Heat waves in urban environments - A Destination Earth use case in the Climate Adaptation Digital Twin
Authors: Lacima Aleks , Grayson Katherine , Chavez Roberto , Versteeg Gert , Gonzalez-Reviriego Nube , Doblas-Reyes Francisco J. , Soret Albert 
Published: February 2023

Current climate projections point towards a severe increase in the intensity, duration and frequency of heat waves under climate change conditions. Such changes are not homogeneous, with certain regions of the planet presenting a higher vulnerability to these extreme events and, therefore, greater adaptation challenges. Among the areas affected by heat waves, urban environments are particularly susceptible to their impacts due to the urban heat island (UHI) effect, which magnifies the severity of heat waves inside cities and significantly increases the health-related risks associated with heat stress.&#160;Simulations produced by Global Climate Models (GCMs) (e.g. CMIP) are of crucial importance to better understand how the Earth&#8217;s climate system will evolve in the coming decades. Unfortunately, their coarse resolution, typically above 100 x 100 km, makes them unable to resolve fine-scale physical processes, including urban-scale phenomena such as the UHI. High-resolution simulations are therefore required to accurately represent physical processes that remain hidden to models with coarser representations of the climate system. GCMs with km-scale grids and sub-hourly output frequency provide the ability to study heat waves at global, mesoscale or even local level, together with an enhanced (i.e. better in physical terms) representation of the large-scale circulation systems (e.g. Rossby waves) that give rise to heat waves.&#160;In the framework of Destination Earth (DestinE), we are developing an urban use case for the Climate Adaptation Digital Twin (ClimateDT) that focuses on the climate impacts produced by extreme temperatures in urban environments. We will present the background and the current state of development of the use case, together with its associated challenges. Given the high-resolution simulations envisioned for the ClimateDT are not yet available, we will use NextGEMS cycle 2-3 data, which have similar characteristics, to present several climate indicators related to heat waves and human thermal comfort (e.g. UTCI, HWMI, EHF), with a particular focus on large metropolitan areas and their immediate surroundings, though results at global scale will be also assessed. Nonetheless, the previously mentioned high spatial and temporal resolutions imply unprecedented volumes of data, which, due to limited storage capacity, need to be streamed at model runtime, without the users ever having access to the full model output, but only to a fraction of it over a limited period of time. Therefore, the innovative streaming framework introduced by DestinE requires the use of one-pass algorithms to create statistical summaries of the simulated climate fields, which in turn places particular constraints on the development of the use case.Together with other relevant statistics, these indicators will allow us to study the spatial and temporal variability of heat waves inside urban areas, a significant knowledge gap in current climate projections. The ultimate goal of our work is to provide useful knowledge to urban planners, both in terms of storylines and climate data, which can be of use towards designing more resilient cities that are better adapted to the impacts of heat waves.