Revisiting the urban block in the light of climate change : A case study of Budapest

Abstract: Since the beginning of the twentieth century, the urban block configuration of free-standing buildings has been deemed superior to other built forms, as it provided the necessities of modern healthy living: sunlight and fresh air. In light of climate change, the dissertation aims to reevaluate these long held beliefs and to reintroduce microclimate as an urban design consideration. The study takes four metropolitan block typologies of Budapest as cases to clarify the microclimate influence of key urban design parameters by means of a numerical simulation study. The effects of built form, orientation, vegetation and facade properties are evaluated for a typical summer day. The cases are assessed on the basis of diurnal potential air and mean radiant temperatures within the urban canopy layer. Numerical modeling is performed by ENVI-met and analysis is conducted with MATLAB. The findings indicate that built form and vegetation are key factors governing the microclimate. During the day, intraurban cool island develops between dense configurations and in tree-shaded urban canopies. Orientation is decisive in configurations with large open spaces, where east-west alignment corresponds with peak radiant and air temperatures. Apart from albedo, facade properties have little effect on the microclimate. The rise of air temperature with facade albedo is the outcome of canopy floor heating, resulting from the increased ratio of reflected shortwave radiation. A short-term field experiment was conducted in Budapest to complement the numerical simulation study and to evaluate ENVI-met. The measurement campaign utilized six air temperature and humidity loggers. Additionally, wind speed, air temperature and humidity were recorded at the pedestrian level during an anticyclonic period. In courtyards, thermal stratification developed by day with cool island intensities up to 7 C. In the case of open configurations, neither cool island, nor stratification was observed. The comparison of measured and predicted air temperatures revealed the strengths and weaknesses of the numerical model. In general, predicted temperatures had a decreased diurnal range with maximum values systematically underestimated. ENVI-met also failed to reproduce the thermal stratification in courtyards. Despite these shortcomings, the predicted trends and the relative microclimate differences between the configurations agreed with observations reasonably well.