Surface Urban Heat Island in Bolzano (Italy): Evaluating the Role of Morphometric and Biophysical Characteristics

Urbanization continues to accelerate, driving global warming change and, at more local scale, land cover changes. In cities, new surface materials, buildings, roads and changes to the surface morphology alter airflow and heat exchange between the urban surface and the atmosphere. As a result, cities are almost always warmer than their surroundings rural area in a phenomenon known as Urban Heat Island (UHI) that could represent a hazard for city inhabitants. Consequently, it is important to evaluate the magnitude of the UHI and understand the urban characteristics involved in its formation process.

The aim of the present study is to assess the Surface Urban Heat Island (SUHI) in Bolzano urban area evaluating its correlation with the urban morphology and its biophysical characteristics. The indices considered to describe the urban morphology are Building Coverage Ratio (BRC), Building Volume Density (BVD), Mean Building Height (MBH), Green Space Ratio (GRS), and Sky View Factor (SVF) at 30 m resolution. The biophysical indices considered are albedo, Normalized Difference Built-up Index (NDBI), Normalized Difference Vegetation Index (NDVI), and Land Surface Temperature (LST) at 30 m resolution.

The morphological indices were calculated starting from building, green area, land cover data, and DEM, whereas biophysical indices were derived from Landsat 8/9 OLI/TIRS satellite images. Two images, one for the summer season and one for the winter season, were selected based on air temperature and absence of clouds: 07/19/2022 during a 7-days period of very high temperatures and 02/14/2021 during a 7-days period of very low temperatures. Subsequently, a linear model analysis was fitted, setting the Urban Heat Island Intensity (UHII) as the dependent variable and the morphological and biophysical indices as independent variables.

Results showed how some indices were positive or negative correlated with the UHII both in summer and winter, whereas other had a different behavior depending on the season.
Results regarding summer period highlighted UHII positive correlations with most of the morphological indices and negative correlation with most biophysical indices. In contrast, in winter, all the biophysical indices were positive correlated with the UHII. Moreover, most morphological indices were positive correlated with it.

Understanding which urban characteristics impact more in the SUHI formation is crucial for improving city environment and people health and this study set a first step into it.

This study was carried out within the RETURN Extended Partnership and received funding from the European Union Next-GenerationEU (National Recovery and Resilience Plan – NRRP, Mission 4, Component 2, Investment 1.3 – D.D. 1243 2/8/2022, PE0000005) – SPOKE TS 1.