Urban Heat Island and thermo-physiological stress by Local Weather Types in Lisbon

Today’s urban areas are excessive heat sources. The urbanization process has led to the development of positive temperature anomalies, called Urban Heat Islands (UHI). The future climate projections add an increasingly concern to the already heated and polluted urban environments, especially the increase in air temperature and in the frequency, intensity and duration of heat wave events. Hence, the degradation of the thermal conditions is already affecting human thermo-physiological comfort (TC) and health. In this investigation the UHI effect in Lisbon was detailed analyzed according to different thermal seasons (created based on the annual cycle of maximum and minimum air temperatures) and meteorological conditions, these later grouped into Local Weather Types (LWT). Over 60 000 hourly air temperature grids between 2008 and 2014 with a spatial resolution of 100m were extracted from the Copernicus Earth Observation Program. The UHI was calculated based on the widely used land use/land cover scheme, Local Climate Zones (LCZ). Furthermore, the UHI daily cycle by LWT and LCZ was also analyzed. Results show that on rainy conditions with higher cloud coverage the UHI effect is less pronounced (median intensity close to 0ºC), while on sunny conditions with weak to moderate winds and almost no clouds, especially very cold winter days and very hot summer days, median UHI reach 1.5ºC. The analysis of the UHI daily cycle proves that the UHI effect is mainly a nighttime phenomenon, while during the morning a slight Urban Cool Island (UCI) appears on most LWT. However, the analysis of UHI’s patterns and intensities only shows and compares the distribution of air temperature across different land uses, but the human body’s thermal sensation depends not only on air temperature, but also on wind, humidity, and radiation fluxes Therefore, the TC on 163 days between 2008 and 2014 from a particular LWT (the hottest summer days) was modeled in Lisbon. Thirteen microscale samples were selected according to the LCZ scheme, and 2 different TC indices based on the heat balance of the human body (Physiologically Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI)) and Mean Radiant Temperature (MRT) were modeled during the day (12:00 to 15:00h) and night (00:00 to 03:00h) on a freely available and user-friendly software (SkyHelios v. 1.5). Results depict a moderate heat stress on LCZ A during the day (average PET/UTCI/MRT of 34ºC, 32ºC and 45ºC respectively) while the  sun-exposed and poorly ventilated areas on the remaining samples registered higher PET, UTCI (strong to extreme heat stress) and MRT values. During the night, PET results present a slight cold stress in all samples, while UTCI simulations show no thermal stress. This investigation will ultimately help to identify critical areas in the city that need interventions on surface materials, urban morphology, urban greenery and anthropogenic heat emissions in order to mitigate extreme heat conditions and its health risks associated.