plots). It successfully predicted category 3 24 hours later, in close agreement with the observed IBTrACS intensity. Hurricane Franklin occurred simultaneously to the north-east of Florida over the Atlantic. Simulated satellite images of reflectance clearly show the narrow eye structure of Idalia and Franklin predicted by the prototype DT, as well as fine-scale structure in the spiralling cloud bands surrounding both hurricanes (see the simulated visible image).
However, we found that the increased resolution does not always lead to an improved prediction of tropical cyclones. For example, tropical cyclone Otis, which made landfall near Acapulco on 25 October 2023, was a major forecast bust. Its rapid intensification from tropical storm intensity to category 5 in less than 24 hours was missed by all operational models. Unfortunately, the prototype global Extremes DT also fell short of predicting the event, failing to even reach category 1.
Planned improvements
The failure to properly forecast Otis could be due to a combination of factors that other activities within Destination Earth have the potential to address. Indeed, the difficulty of initialising a weak and small tropical depression with a coarse resolution analysis might have played a crucial role in the forecast failure. We are working on a higher-resolution 4D‑Var system assimilating more observations with the potential to provide improved forecast initial states at higher resolutions. We are also actively exploring new physics for convection to address errors in the representation of mesoscale convective systems, which tend to be too weak in the IFS at a resolution of both 9 km and 4.4 km. The intense convective activity embedded in the tropical depression from which Otis developed could have played a pivotal role in the onset of the event.
Work is continuing to produce daily simulations with the global Extremes DT on EuroHPC systems. Simulations will first be performed on the LUMI supercomputer in Finland, and then on Leonardo in Italy. Work is also ongoing to connect the Extremes DT with the DestinE data lake, implemented by EUMETSAT, and the DestinE core platform, implemented by ESA, which will allow users to access the data of DestinE DTs. The objective is to have all these essential building blocks in place to demonstrate production capability for the Extremes DT on EuroHPC at the end of Phase 1 in June 2024, when the DestinE system will open to users.
In parallel, work to continuously advance the global extremes DT will continue through the remainder of Phase 1 of DestinE and in Phase 2, by improving the representation of physical processes, by activating selected prognostic aerosol species, and by enhancing the accuracy of the initial conditions for Earth system components.