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Limited-area Numerical Weather Prediction Modelling Efforts in Europe

Limited-area Numerical Weather Prediction Modelling Efforts in Europe

HARMONIE, ALADIN, AROME, COSMO, ALARO, ICON - the names of European weather prediction models can be confusing. What are the differences and what is their history? This article gives an overview.

by Max H. Balsmeier - July 1st, 2023

map of NWP consortia
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Limited-area numerical weather prediction (NWP) models, also called regional models, are models that are used to produce accurate forecasts for a domain as large as several countries or as small as a county or city. In contrast to global models, which span the whole Earth, they are easier to implement with higher resolutions.

Regional models are usually developed as collaborative efforts of national meteorological services, institutes and universities, almost always publicly funded. The first such consortium was the HIRLAM consortium (High-Resolution Limited Area Model). It was founded in 1985 by the Nordic countries. Later, more countries joined. The model developed and run by this consortium has the same name - HIRLAM - and was run for many years in the member states' national meteorological services. In some countries it is still used, but since it is not under active developemnt anymore, it will be phased out soon. HIRLAM is a hydrostatic gridpoint model, meaning it calculates vertical motion - in deep convective clouds, for example - only in a very simplified manner. It has mainly been applied at resolutions of approximately 10 km. In 2022, HIRLAM member states included Denmark, Estonia, Finland, Iceland, Ireland, the Netherlands, Norway, Spain, Sweden and Lithuania, with France being an associate member.

The second European NWP-consortium, called ALADIN (Aire Limitée Adaption Dynamique Développement International) was founded under the head of Météo-France in 1991, including, apart from France, the Czech Republic, Hungary, Poland, Romania and Slovakia. Slovakia decided not to participate within the same year, while Austria joined. The main target was to develop a limited-area version of the French global model ARPEGE (Action de Recherche Petite Echelle Grande Echelle), which had been branched off the ECMWF's (European Centre for Medium-Range Weather Forecasts) global model, the Integrated Forecasting System (IFS). Until 2008, Morocco, Bulgaria, Slovakia, Slovenia, Croatia, Belgium, Portugal, Moldova, Tunisia, Algeria and Turkey joined. The ALADIN model is a non-hydrostatic spectral model, meaning it computes the meteorological variables (temperature, pressure, etc.) not at gridpoints but in a mathematically more complex way. Since 2008, Météo-France uses AROME (Application of Research to Operations at the Mesoscale), which is based on ALADIN. AROME takes its boundary conditions from the French global model ARPEGE and has replaced the ALADIN model in many of the member states. ALARO (short for ALADIN-AROME) is a combination of ALADIN and AROME and optimized for resolutions in the 4 - 8 km range. It is in operational use, for example, in Belgium.

The HIRLAM members, with HIRLAM being a model created for coarser resolutions than today's regional NWP models, are also in need of a more up-to-date model. With ALADIN and HIRLAM being in a collaboration since 2005, an AROME version called HARMONIE (HIRLAM ALADIN Research on Meso-scale Operational NWP in Europe) is used for this purpose. The main difference between AROME and HARMONIE is that AROME is nested in ARPEGE, while HARMONIE is nested into the IFS.

The third NWP-consortium founded in Europe is the Consortium for Small-scale Modelling under the coordination of the German Weather Service (Deutscher Wetterdienst, DWD). It was formed in 1998 by Germany and Switzerland, being joined by Italy and Greece shortly after. Until 2017, Poland, Romania, Russia and Israel joined. The COSMO model originated in the Lokalmodell (LM), which entered operational use in 1999 (with an embarassing event around one month later, where it crashed during the Storm Lothar at Christmas 1999). It is a non-hydrostatic gridpoint model.

In 2021, the HIRLAM-ALADIN cooperation evolved into ACCORD (A Consortium for Convection-scale Modelling Research and Development). ALADIN, being a spectral model, gets computationally excessively expensive at fine resolutions, meaning the model will require too much resources when run with a horizontal resolution of less than 1 km. This is one of the disadvantages of spectral models and one of the motivations for developing a new gridpoint model within the ACCORD consortium.

The current operational model of DWD as well as of large parts of Germany's research community is now the ICON model. The ICON model was developed as a global model first, however, regional stand-alone implementations are possible. One such implementation is ICON-D2, covering Germany and most neighbour states at a horizontal resolution of approximately 2 km. Since DWD is mainly coordinating COSMO, it encourages members to migrate to ICON as well. The final version of the COSMO model has been published in December 2021, so further developments are now restricted to the ICON model. As of 2023, however, most COSMO member states still use the COSMO model for their operational predictions.

Unfortunately, despite having been publicly funded, none of these models' source codes is available under an open-source license.