Modelling urban heat island intensity and the effect of vegetation at microscale to attenuate heat stress factors: The case of İstanbul

In the thesis conducted within the scope of the Istanbul case, the relationship between micro- scale thermal comfort conditions and large-scale climatic indicators was examined, focusing on temporal extremes represented by Extreme Heat Events (EHE) and spatial variations derived from Land Surface Temperature (LST) data. In this context, the interactive role of street morphology and street tree in identifying regional and local heat risk areas was evaluated in four sections. In the first section, the Physiological Equivalent Temperature (PET) values on August 5, 2021, an EHE day, were found to be significantly higher compared to the 15-year average, indicating a shift from moderate and slight heat stress to strong and extreme heat stress levels. Extreme thermal stress conditions were observed on the right sides of streets oriented between 105° and 165°. In the second section, it was found that east–west oriented streets were exposed to conditions beyond extreme heat stress due to prolonged solar exposure on their northern façades. When examining the effects of tree species on PET across different street orientations, species with broad crown diameters reduced PET values by 5 to 6 °C in the morning hours. However, this reduction was not sufficient to drop PET below 41 °C, and extreme heat stress conditions persisted. The third and fourth sections revealed that rising temperatures negatively affected thermal sensation regardless of spatial typology, yet the presence of cooling elements such as trees and water features positively influenced perception. The neutral PET range was calculated as 17 °C in the third section and as 22 °C in the fourth. In the fourth section, areas with high LST consistently exhibited high PET values, whereas in medium LST areas, PET values remained more moderate. In contrast, in low LST areas, high heat stress was also observed, which was attributed to low wind speeds and shallow canyon morphology. Depending on the morphological characteristics of tree species, street orientation, and openness ratios, global radiation was reduced by up to 80 percent, and PET values decreased by 5 to 15 °C. Based on various greening scenarios, it was determined that in the northern façades of east–west oriented streets within high LST areas, trees with a 6- meter crown diameter planted at 5-meter intervals improved PET by over 50 percent. Similarly, in medium LST areas, trees with the same crown diameter planted at 2-meter intervals yielded comparable improvements. However, these scenarios were still insufficient to reduce thermal conditions below the strong heat stress level. It was recommended that in high and medium LST areas, cooling strategies be further developed through the integration of additional landscape elements. In low LST areas, prioritization within urban renewal projects was advised. Furthermore, it was suggested that heat action plans and citizen information systems be integrated into urban management frameworks.