Optimally choosing small ensemble members to produce robust climate simulations.

This study examines the subset climate model ensemble size required to reproduce certain statistical characteristics from a full ensemble. The ensemble characteristics examined are the root mean square error, the ensemble mean and standard deviation. Subset ensembles are created using measures that consider the simulation performance alone or include a measure of simulation independence relative to other ensemble members. It is found that the independence measure is able to identify smaller subset ensembles that retain the desired full ensemble characteristics than either of the performance based measures. It is suggested that model independence be considered when choosing ensemble subsets or creating new ensembles.

Figure 1: Difference between the full 30 member ensemble and the given sub-ensemble for the spatial average RMSE, mean and standard deviation (s.d.). The sub-ensembles are formed from the top ranking members determined using either an independence (IND), or performance (climatological, PERc, or impact, PERi) based measure. The gray region represents 1%, 1% and 5% of the full ensemble value respectively.