Investigating the future urban climates of European cities using an ensemble of convection permitting regional climate models

Urban areas modify natural land surfaces through the artificialization and sealing of surfaces, which has an impact on their exchanges of energy, water and momentum with the atmosphere. These modifications create specific meteorological conditions in cities, generally referred to as “urban climate”. The most notable – and most studied – climate effect of urbanisation is the increase in nighttime near surface air temperature, known as the Urban Heat Island (UHI).

Future climate change projections are most often derived from Global Climate Models (GCMs), which are downscaled to the regional scale using statistical tools, or by limited-area regional climate models (RCMs). Because of their horizontal resolution, which often remains too coarse, RCMs cannot represent most urban areas adequately and, as a result, little is known about projected changes in UHIs in the future. A few studies have examined the projected evolution of UHIs in the context of climate change using different approaches ranging from GCMs and RCMs to high-resolution land surface models, but with little agreement between studies and great sensitivity to the city analyzed, its climate and, more particularly, the downscaling approach used.

The latest generation of RCMs, known as Convection Permitting Regional Climate Models (CPMs), now reach horizontal resolutions of the order of a few kilometers, and can be coupled with various urban parameterizations to improve the representation of the urban climate in climate change projections.

Here we use transient (i.e. GCM-driven) climate simulations from the EURO-CORDEX initiative (12.5 km RCM) and the CORDEX Flagship Pilot Study on Convection (3 km CPM). We study the future UHIs of several European cities at the end of the century (2090-2099) under a scenario of very high greenhouse gas emissions (RCP8.5) and in which the cities are fixed in their historical states.

In particular, our analysis focuses on:

• The concordance between the previous generation of climate projections from RCMs and the most recent one using CPMs

• The potential differences due to the multiple urban parameterizations used between RCMs and CPMs, and within the CPM ensemble

• The expected changes in UHIs under projected regional climate conditions at the end of the 21^st century, and their potential effects on certain impact indicators

• The possibility of highlighting the physical drivers of potential future UHI changes

Acknowledgment: This work was conducted in the CIRCE project (City-oriented Impacts of Regional Climate for Europe) funded by the European Commission under the Marie Skłodowska-Curie Actions (MSCA) (Grant agreement ID: 101067769).