Climate change impact assessment on urban rainfall extremes and urban drainage: Methods and shortcomings P. Willems a,b, ⁎, K. Arnbjerg-Nielsen c , J. Olsson d , V.T.V. Nguyen e a Katholieke Universiteit Leuven, Hydraulics Division, Kasteelpark Arenberg 40, BE-3001 Leuven, Belgium b Vrije Universiteit Brussel, Dept. of Hydrology and Hydraulic Engineering, Pleinlaan 2, BE-1050 Brussels, Belgium c Technical University of Denmark, Department of Environmental Engineering, DK-2800 Lyngby, Denmark d Swedish Meteorological and Hydrological Institute, Research & Development (hydrology), SE-601 76 Norrkoping, Sweden e Mc Gill University, Department of Civil Engineering and Applied Mechanics, Montreal, Quebec, Canada H3A 2K6 article info abstract Article history: Received 14 July 2010 Received in revised form 3 April 2011 Accepted 8 April 2011 Cities are becoming increasingly vulnerable to flooding because of rapid urbanization, installation of complex infrastructure, and changes in the precipitation patterns caused by anthropogenic climate change. The present paper provides a critical review of the current state-of-the-art methods for assessing the impacts of climate change on precipitation at the urban catchment scale. Downscaling of results from global circulation models or regional climate models to urban catchment scales are needed because these models are not able to describe accurately the rainfall process at suitable high temporal and spatial resolution for urban drainage studies. The downscaled rainfall results are however highly uncertain, depending on the models and downscaling methods considered. This uncertainty becomes more challenging for rainfall extremes since the properties of these extremes do not automatically reflect those of average precipitation. In this paper, following an overview of some recent advances in the development of innovative methods for assessing the impacts of climate change on urban rainfall extremes as well as on urban hydrology and hydraulics, several existing difficulties and remaining challenges in dealing with this assessment are discussed and further research needs are described. © 2011 Elsevier B.V. All rights reserved. Keywords: Climate change Rainfall extremes Downscaling methods Urban drainage 1. Introduction For more than a century sewer systems have been constructed at large scale across cities worldwide. These sewer systems have reduced the vulnerability of the cities in general, but at the same time could make them more vulnerable to rainfall extremes, partly due to the lack of consideration to what occurs when the design criteria are exceeded. Next to this increase in the vulnerability, there is strong evidence that due to the global warming the probabilities and risks of sewer surcharge and flooding are changing. In their Fourth Assessment Report (AR4) the Intergovernmental Panel on Climate Change (IPCC, 2007) indeed reports for the late 20th century a worldwide increase in the frequency of extreme rain storms as a result of global warming. Based on climate model simulations with different future greenhouse gas emission scenarios, IPCC (2007) furthermore concluded that it is very likely (defined as more than 90% likelihood) that this trend will continue in the 21st century. Water managers therefore have to start accounting for these effects. Consequently, the number of hydrological impact studies of climate change strongly increased in recent years. These studies, however, most often focus on risk of floods and droughts on river catchment scale. The number Atmospheric Research 103 (2012) 106–118 ⁎ Corresponding author at: Katholieke Universiteit Leuven, Hydraulics Division, Kasteelpark Arenberg 40, BE-3001 Leuven, Belgium. Tel.: +32 16 321658; fax: +32 16 321989. E-mail addresses: Patrick.Willems@bwk.kuleuven.be (P. Willems), karn@env.dtu.dk (K. Arnbjerg-Nielsen), jonas.olsson@smhi.se (J. Olsson), van.tv.nguyen@mcgill.ca (V.T.V. Nguyen). 0169-8095/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2011.04.003 Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos