IMPACT ANALYSIS OF BLUESPACE ON URBAN HEAT ISLAND IN COMPOSITE CLIMATE

Abstract

Earth’s average global temperature has risen by 0.91 °C in the last 100 years. This temperature rise is even more pronounced in urban areas as they exhibit higher temperatures than their surrounding rural areas. The rise in urban air temperature has forced researchers to look for nature-based solutions to resolve the problem sustainably. Urban waterbody plays a multidimensional role in the city's well-being by catering to its economical, ecological, and socio-cultural needs. It can offer a potential solution for urban heat attenuation, but its effect on outdoor thermal comfort is contentious. This study investigates the thermal impact of a waterbody on its surroundings by adopting a human-centric approach. It analyses the impact of waterbody on land surface temperature, ambient air temperature, and outdoor thermal comfort, diurnally and seasonally, to establish a correlation between ‘Impact on UHI’ and ‘shape and area of the waterbody and its distance’ and suggesting suitable Local Climate Zones to attenuate the urban heat. Waterbody impact on land surface temperature, ambient air temperature, PET, and UTCI are evaluated and compared to understand its thermal impact on nearby surroundings better. This study employs Landsat-8 OLI/TIRS data for land surface temperature estimation and CFD based simulation model, Envi-met, for microclimate analysis. Outdoor thermal comfort is estimated through UTCI and PET indices which are calculated through simulated data. The results indicate that waterbody lowers land surface temperature during daytime and nighttime in the summer season, whereas it has a varied effect during the winter season. The maximum impact distance of the waterbody is about 300 m during summer as well as winter. The maximum impact amplitude of bluespace during summer was 3.52 ℃, whereas it was restricted to -1.39 ℃ during winter. These results indicate that waterbody has the potential to lower the LST substantially during summer, but the warming effect exhibited during winter is comparatively less in magnitude. This study highlights that LCZs with well-defined canyons oriented towards the waterbody exhibits maximum impact range, whereas the highest waterbody impact gradient is observed in compact LCZs. Waterbody exhibits a similar impact on air temperature as well. It lowers the canopy layer air temperature during the day and night in the summer season, albeit with varying intensity, whereas it has a varied impact during the winter season. The maximum impact amplitude is observed to be 3.09 ℃ during summer and -0.4 ℃ during the winter season. The maximum impact range is limited to 140 m. This investigation highlighted two other thermal behavioural aspects of bluespace. First, a larger waterbody has a greater cooling distance, and amplitude and secondly, higher humidity decreases the cooling effect of the waterbody. Air with less humidity increases the evaporation rate, while humid air has the opposite impact. This investigation also provides an insight into the impact of bluespace on the behavioural pattern of outdoor thermal comfort in different LCZs and their variability seasonally and diurnally. The results of this research state that compact LCZs exhibit lower PET and UTCI during the daytime in summer. LCZs with restricted wind movement exhibit higher values of PET and UTCI during the night. LCZs with dense vegetation show extremely high UTCI during summer nights. These regions exhibit nocturnal longwave radiative cooling because of the smaller sky view factor, while excess moisture increases the thermal capacity of the soil and slows down surface cooling. The intensity of incoming solar radiation is significantly less during the winter season, and airflow takes precedence in determining thermal comfort. LCZs with restrictive airflow exhibit maximum PET and UTCI during winters.