Benefits of increasing the reforecast ensemble size of the Météo-France seasonal prediction system

The current Météo-France seasonal prediction system (MF System 8) has 25 members for hindcast from 1993 to 2016 and 51 members for real-time forecast. In order to investigate the benefits of increasing the ensemble size within our Copernicus Climate Change Services (C3S) seasonal prediction contract, we extended the system 8 hindcast to 51 members for the four main start dates (February, May, August, November). We compare the forecast skill between the official 25-member hindcast and the 51-member extended hindcast.

We focus on the European region at the lead-time 1 for the next trimester. To describe the performance of forecasts, we use correlation and relative operating characteristic (ROC) on the mean 2-meter temperature (T2M) and the mean precipitation (RR) averaged over Europe. Similarly, we also evaluate the forecast of modes of variability (East Atlantic, North Atlantic Oscillation, and Scandinavian Blocking) which impact the European climate.

The scores with 51 members are similar and not necessarily better than with 25 members. Moreover, we use 1000 random draws of 25 members out of 51 to determine the uncertainty of the official forecast scores. These scores can be at the edge of the confidence interval, while the 51-member scores are close to the median of the 1000 random draws. We did the same analysis for different regions that there is less uncertainty on the scores in the Tropics (e.g. Northeast Brazil) than in the mid-latitudes (e.g. Europe). These results suggest that it is not necessary to increase the ensemble size for verification of the seasonal forecasts beyond the available 25 members.