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This comment focuses on the statistical limitations of a model grading, asapplied by D. Waugh and V. Eyring (2008) (WE08). The grade <i>g</i> is calculatedfor a specific diagnostic, which basically relates the difference of means of modeland observational data to the standard deviation in the observationaldataset.We performed Monte Carlo simulations, which show that this method has the potential to lead tolarge 95%-confidence intervals for the grade. Moreover, the differencebetween two model grades often has to be very large to become statisticallysignificant. Since the confidence intervals were not considered in detailfor all diagnostics, the grading in WE08 cannot be interpreted,without further analysis. The results of the statistical tests performed in WE08agree with our findings. However, most of those tests are based on specialcases, which implicitely assume that observations are available without anyerrors and that the interannual variability of the observational data and themodel data are equal. Without these assumptions, the 95%-confidence intervals become even larger.Hence, the case, where we assumed perfect observations (ignored errors), providesa good estimate for an upper boundary of the threshold, below that agrade becomes statistically significant. Examples have shown that the 95%-confidence interval may even span the wholegrading interval [0, 1]. Without consideringconfidence intervals, the grades presented in WE08 do not allow to decide whethera model result significantly deviates fromreality. Neither in WE08 nor in our comment it is pointed out,which of the grades presented inWE08 inhibits such kind of significant deviation.However, our analysis of the grading method demonstrates the unacceptably highpotential for these grades to be insignificant.This implies that the grades given by WE08 can not be interpreted by the reader.We further show that the inclusion of confidence intervalsinto the grading approach is necessary,since otherwise even a perfect modelmay get a low grade

Comment on &quot;Quantitative performance metrics for stratospheric-resolving chemistry-climate models&quot; by Waugh and Eyring (2008)

Comment on "Quantitative performance metrics for stratospheric-resolving chemistry-climate models" by Waugh and Eyring (2008)