Development and validation of the Climate Model Confidence Index (CMCI): measuring ability to reproduce historical climate conditions

Abstract

This study further develops and finally validates the Climate Model Confidence Index (CMCI) as a simple and effective metric for evaluating and ranking the ability of climate models to reproduce historical climate conditions. Modelled daily climate data outputs from two different statistical downscaling techniques (PCIC: Pacific Climate Impacts Consortium; SDSM: Statistical Down-Scaling Model) are compared with observational data recorded by Environment Canada weather stations located in Kelowna, BC (Canada), for the period from 1969 to 2005. Using daily data (N > 13,000), Student’s t-tests determined if there were statistically significant differences between the modelled and observed means while ANOVA F-tests identified differences between variances. Using aggregated annual data (N = 37), CMCI values were also calculated for the individual model runs from each statistical downscaling technique. Climate model outputs were ranked according to the absolute value of the t statistics. The 20 SDSM ensembles outperformed the 27 PCIC models for both minimum and maximum temperatures, while PCIC outperformed SDSM for total precipitation. Linear regression determined the correlation between the absolute value of the t statistics and the corresponding CMCI values (R2 > 0.99, P < 0.001). Rare discrepancies (< 10% of all model rankings) between the t statistic and CMCI rankings occurred at the third decimal place and resulted in a one rank difference between models. These discrepancies are attributed to the precision of the t tests which rely on daily data and consider observed as well as modelled variance, whereas the simplicity and utility of the CMCI are demonstrated by only requiring annual data and observed variance to calculate.