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.