A storm model for France, then for the tropics
At the start of the 2000s, Météo-France's operational models still guessed thunderstorms rather than computed them: with grid cells of about ten kilometres, a cumulonimbus was far smaller than a cell and had to be represented by a parameterisation, a recipe that estimates its average effect. The AROME project, run from 2003 to 2008, set out to build a model fine enough to compute these clouds itself, to forecast storms, heavy rain, fog and wind driven by relief. Its name stands for "Application of Research to Operations at Mesoscale".
Rather than starting from scratch, the project assembled two existing pieces. The dynamical core, the part that solves the equations of air motion, came from ALADIN-NH, the non-hydrostatic version of the limited-area model shared by Météo-France with the ALADIN consortium of European and North African weather services. The physics, which handles clouds, turbulence and radiation, came from Meso-NH, a research model of the French atmospheric research community (CNRM and CNRS laboratories). The surface came from SURFEX, the land and sea surface platform built on the same research. AROME-France became operational in December 2008, after about six years of development and validation.
The first versions ran at 2.5 km, with a new analysis every three hours; just before 2015 the model had 60 levels. In April 2015 two major steps came together: the mesh went to 1.3 km and 90 levels, and the analysis cycle went from three hours to one. The same year AROME-PI, a nowcasting version run every hour, was put into service, and an ensemble version, PE-AROME, became operational in December 2016. In October 2024 the hourly analysis switched to an ensemble-based scheme, 3DEnVar, and in March 2026 Météo-France announced a new forecast chain that triples the observations entering the model over France, from 1.4 to 4.2 million a day, and takes the ensemble from 76 to 108 scenarios a day.
The overseas territories followed. Until the mid-2010s they relied on global models and, for Réunion, the Caribbean and French Guiana, on 8 km versions of the hydrostatic ALADIN model; in February 2016 Météo-France put into service AROME-OM ("AROME Outre-Mer", AROME overseas), five 2.5 km domains over the Caribbean, French Guiana, the south-west Indian Ocean, New Caledonia and French Polynesia. Tropical cyclones were a main motive: Météo-France's centre in Réunion has been the World Meteorological Organization's Regional Specialized Meteorological Centre for tropical cyclones in the south-west Indian Ocean since 1993. A coupled ocean mixed layer was added in December 2017, the range was extended to 78 hours during cyclone warnings around 2018-2019, an experimental AROME-OM ensemble has run since 2020, and the Indian Ocean domain was stretched about 20 % further north in 2026 to better follow cyclones threatening Mayotte.
The science: computing clouds instead of guessing them
AROME solves the fully compressible, non-hydrostatic equations of the atmosphere. "Non-hydrostatic" means that vertical accelerations are computed rather than assumed negligible: global models such as ARPEGE assume that the weight of the air balances the pressure at every point, which holds for large weather systems but fails inside a storm updraft or a mountain wave. On top of wind, temperature, humidity and surface pressure, the model carries the vertical velocity, a non-hydrostatic pressure departure, the turbulent kinetic energy and five kinds of water: cloud droplets, ice crystals, rain, snow and graupel.
Like ARPEGE and ECMWF's IFS, whose code it shares, AROME is a spectral model: fields are represented as sums of waves, which lets horizontal derivatives be computed very accurately. Time stepping is semi-Lagrangian and semi-implicit, a combination that follows air parcels along their trajectories and treats the fastest waves implicitly, so that the model can take steps of about 50 s at 1.3 km without becoming unstable. Over France the grid is a Lambert conformal projection with a 1.3 km mesh, overseas a 2.5 km mesh. The 90 levels follow the terrain near the ground and the pressure aloft (a hybrid coordinate); they are packed close together in the lowest layers, the first one about 5 m above the surface, and the top lies at 10 hPa.
The physics comes from Meso-NH. Deep convection is not parameterised at all: storms grow on the grid, their clouds and precipitation handled by the ICE3 microphysics scheme, which tracks the five water species and the processes between them. Turbulence in the boundary layer is computed by a scheme that forecasts turbulent kinetic energy, and shallow, non-precipitating convection, such as fair-weather cumulus, by an eddy-diffusivity mass-flux scheme. As of 2017, radiation used the RRTM scheme in the infrared and the Fouquart-Morcrette scheme in the solar spectrum. The ground and sea are handled by SURFEX, which splits each cell into soil and vegetation, sea, lakes and towns, with relief taken from the GMTED global elevation database.
