The multivariate role of surface-atmosphere interaction in urban heat stress

Heat stress is associated with increased health risks, with disproportionately higher mortality and morbidity rates among vulnerable individuals.
The role of surface-atmosphere interaction during extreme heat events has been increasingly studied in recent years, focusing on its impact in terms of surface and near-ground temperature. This has revealed coupling dynamics that can enhance extreme temperatures when the soil is dry¹. Soil moisture has also been shown to affect thermal circulation dynamics in urban areas².

 

What is less clear is the effect of surface-atmosphere interaction on heat stress as a multivariate indicator of physiological strain. While evaporation decreases air temperature, it also has implications for the dynamics of the boundary layer, and it can be a meaningful source of air moisture. Heat stress is therefore affected by counteracting mechanisms in a complex way, but to date, this has only been investigated through simple heat stress indicators.

 

We review existing evidence of surface-atmosphere interactions during heatwaves, from soil desiccation enhancing high temperatures, to high evaporation from the soil leading to an increase in heat stress.

Using statistical tools and reanalysis datasets, we investigate the coexistence of these two contrasting effects during summers in the Euro-Mediterranean region by linking evaporation and heat stress. The latter is defined through the Universal Thermal Climate Index (UTCI), which is regarded as one of the best estimates of the actual human physiological strain.

Subsequently, the WRF model is used to simulate and quantify the impacts of the physical processes involved, with a focus on the heat stress in urban areas, estimated with the WRF-Comfort module.

 

We show that simple indicators disagree with more sophisticated ones, such as UTCI, in the identification of extreme events, with a mismatch for approximately one-fourth of the cases. Our preliminary findings suggest that evaporation from the soil can play a crucial role in enhancing heat stress during these extreme events, with up to a 1°C increase in UTCI in some locations compared to a low evaporation condition.

 

These results provide valuable insights into surface-atmosphere interaction during the summer months, which may enable the identification of impactful events often neglected by simple univariate approaches, allowing for updates to early warning systems and adaptation strategies.

 

References
1. Miralles, D. G., Teuling, A. J., Van Heerwaarden, C. C. & Vilà-Guerau De Arellano, J. Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation. Nature Geosci 7, 345–349 (2014).
2. Tabassum, A., Hong, S.-H., Park, K. & Baik, J.-J. Impacts of Changes in Soil Moisture on Urban Heat Islands and Urban Breeze Circulations: Idealized Ensemble Simulations. Asia-Pac J Atmos Sci 60, 541–553 (2024).