An integrated approach to project the future urban climate response: Changes to Lisbon's urban heat island and temperature extremes

Most state-of-the-art climate simulation ensembles misrepresent the effects of urbanization on local climate due to computational restrictions, posing a major limitation for urban climate projections. We circumvent this shortcoming by employing the SURFEX land-surface model coupled to TEB (Town Energy Balance) urban canopy model, forced by EURO-CORDEX regional climate simulations. We demonstrate the added-value of CORDEX-SURFEX-TEB compared to EURO-CORDEX in reproducing Lisbon's temperature observations, halving the bias and improving the PDF matching by similar to 5%. Furthermore, the proposed framework allows characterizing the urban heat island (UHI) effect. Over Lisbon, the annual-mean UHI was similar to+3 degrees C (city warmer than rural) during nighttime, and around -0.5 to -1 degrees C (city cooler than rural) during daytime. The CORDEX-SURFEX-TEB climate projections for Lisbon, under fixed land-use and RCP8.5 scenario, showed a warming of similar to+1 degrees C by 2021-2050 and similar to+3.5 degrees C by 2070-2099. Despite the large multi-model spread, these long-term temperature changes were roughly constant for all hours and for urban and rural environments (corresponding to roughly constant UHI magnitude). Our results also reveal large seasonality of Lisbon's UHI effect and of long-term warming. Finally, the projections show a striking growth in the number of extreme hot days and nights, respectively nearly tripling and quintupling its current value by 2070-2099.