Sensitivity of runoff and projected changes in runoff over Quebec to the update interval of lateral boundary conditions in the Canadian RCM

An analysis was carried out of the sensitivity of runoff simulations from version 4.2 of the Canadian Regional Climate Model (CRCM) to the frequency of lateral boundary condition (LBC) forcing at 6 and 12-hour intervals. The motivation for this study was that some climate model output may only be available at a 12-hour interval and it is important to know if CRCM runs with these outputs are comparable to runs made with 6-hourly forcing. The LBC sensitivity was assessed over two different regional domains (North America and Quebec) for annual runoff simulated over 21 river basins located in the Quebec/Labrador peninsula. The sensitivity results were compared with the CRCM's internal variability and natural climate variability to reach conclusions about the relative importance of LBC update frequency. The results show that LBC frequency can have a significant influence on mean annual runoff over the investigated basins when the simulation domain is relatively small, as in the case of the Quebec, but not for the larger North American (AMNO) domain runs. The entire ensemble of five members of the Canadian Coupled Global Climate Model (CGCM3) can therefore be safely used to generate dynamically downscaled projections over the basins, even though three of the members were archived at a 12-hourly interval. Climate projections for the 2041-2070 horizon (with SRES-A2), from a five-member ensemble of CRCM 45-km runs performed over the AMNO domain (driven by each of the five CGCM3 members), project an increase of annual runoff over all investigated river basins with the largest changes towards the north. This ensemble also provides an estimate of uncertainty of projected basin runoff change related to natural variability, but there remains a need to consider additional projections (more RCMs, more driving GCMs) to produce a more complete assessment of uncertainty.