COSMO forecasting system

Forecasting systems calculate future atmospheric conditions. MeteoSwiss uses the COSMO (Consortium for Small-scale Modeling) numerical weather forecasting model for the production of regional and local forecast products in the topographically challenging Alpine region. Together with forecasts by the European Centre for Medium-Term Weather Forecasts (ECMWF), the various configurations of COSMO used by MeteoSwiss form the basis for the daily weather forecasts produced by MeteoSwiss, as well as the meteorologists' warnings of extreme weather conditions such as storms and precipitation events.

Thanks to complex computer models, it is possible to simulate how the weather will develop. MeteoSwiss uses the COSMO numerical forecasting model for this purpose. The model calculates high-resolution weather forecasts for the Alpine region several times a day. 

The COSMO forecasting model calculates future atmospheric conditions in detail from ground level up to an altitude of around 20 kilometres. These calculations also cover the development of land-surface properties, snow cover, and lake temperatures. The model is being developed in close cooperation with international partners.

COSMO: a numerical weather prediction model

A numerical weather forecasting model describes processes which take place in the atmosphere and on the Earth's surface. It is based on physical laws, such as the conservation of energy, mass and momentum, as well as simulating phenomena such as the phase transitions of water and radiation processes.

Based on appropriate initial and boundary fields, future atmospheric conditions can be calculated. This allows for a variety of atmospheric processes to be described on different temporal and spatial scales (e.g. development of an area of low pressure, foehn, snowfall wind, convection). The calculations are made on a three-dimensional grid. The vertical distances between the grid points are smaller at lower altitudes than at high altitudes to allow for a more detailed description of phenomena close to the ground.

The COSMO models: COSMO-1 and COSMO-E

Low-resolution numerical weather models that cover the entire globe are not precise enough to be able to create regional or even local forecasts. The weather forecasts for Switzerland – with its complex topography – are therefore based on small-scale models. MeteoSwiss uses models with varying levels of resolution depending on the different tasks at hand. Resolution is defined by the distance between grid points for which the weather parameters are calculated.


High-resolution model for short-range weather forecast for the current and next day. The distance between grid points is 1.1 km, and the area covered by the model is the entire Alpine region.


This ensemble model calculates a probabilistic forecast based on 21 individual model runs. This kind of probabilistic forecasting model allows meteorologists to determine the most probable weather development as well as the reliability of a forecast. In addition, it is also possible to calculate the probability of particular weather events occurring. COSMO-E calculates the weather up to five days ahead. The distance between the grid points is 2.2 km, and the area covered by the model is also the entire Alpine region.

Assimilation of weather observations with COSMO

Optimum starting fields for initiating a model are obtained by combining observation data, previous model runs and climatological information. The incorporation of this data in the current model run is referred to as assimilation. The assimilation cycle for COSMO-1 and COSMO-E generates a new analysis once an hour. These analyses form the initial conditions for the forecasts. In this process, the assimilation cycle uses all available observations at ground level as well as, and especially, in the atmosphere, in order to be able to describe the atmospheric conditions as accurately and as physically consistently as possible. Radar data are also assimilated, as these are particularly valuable for the first few hours of a forecast.

Calculation of the COSMO models

The generation of forecasts using numerical weather forecasting models places high demands on computing power. For a 24-hour forecast created using the COSMO-1 model, more than four quadrillion computing operations are typically required. All calculations are performed at the Swiss National Supercomputing Centre (CSCS) on the massive-parallel "Piz d'Es-cha" computer. COSMO-1 forecasts are initiated eight times a day, every three hours, and make predictions 33 hours ahead, and up to 45 hours ahead for the 03 UTC run. The COSMO-1 forecasting products are available 100 minutes after initiation (i.e. at 01:40 UTC, 04:40 UTC, etc.). COSMO-E produces forecasts for 5 days ahead, and are computed twice a day, starting at 00 UTC and 12 UTC. The COSMO-E forecast products are available from 04:00 UTC and 16:00 UTC.

COSMO: successful international cooperation

To allow weather forecasts to become even more accurate in future, MeteoSwiss is developing the COSMO numerical forecasting model on an ongoing basis in collaboration with international partners. The national weather services of Germany, Greece, Israel, Italy, Poland, Romania, Russia and Switzerland are working together closely within the framework of the Consortium for Small-scale Modeling (COSMO). This consortium was founded in October 1998 with the objective of developing and continuously improving a non-hydrostatic, regional atmospheric model. The COSMO model is used for operational and research purposes.

Further information