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. In order to be able to provide optimal probability forecasts for as many uses as possible, MeteoSwiss deploys two different ensemble configurations of COSMO. Together with the ECMWF forecasts, these form the basis for the daily weather forecasts produced by MeteoSwiss, as well as warnings of extreme weather conditions such as storms and precipitation events. COSMO-1E comprises an ensemble of 11 forecasts at a resolution of 1.1 km, calculated eight times a day for central and southern Europe. COSMO-2E is calculated four times a day from 21 realisations at a resolution of 2.2 km.
COSMO forecasting system
Numerical weather forecasts with the COSMO weather forecasting model
Thanks to complex computer models, it is possible to simulate how the weather will develop. A numerical weather forecasting model describes the weather-forecast-related 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. Using the 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. These calculations also cover the development of land-surface properties, snow cover, and lake temperatures.
MeteoSwiss uses various configurations from the COSMO numerical forecasting model and calculates high-resolution weather forecasts for the Alpine region several times a day. The COSMO model is being developed in close cooperation with international partners. All the MeteoSwiss calculations are performed at the Swiss National Supercomputing Centre (CSCS) on the massive-parallel "Pigne d’Arolla” supercomputer.
COSMO-1E and COSMO-2E: Probabilistic forecasts for the Alpine region
The COSMO-1E and COSMO-2E forecasting systems based on the COSMO model calculate the future evolution of the atmospheric conditions for the entire Alpine region, with Switzerland in the centre of the model domain. The models are calculated as ensembles, which means that, for one forecast, various iterations are produced that depict the possible future atmospheric conditions. This allows the probability of particular weather events to be deduced, as well as the most probable weather developments for the coming days. Moreover, the quality of short to medium range forecasts for extreme or highly localised weather events is improved in comparison to a (deterministic) forecast, and the reliability of the forecasts can also be estimated.
In contrast to the low-resolution global IFS ENS model of the European Centre for Medium-Range Weather Forecasts (ECMWF), which has a resolution of around 20 km, COSMO-1E and COSMO-2E have high spatial resolutions of 1.1 km and 2.2 km respectively. This allows better probability forecasts, particularly for extreme events such as storms and heavy precipitation, to be produced for Switzerland, with its detailed and complex topography
The boundary conditions, or, in other words, the weather-related information from outside the model domain, are received by the two ensemble systems from the IFS ENS global ensemble system. The initial conditions for the forecast – the analyses – are ascertained using measurement data derived with the help of an ensemble data assimilation system. Thanks to this process, not only can a consistent, optimal ensemble of atmospheric conditions be determined, but the uncertainty of this best-estimate ensemble for the current atmospheric conditions can also be worked out from the distribution of the initial conditions.
Why do we calculate ensembles?
Minor errors in the recording of the current atmospheric conditions can have a major impact on the reliability of weather forecasts. Even the boundary data from the global model, the ECMWF’s IFS ENS, which is used to drive COSMO-1E and COSMO-2E, have inherent errors which lead to uncertainties in the regional forecasting. Finally, the model itself is not perfect, which also results in uncertainty in the forecasts.
The aim of ensemble or probabilistic forecasting is to represent and take all of these uncertainties into consideration to the greatest extent possible. To do this, numerous forecasts are calculated, each with slightly different initial and boundary conditions as well as differing stochastic model peturbations. Based on this multitude of forecasts, otherwise known as ensemble members, the likelihood of a certain event occurring can be calculated. The spread of the ensemble also provides an assessment of the predictability of the weather conditions, allowing the reliability of the forecast to be quantified.
The high-resolution ensemble forecasting system, COSMO-1E, with its 11 ensemble members, is calculated eight times a day with a horizontal grid size of 1.1 km. Every three hours (00:00, 03:00, ... 21:00 UTC), the evolution of the weather is newly calculated for a forecast period of up to 33 hours. The model run that starts at 03:00 UTC has an extended forecast time (up to 45 hours), so that the forecast covers the whole of the following day. On the horizontal plane, the model domain of COSMO-1E covers 1075 x 691 grid points. It covers the entire Alpine region, with Switzerland at the centre of the model domain. Vertically, COSMO-1E extends through 80 layers, up to an altitude of 22 km. The model topography reaches 4,268 metres above sea level at its highest point. M.. At each of the 73,569,600 grid points, the weather is predicted at 10-second intervals.
COSMO-2E, the ensemble forecasting system with the longer forecast period of five days, consists of 21 ensemble members and is calculated four times a day (00:00, 06:00, 12:00 and 18:00 UTC) with a horizontal grid size of 2.2 km.
With 538 x 346 grid points, the model domain likewise covers the whole Alpine region. On the vertical plane, COSMO-2E rises through 60 levels, up to an altitude of 22 km. The forecast for the 13,618,800 grid points is calculated at 20-second intervals.
Forecast products are available for both models within the following corner points:
- NW corner 49.52° N, 0.16° E;
- NE corner 49.73° N, 16.75° E;
- SW corner 42.67° N, 1.33° E;
- SE corner 42.85° N, 15.94° E
This represents 1075 x 691 grid points for COSMO-1E and 538 x 346 grid points for COSMO-2E and contains the entire domain with the exception of a small boundary line which is unusable due to boundary effects.
COSMO: successful international cooperation
To further increase the accuracy of weather forecasts in the future, MeteoSwiss is continually developing the numerical forecasting model 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 modelling (COSMO). This consortium was founded in October 1998 with the objective of developing and continuously improving a non-hydrostatic, regional atmospheric model. This model is used for operational and research purposes.