Probabilistic forecasts with COSMO-LEPS

The European Centre for Medium-Range Weather Forecasts (ECMWF, Reading GB) calculates such ensembles globally, albeit with a very coarse resolution (horizontal mesh-size of ~50 km). COSMO-LEPS, initally developed at ARPA-SIM in Bologna, Italy, provides daily probabilistic forecasts at a very high resolution (horizontal mesh-size of 10 km) based on a 16-member ensemble for central and southern Europe (Figure 2). The driving initial and lateral boundary fields for COSMO-LEPS are representative members of the global ECMWF ensemble. The reason for selecting 16 representative members out of the 51 member global ensemble is purely economical: it is currently not possible to calculate as many as 51 forecast at this high horizontal resolution. Research results however show that the high resolution of the COSMO-LEPS forecasts provides better results for the Alpine region, especially for extreme events such as storms and heavy precipitation.

Based on the COSMO-LEPS forecasts, new products have been developed at MeteoSwiss, such as probability maps for precipitation (cf. figure 1). These maps depict the probability for a specific event to take place. The indication of e.g. '70% for precipitation greater than 20 mm' effectively means that this value is exceeded 7 out of 10 times. Another new product is the so-called meteogram, which summerizes the forecasts for a specific location for the next few days (cf. figure 3). It shows the 'ensemble mean (red line)', the centred region within which the observation will lie with a probability of 50% (grey area), the minimum and the maximum of the ensemble, respectively (dashed lines), as well as the COSMO forecast (blue line). The scatter of the different curves is a direct measure of the uncertainty of the forecast.