A grid computes nothing smaller than a few of its cells: at 1.3 km, AROME draws a sea breeze front, the acceleration at a headland or a squall line well, but not the individual gust under a cloud or the thermal over a beach. And because storms are computed rather than averaged, their exact place and hour depend on small details of the starting state: AROME tells where and when cells can develop more than exactly which bay a given cell will hit. Overseas, the sea surface beneath the model is not fixed: a one-dimensional ocean mixed layer lets the water cool as a cyclone stirs it, which changes the energy the storm receives.
How a run is computed
Over France, AROME starts each forecast from its own analysis, a best estimate of the present atmosphere at 1.3 km. It ingests everything the global ARPEGE model uses, surface stations, ships, buoys, radiosondes, aircraft, satellite radiances and GNSS signals, plus observations specific to fine scales: reflectivities from weather radars, turned into humidity profiles, Doppler radial winds from radars across Europe, and denser surface and aircraft data. Since the 2026 upgrade, about 4.2 million observations enter the model each day.
The analysis is renewed every hour. Since October 2024 it uses a 3DEnVar scheme: a variational analysis, which finds the atmospheric state that best fits both the previous one-hour forecast and the new observations, weighted by their expected errors, where the shape of the forecast errors is taken from an ensemble of 50 parallel assimilations run at 3.2 km every three hours. The corrections thus follow the weather of the day, stretched along a front or concentrated around a storm, instead of the fixed average shapes of the 3D-Var used from 2008 to 2024. In the published evaluation, the new scheme clearly beat 3D-Var on winter storms, fog and heavy rain events.
The long forecasts are launched eight times a day, every three hours. At the edges of its domain AROME takes its boundary conditions from the latest ARPEGE run, whose stretched global grid reaches 5 km over France, and each forecast is nudged during its first hour towards the following hourly analysis (incremental analysis update) to include the freshest observations. The runs are computed in Toulouse on Belenos and Taranis, two Atos BullSequana XH2000 supercomputers in service since 2021, each with a peak power of 10.74 petaflops; Météo-France has launched the procurement of their successors.
AROME-OM works differently: it has no assimilation of its own, because the tropics have too few dense observations, such as radars, to feed one. Each run starts from the analysis of ECMWF's global IFS, blended with the previous AROME-OM six-hour forecast so that the model's own fine-scale structures are not lost, and takes IFS boundary conditions every hour; ARPEGE supplies the surface state and Mercator Ocean's global ocean model the initial state of the ocean mixed layer. The five domains run four times a day, at 00, 06, 12 and 18 UTC, to 48 hours, and to 78 hours on request during tropical cyclone warnings. This dynamical adaptation of IFS gave better results in the tropics than a 3D-Var assimilation, according to Météo-France's researchers.
Since 1 January 2024 all of Météo-France's public data are free of charge. AROME's fields are published on regular latitude-longitude grids, 0.01° over France and 0.025° over France and overseas, in GRIB2 format, through the meteo.data.gouv.fr portal and Météo-France's public API, under the Licence Ouverte 2.0 (Etalab), which allows reuse with attribution.
At sea: the coast in detail, for two days
AROME's strength is the wind that the coast and the relief make. At 1.3 km it sees headlands, bays and valleys: it draws the onset and veering of the sea breeze along a coast, the acceleration off Cap Corse or through the Strait of Bonifacio, the Mistral and Tramontane channelled by the Rhone valley and the gap between the Pyrenees and the Massif Central, the calm in the lee of an island. These are exactly the features a 10 to 25 km global model smooths out, and they often decide a coastal passage.
Because it computes storms explicitly, AROME is the model to look at when squalls and thunderstorms threaten, in a Mediterranean autumn or a summer afternoon over the land. It shows where cells can grow, their gust fronts and the wind shifts they bring, but the exact cell is uncertain: read a storm in AROME as a risk over an area and a time window, not as an appointment, and check the PE-AROME ensemble or the radar when it matters. The wind the app uses is the mean wind at 10 m, not the gusts, which under a squall can be much stronger.
