MeteoSwiss operates and further develop a high-precision numerical weather prediction system, COSMO, in order to automatically generate regional and local forecasts products in complex topography. A detailed image of the future state of the atmosphere is computed, from the low stratosphere to the surface, including the evolution of the snow cover, the lake temperature and the soil characteristics. Standards products are available up to three days in advance on a domain covering central and western Europe (so called COSMO-7), and more detailed products are available up to 24 hours in advance on a domain including the Alpine arc (so called COSMO-2, see figure 1). These products provide a quantitative guidance to the forecasters for their daily forecasts, contribute to the security of the Swiss population by the generation of warnings, e.g. in case of high-impact weather, floods or incidents in nuclear power plants, and respond to the specific needs of many professional users. The COSMO system is permanently monitored, a prerequisite for a high forecast quality.
What is a numerical weather prediction model?
A numerical weather prediction model is based on the physical laws describing the atmospheric and surface processes (e.g. conservation of energy, transformation of phase, black body radiation), suitable initial and boundary conditions, and a numerical method to solve in time the resulting system of complex mathematical equations. These equations describe a variety of atmospheric processes on different temporal and spatial scales, for example the development of low pressure systems, snow fall, Föhn, summer convection. The calculations are performed on a three-dimensional grid, where the vertical spacing of the computational levels is inhomogeneous in order to better resolve the low level atmospheric conditions.
Suitable initial conditions for starting a forecast are produced by combining actual atmospheric observations, previous model guidance, and climatic information in a way compatible with the model equations; this is the so-called assimilation process. In a typical 24 hours assimilation period, COSMO-7 ingests about 120 vertical soundings, about 8000 aircraft observations, about 28000 surface observations and about 1000 wind profilers. In addition, COSMO-2 ingests radar-derived rainfall to better represent convective activities.
A forecast with a numerical weather prediction model is computationally very demanding and time critical. A typical 24 hours COSMO-2 forecast requires more than 1014 mathematical operations (this is 100’000 billions!) and must be computed within 20 minutes. That’s why the COSMO system is composed of 3 nested numerical models: one global model simulating the whole atmosphere but with medium precision, and two limited area models each with increased precision. That's also why the evolution of the models always went side by side with the development of the fastest supercomputers in the last twenty years.
Figure 1: The three nested numerical weather prediction models of the COSMO system. The European Center for Medium Range Weather Forecast operates a global model describing the synoptic scales. MeteoSwiss operates the regional scale COSMO-7 and the local scale COSMO-2 models.
Figure 2: The benefit of higher resolution in complex topography. Simulation of 30 meter above ground wind field in Canton de Vaud, with COSMO-7 (right plot) and COSMO-2 (left plot).
COSMO in operations at the CSCS
MeteoSwiss operates two nested COSMO models (see figure 1). The regional COSMO-7 is driven by the global model of the European Centre for Medium Range Weather Forecast (ECMWF, UK), covers most of western and central Europe, and is computed on a grid with a spacing of about 6.6 km. The local COSMO-2 is driven by COSMO-7, covers the Alpine region with Switzerland at the center, and is computed on a grid with a spacing of about 2.2 km. Both models use a grid with 60 vertical layers; the lowest level is at about 10 meter above ground and the mean layer thickness in the lowest 1000 meter above ground is better than 100 meter.
72 hours COSMO-7 forecasts are computed twice daily, at 01:45 am and 13:45 pm winter time. 24 hours COSMO-2 forecast are computed 8 times daily, each 3 hours, starting at 01:45 winter time; this latter configuration allows an optimal usage of the latest available observations and is beneficial for nowcasting and very short range forecasting. Hourly analyses, representing the current state of the atmosphere, are also produced by both models and indefinitely kept in an archiving system.
All computations are performed at the Swiss National Supercomputing Centre (CSCS, Manno), on a 896 cores Cray XT4.
A successful collaboration inside COSMO
COSMO is developed within an international collaboration. The six national weather services of Germany, Greece, Italy, Poland, Romania and Switzerland closely collaborate inside the 'Consortium for Small-Scale Modelling' (COSMO). This consortium was formed in October 1998; it's general goal is to develop, improve and maintain a non-hydrostatic limited-area atmospheric model, to be used both for operational and for research applications by the members of the consortium.
