Subsampling a multimodel seasonal forecast ensemble in the Mediterranean region

Regional Climate Outlook Forums gather experts in climate prediction from operational centers, sharing knowledge and bridging the gap between research, operational centers and users, with the aim to produce seasonal forecast outlooks. Traditionally, the production of information followed a subjective approach of analysis and discussion. Although this procedure adds value from expert knowledge, the process is difficult to trace and reproduce, and not suitable for generating products and applications for decision making. Identifying this issue, WMO encourages Regional Climate Centers and RCOFs to develop objective seasonal forecast procedures. As a step towards that goal in the context of the Mediterranean Climate Outlook Forum (MedCOF), we explored ways of developing an objective approach that adds value to raw model forecasts in the Mediterranean region.
Some works have shown potential improvements in skill by selecting specific members of a multimodel ensemble. Here, we propose a methodology based on comparing the trajectory of ensemble members with the observations, retaining those members whose trajectories were the closest to observed values. For that, we used lead 2 and lead 3 forecasts, which in some cases (particularly for Dec-Feb winter) showed comparable skill to lead 1 forecasts. Then, we compared the state of each ensemble member with the latest ERA5 observations available before the period to be forecast. We kept those members who were closer to observed values and calculated the forecast for months 2-4 (lead 2) or 3-5 (lead 3) of the model run. Comparisons were based on precipitation and sea level pressure observations, but other sources of information could be used, depending on relevant elements for predictability in a particular area. We found local skill improvements by using this method.