Its limits are range and scale. AROME looks only about two days ahead, and the large systems, a depression, a front, a ridge, come in through its boundaries from ARPEGE or IFS: when a front arrives early or late in the global model, it arrives early or late in AROME too. Far from the coast, over open water, it adds little to a good global model. A useful reading is to take the timing of the large systems from the global models and their ensembles, and the local shape of the wind near the coast from AROME; when AROME and the global models disagree on the coast, AROME is usually closer to what the relief does.
Overseas, AROME-OM draws the trade wind around the islands: the accelerations along the coasts and between islands, the calm and the reversals in the lee of high islands, the afternoon showers over the land. Météo-France's verification over 2018-2019 found its 10 m wind closer to observations than that of ECMWF's IFS over all five domains, and its heavy rain better detected. In a tropical cyclone, however, it starts from IFS's analysis of the storm, without observations of its own: its track and intensity are a forecast to compare, never a substitute for the official advisories of the regional cyclone centres.
In Saily WR
The app reads AROME through Météo-France's public API and keeps only the wind at 10 m, as its two components (towards the east and towards the north). Over France it takes the 0.01° grid, about 1.3 km, between 37.5N and 55.4N and between 12W and 16E, which covers France and its neighbours; it downloads the eight runs of the day, at 00, 03, 06, 09, 12, 15, 18 and 21 UTC, with hourly steps from H+1 to H+51. For the five overseas domains it takes the 0.025° grid, about 2.5 km, four runs a day at 00, 06, 12 and 18 UTC, hourly from H0 to H+48.
Each AROME domain counts as one model. When both the start and the end of your route lie inside a domain, the app computes an isochrone route with your boat's polar sailed in that domain's wind, interpolated in space between grid points and in time between hours, and shows it next to the routes of the other models that cover the same water. Given its two-day horizon, AROME is the model for day sails and coastal passages, and the one to compare against the global models when the route hugs a coast or threads between islands.
A server fetches each new run as soon as Météo-France publishes it, and the Data page shows live which run each AROME domain is on, how far ahead it goes and the area it covers. The sea current (SMOC, RTOFS or none) and land avoidance, with the OpenStreetMap coastline and an optional minimum depth, are applied to AROME's route in the same way as to every other model's.
The domains
AROME France
The France domain runs from 37.5N to 55.4N and from 12W to 16E: the whole of France and Corsica, the English Channel and the southern half of the North Sea, the Bay of Biscay and the Atlantic out to 12W, the British Isles south of 55.4N, the Benelux, Germany and the Alps, most of Spain with the Balearics, the Gulf of Lion, the Ligurian and Tyrrhenian Seas with Sardinia, and Italy west of 16E. It is the only AROME domain with its own data assimilation, from radars and dense observations, and the only one at 1.3 km and eight runs a day.
It resolves the winds that make sailing in these waters: sea breezes along the Atlantic and Channel coasts, the acceleration around Brittany's headlands, the Mistral and Tramontane fanning out over the Gulf of Lion, the wind funnelled between Corsica and Sardinia, and the Mediterranean thunderstorms of late summer and autumn. Near its western edge the Atlantic weather enters from ARPEGE, so on the open Atlantic its forecast is only as good as the global one it inherits.
AROME Antilles
The Antilles domain covers 9.7N to 22.9N and 75.3W to 51.7W: the whole arc of the Lesser Antilles from the Virgin Islands to Trinidad and Tobago, with Guadeloupe, Martinique, Saint Martin and Saint Barthelemy. It also takes in Puerto Rico, Hispaniola, the Turks and Caicos, a strip of the Venezuelan coast and a wide band of Atlantic to the east of the islands.
Here the trade wind blows almost all year from the east, and the high, volcanic islands shape it: it accelerates along their tips and in the channels between them, dies in long wind shadows to the west of Guadeloupe's Basse-Terre, Dominica or Martinique, and turns into afternoon showers over the relief. AROME's 2.5 km mesh draws these wakes and channel effects, which a global model at 10 km or more blurs. From June to November the domain lies on the path of Atlantic hurricanes, for which its range can be extended to 78 hours.
AROME French Guiana
The French Guiana domain covers 1.05N to 8.95N and 56.75W to 46.30W: French Guiana itself, most of Suriname, the coast of Amapa in Brazil down to the north of the Amazon mouth. Offshore it covers the Atlantic and the approaches to Cayenne, Kourou and the Maroni and Oyapock estuaries.
