Urban heat island under climate change. The case study of Milano, Italy.

Poli-HE, a coupled hydrological-energy budget model was developed to simulate the surface water, and energy fluxes between soil and shallow atmospheric boundary for the city of Milano, Northern Italy. So doing, we describe the urban heat island effect, i.e. differences in land surface temperature (LST) between paved/urban, and green/natural areas, i.e. parks, and suburban agricultural patches.

Energy and water balance equations are linked through soil water content (W), and latent heat flux (LE), calculated as a function of the LST. W in turn drives (actual) evapotranspiration, thus driving (water) mass balance. Input variables were used from i) meteorological stations, air temperature (Ta), net radiation (Rn), rainfall (R), and ii) satellite images, giving leaf area index (LAI) and land surface temperature were used for model tuning.

The results show large differences in LST between urban/green areas, high during summer, viz 3-4 °C, lower in winter, viz 0.3°C. During 2010-2021 max surface temperature in Milano was +37.3°C in urban areas, and to +33.6°C in the green areas.

The model was then used for future projections of LST, using outputs of the Global Circulation Model EC-Earth3.0, constrained to shared socio-economic pathways SSP1-2.6 and SSP5-8.5 of the CMIP6. We analysed near (2030-2041), medium (2050-2061) and long-term (2080-2091). On average in the city of Milano, LST is projected between +0.14°C (SSP 2.6 in the medium-term), and +6.35°C (SSP 8.5 in the long-term). The jump of LST between urban/green areas would reduce against average increase of air temperature, and LST, i.e. large increase of air temperature will be less and less dampened by the present green area cover.