A holistic study of the urban climate and thermal bioclimate before, during and after the extreme heat wave of July-August, 2021, in the Athens urban area in Greece

Situated in the eastern Mediterranean basin, Greece is identified as a climate change hotspot, characterized by pronounced increasing trends in temperature extremes and heat stress. People accommodated in cities are particularly at heat-related risk due to the amplifying impact of the built-up environment in the human thermal comfort conditions. Recent studies highlight the applicability of the local climate zone (LCZ) concept in investigating the diversified heat exposure within cities. For this, in the frame of the ADAPT2CC research project, the current study presents the first attempt at introducing the LCZ framework into a multi-scale numerical modeling system for examining the urban climate and thermal bioclimate in the Athens urban area (AUA), Greece, before, during and after an extreme heat wave (HW) episode that occurred during July and August of 2021. The modeling system is based on the online coupling between the state-of-the-art Weather Research and Forecasting (WRF) model and the advanced urban scheme BEP/BEM (Building Energy Parameterization/Building Energy Model). Further, the system is coupled offline with the RayMan Pro model for assessing the heat stress conditions in a physiologically consistent manner. The performance of the modeling system is evaluated against ground-based and satellite observations, with the results demonstrating a high capability of the system in reproducing the meteorological and human-biometeorological conditions during the examined period (July 25 to August 08, 2021). The numerical atmospheric analysis underlines the important contribution of physical factors (altitude) and background atmospheric circulations (sea breeze and Etesians) to the spatiotemporal variation of the urban temperatures under both non-HW and HW conditions. Concerning the heat stress conditions, the use of the modified physiologically equivalent temperature (mPET) is suitable for human-biometeorological analysis especially during HW days, when increased solar radiation and low wind speeds prevail. Overall, the long-lasting heat wave of July-August, 2021, put a great heat stress burden to the AUA population, especially in the south and central regions of the study area. In the course of a typical sea breeze day under HW conditions, the local atmospheric circulation lead to lower temperature and mPET values in the south areas of Athens compared to the city’s central regions across all LCZs. However, this cooling effect is not effective to also result in a more comfortable human thermal perception (i.e., from very hot to hot). This outcome, as well as the whole set of results of the study, could be very useful for urban design applications aiming at mitigating the impact of heat waves and urban heat in AUA.