Close to the equator, the domain lives with the trade winds and the yearly passage of the intertropical convergence zone, the belt of showers and storms where the trade winds of both hemispheres meet. Winds are often moderate, and the weather is driven by convection: squall lines, sea breezes along the low coast and heavy rain over the forest. This is where an explicit storm model like AROME brings most over a global one, and where its exact timing of a squall is also the least certain.
AROME Indian Ocean
The Indian Ocean domain covers 25.9S to 3.45S and 32.75E to 67.6E, from the Mozambique Channel to Rodrigues: Réunion, Mauritius and Rodrigues, Mayotte and the Comoros, the whole of Madagascar, the coasts of Mozambique and southern Tanzania, and, since the domain was stretched about 20 % further north in 2026, the latitudes of the Seychelles' main islands. It is the largest AROME-OM domain.
The south-east trade wind dominates, and high islands bend it: around Réunion it splits and accelerates at the island's tips, leaving a calm on the west coast, and Madagascar's capes and mountains channel and block it in the same way. From November to April the domain is the playground of tropical cyclones, watched by Météo-France's centre in Réunion, the regional cyclone centre for the basin; this is where AROME-OM's coupled ocean layer and its range extended to 78 hours matter most. In the Mozambique Channel and north of Madagascar the seasonal shift between the trade winds and the north-west monsoon flow also shapes the passages.
AROME New Caledonia
The New Caledonia domain covers 26.00S to 13.75S and 158.5E to 171.5E: Grande Terre and its lagoon, the Loyalty Islands, the Isle of Pines, the Chesterfield reefs at its western edge. To the north-east it takes in most of Vanuatu.
The south-east trade wind blows along Grande Terre's long mountain chain, which splits the weather between a wetter east coast and a drier, sheltered west coast where the lagoon lies; in the afternoon the sea breeze adds to the trade wind and strengthens it inside the lagoon. AROME's 2.5 km mesh resolves the chain and these daily cycles. From November to April tropical cyclones and depressions cross the domain, between Vanuatu and New Caledonia.
AROME French Polynesia
The French Polynesia domain covers 25.25S to 12.6S and 157.5W to 144.5W: the Society Islands, Tahiti, Moorea, Bora Bora and the Leeward Islands, the western Tuamotu atolls such as Rangiroa, and the northern Austral Islands. The Marquesas, the eastern Tuamotu and the Gambier lie outside it. In 2017 this domain ran only twice a day; it now runs four times a day like the others.
The weather alternates between the east to south-east trade wind and the passage of the South Pacific convergence zone and of fronts from the south, which bring wind shifts, squalls and rain. Around Tahiti and Moorea the high relief makes accelerations and wind shadows that a global model cannot draw, while over the low atolls of the Tuamotu the wind is close to that of the open ocean and AROME mainly adds the detail of squalls and showers.
Sources
- The AROME-France convective-scale operational model, Seity et al., Monthly Weather Review (2011)
- The operational 3DEnVar data assimilation scheme for the Météo-France convective scale model AROME-France, Brousseau et al., EGUsphere preprint (2025)
- Recent progress in convective scale Arome NWP system and on-going research activities, Météo-France, WGNE-31 (2016)
- Joint WMO technical progress report on the global data processing and forecasting system and numerical weather prediction research activities for 2017, Météo-France, WMO (2018)
- Application and verification of ECMWF products 2019: France, Météo-France, ECMWF (2019)
- Operational implementation of the AROME model in the Tropics: multiscale validation of rainfall forecasts, Faure, Chambon and Brousseau, Weather and Forecasting (2020)
- Physics-dynamics aspects of the AROME model and its coupling with ocean model NEMO and wave model WW3, Malardel, ECMWF Annual Seminar (2020)
- Towards a dedicated convection-permitting ensemble in the operational NWP systems at Météo-France for the tropics, Nuissier et al., ECMWF Workshop on Model Uncertainty (2022)
- The AROME Overseas model, Météo-France (2017)
- Better-performing weather models, Météo-France (2026)
- Météo-France's supercomputers, Météo-France (2024)
- Weather models (wind, atmospheric pressure): product content description, Shom (2025)