Abstract:There is now a large published literature on the strengths and weaknesses of downscaling methods for different climatic variables, in different regions and seasons. However, little attention is given to the choice of downscaling method when examining the impacts of climate change on hydrological systems. This review paper assesses the current downscaling literature, examining new developments in the downscaling field specifically for hydrological impacts. Sections focus on the downscaling concept; new methods; comparative methodological studies; the modelling of extremes; and the application to hydrological impacts.Consideration is then given to new developments in climate scenario construction which may offer the most potential for advancement within the 'downscaling for hydrological impacts' community, such as probabilistic modelling, pattern scaling and downscaling of multiple variables and suggests ways that they can be merged with downscaling techniques in a probabilistic climate change scenario framework to assess the uncertainties associated with future projections. Within hydrological impact studies there is still little consideration given to applied research; how the results can be best used to enable stakeholders and managers to make informed, robust decisions on adaptation and mitigation strategies in the face of many uncertainties about the future. It is suggested that there is a need for a move away from comparison studies into the provision of decision-making tools for planning and management that are robust to future uncertainties; with examination and understanding of uncertainties within the modelling system.

Heavy rainfall extremes are intensifying with warming at a rate generally consistent with the increase in atmospheric moisture, for accumulation periods from hours to days.• In some regions, high-resolution modeling, observed trends and observed temperature dependencies indicate stronger increases in short-duration, sub-daily, extreme rainfall intensities, up to twice what would be expected from atmospheric moisture increases alone.• Stronger local increases in short-duration extreme rainfall intensities are related to convective cloud feedbacks but their relevance to climate change is uncertain due to modulation by changes to temperature stratification and large-scale atmospheric circulation• The evidence is unclear whether storm size will increase or decrease with warming; however, increases in rainfall intensity and the spatial footprint of the storm can compound to give significant increases in the total rainfall during an event.• Evidence is emerging that sub-daily rainfall intensification is related to an intensification of flash flooding, at least locally. This will have serious implications for flash flooding on much of the planet and requires urgent climate-change adaptation measures.

Identifying mechanisms driving spatially heterogeneous glacial mass-balance patterns in the Himalaya, including the "Karakoram anomaly", is crucial for understanding regional water resource trajectories. Streamflows dependent on glacial meltwater are strongly positively correlated with Karakoram summer air temperatures, which show recent anomalous cooling. We explain these temperature and streamflow anomalies through a circulation system -the Karakoram Vortexidentified using a regional circulation metric that quantifies the relative position and intensity of the westerly jet. Winter temperature responses to this metric are homogeneous across South Asia, but the Karakoram summer response diverges from the rest of the Himalaya. We show that this is due to seasonal contraction of the Karakoram Vortex through its interaction with the South-Asian monsoon. We conclude that interannual variability in the Karakoram Vortex, quantified by our circulation metric, explains the variability in energy-constrained ablation manifested in river flows across the Himalaya, with important implications for Himalayan glaciers' futures.

[1] Using the results from multimodel ensembles enables the assessment of model uncertainty in present and future estimates of extremes and the production of probabilities for regional or local-scale change. Six regional climate model (RCM) integrations from the PRUDENCE ensemble are used together with extreme value analysis to assess changes to precipitation extremes over Europe by 2070-2100 under the SRES A2 emissions scenario, investigating the contribution of the formulations of global (GCM) and regional climate models to scenario uncertainty. RCM ability to simulate precipitation extremes is evaluated for a UK case study. RCMs are shown to underestimate 1 day return values but reasonably simulate longer-duration (5 or 10 day) extremes. A multimodel approach by which probabilities can be produced for regional or local-scale change in extremes is then developed. A key result is that all RCMs project increases in the magnitude of short-and long-duration extreme precipitation for most of Europe. Individual model projections vary considerably but are independent of changes in mean precipitation. The magnitude of change is strongly influenced by the driving GCM but moderated by the RCM, which also influences spatial pattern. Therefore, when designing future ensemble experiments (1) the number of GCMs should at least equal the number of RCMs and (2) if spatial pattern is important then integrations from different RCMs should be incorporated. For impact studies, both the resolution and number of models in the ensemble will influence projections of change. The use of a multimodel approach therefore provides more robust estimates.

No abstract