The Modified Rainfall Anomaly Index (mRAI)—is this an alternative to the Standardised Precipitation Index (SPI) in evaluating future extreme precipitation characteristics?

Precipitation extremes affect various economic sectors and may result in substantial costs for societies. Future projections of such extreme occurrences are needed to successfully develop robust regional adaptation strategies. Model ensemble-based approaches provide a higher level of confidence since they compensate to some degree for the uncertainties of individual climate model projections. An ensemble of twelve regional climate projections from five regional climate models was used to evaluate the suitability of a modified version of the Rainfall Anomaly Index (mRAI) as an alternative to the Standardised Precipitation Index (SPI) in assessing future precipitation conditions. We compared frequency distributions and trends of the mRAI with the SPI for a test region that is climatologically representative of Central Eastern Europe. Both indices are highly correlated with each other at all tested timescales—both for stations and for regionally averaged data—with Pearson correlation coefficients >>0.9 and Spearman correlation coefficients >0.99. There are no significant differences in their frequency distributions, although the mRAI shows slightly higher frequencies in the classes of ‘moderately dry’ to ‘very dry’ conditions. The change signals revealed by SPI and mRAI are very similar for mean changes as well as for changes in the extremes. Considering the large bandwidth of change signals of individual regional climate projections, the mRAI provides sufficiently robust results for the evaluation of future precipitation anomaly trends. The notably more complex calculation of the SPI has no appreciable advantage for this application.