Coastal breezes and thermal comfort during a heatwave event in the southwestern Iberian Peninsula: an integrated modelling and observational study.

The frequency and impacts of heatwaves have significantly increased in the last years (1975-2020), with Spain experiencing a substantial rise in the occurrence these extreme events (Núñez-Mora, 2021). In coastal regions, breezes —driven by temperature gradients between land and sea surfaces— can play a crucial role in mitigating extreme temperatures. This study examines the impact of coastal breezes on thermal comfort during a heatwave period in the southwest of the Iberian Peninsula.

Coastal areas have undergone considerable urban development, with approximately 60% of the Spanish population residing in these regions (de Andrés et al., 2017). Consequently, urban heat exposure in these regions is shaped by meteorological variables operating across multiple spatial (meters to hundreds of meters) and temporal scales. Within cities, temperature and humidity exhibit local variations over hundreds of meters, while wind speed and shortwave/longwave radiation show highly microscale heterogeneity influenced by individual buildings and fluctuating over just a few meters.

To assess the impact of coastal breezes on thermal comfort, we analyse observational data from meteorological stations and radiosoundings launched at strategic coastal sites during sea breeze conditions. We also employ the Weather Research and Forecasting (WRF) model with the urban parameterization WRF-Comfort (Martilli et al., 2024). This integrated approach enables us to evaluate the thermoregulatory effects of coastal breezes and compare the simulations against both surface and vertical atmospheric observations.

Understanding the vertical structure of the atmospheric boundary layer (ABL) is crucial, particularly for sea breezes, phenomena theoretically characterised by surface inflow and upper-level return flow. However, this simplified view is rarely captured fully by observations alone, due to the complexity, high variability, and heterogeneity of the various factors influencing the ABL's vertical structure. Therefore, this study leverages the WRF model to investigate the vertical characteristics of the coastal urban boundary layer during heatwave events, complementing the surface and upper-air observational analyses.

Our findings offer insights into mesoscale interactions between urban dynamics and regional climate processes during extreme heat events, highlighting the importance of integrating mesoscale modelling with urban-scale processes and evaluating against comprehensive observational datasets to better understand and potentially mitigate the impacts of weather extremes in coastal urban environments.