Forecasting models constantly evolving

The constant improvement of its weather forecasting systems, a prerequisite for better anticipation and localisation of weather phenomena, is a priority for Météo-France. There are many issues at stake: safety when it comes to alerting public authorities and the population of the arrival of a hazardous phenomenon or forecasting weather conditions at airports, economic when a company needs a very precise forecast to organise its activity.

For weather forecasting up to 3 days ahead, Météo-France mainly uses two complementary models: Arpege for large-scale phenomena, and Arome for localised phenomena up to one day ahead.

A global model: Arpege

This model covers the entire globe with a mesh size ranging from 5 km for Europe to 24 km. It makes it possible to predict large-scale phenomena (depressions, anticyclones, for example) that travel around the globe. Arpege is used for forecasting up to four days ahead over mainland France, overseas territories, maritime domains and areas of interest for France. Beyond three to four days, Météo-France uses the model of the European Centre for Medium-Range Weather Forecasts (ECMWF) based in Reading. This model is developed in partnership with Météo-France and shares common elements with Arpege. It is used for forecasts up to 10-15 days.

Arpege is also available in a "climate" version. Coupled with other models (ocean, sea ice, surfaces, etc), it can be used to simulate climate changes over long periods (month, year, decade).

A fine-meshed regional model: Arome

With a mesh of 1.3 km*, Arome covers an area limited to mainland France and neighbouring countries. It is fed via its lateral edges by simulations from the larger-scale Arpege model. Arome provides much more detailed "zoomed in" information on France that forecasters need to refine their small-scale forecasts, particularly in terms of anticipating and locating potentially dangerous weather phenomena such as thunderstorms. Other versions of Arome cover overseas territories. 

*Resolutions of the operational models since 2015.

supercalculateur

In addition to the model, the forecasting system also includes:

  • upstream, a system for processing weather observation data from around the world that provides a consistent description of the state of the atmosphere at any given time: this is data assimilation;
  • and a supercomputer to run the assimilation module and forecast model. The computer can work either in real time for weather forecasting purposes, or over past periods to reconstruct the behaviour of the atmosphere from observation archives. 
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