Assessment of Unforced Variability as a Source of Uncertainty in Regional Hydroclimate Simulations and Projections

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Nested Regional Climate Models (RCMs) are the main tool used worldwide to generate climate change projections and long simulations at scales suitable for hydrological studies. Funds to pursue my innovative non-directed research are required to investigate issues that are vital for the estimation of uncertainty in regional climate projections and simulations for water management. The general perception is that GCM or RCM internal variabilities (IV) are minor contributors to the overall uncertainty in comparison to uncertainty associated to emissions scenarios or to deficiencies in GCMs. However, De Wit et al. (2007) stated that different RCMs forced by the same GCM generate different mean seasonal precipitation and evaporation patterns and Horton et al. (2006) found that the simulated discharges for a given emissions scenario show large variability induced by both, the driving GCMs and the driven RCMs. These studies demonstrated that a RCM ensemble forced by the same GCM could generate variability in water related variables of the same magnitude as the variability produced by using the same RCM driven by different GCMs. Recently, Musíc and Caya (in preparation) showed that the GCM IV contribution to uncertainty in runoff at the watershed scale can be as large as the one introduced by using a different GCM. Braun et al. (2011) showed that the IV in RCM using large domains could reach values close the GCM IV in analysis of runoff made at the watershed scale. Therefore, the unforced variability in both GCMs and RCMs has to be taken into account in every hydrology studies using RCMs. All of the above studies neglected the contribution from the unforced variability. The present proposal wants to estimate the spatial distribution of the unforced variability and its relative contribution to the overall uncertainty. The ensemble generated by Ouranos (under my leadership) that was designed to address this unforced variability analysis is now complete and available. A complete analysis of the unforced variability will be conducted using this recently available ensemble of regional simulations.