Potential changes in outdoor thermal comfort conditions in Gothenburg, Sweden due to climate change: the influence of urban geometry

Quantitative information about outdoor thermal comfort, on various temporal and spatial scales, is required to design better cities and mitigate heat problems not only in warm but also in temperate climates. The overall objective of this study is to explore the augmentation of global/regional climate changes by urban features such as geometry in a compact mid-rise high-latitude city (Gothenburg). The magnitude of spatial and temporal variations of intra-urban mean radiant temperatures (T-mrt) is quantified using the SOLWEIG (SOlar and LongWave Environmental Irradiance Geometry) model. Hourly time resolution, statistically downscaled meteorological data, based on the ECHAM5-GCM under the A1B emission scenario is used to simulate changes in T-mrt and physiologically equivalent temperature (PET) at the 2080-2099 time horizon. Results show that urban geometry causes large intra-urban differences in T-mrt, on hourly, daytime and yearly time scales. In general, open areas are warmer than adjacent narrow street canyons in summer, but cooler in winter. According to the ECHAM5-based scenario, the daytime T-mrt will increase by 3.2 degrees C by the end of this century. This is 0.4 degrees more than simulated increase in air temperature (2.8 degrees C) and is mainly a result of decreases in summer cloudiness. Occasions of strong/extreme heat stress are expected to triple. This equates to 20-100 h a year, depending on geometry. Conversely, the number of hours with strong/extreme cold stress decreases by 400-450 h. Furthermore, the number of hours with no thermal stress increases by 40-200 h a year. The study confirms the potential for using geometry to mitigate daytime thermal stress. A densely built structure mitigates extreme swings in T-mrt and PET, improving outdoor comfort conditions both in summer and in winter. Furthermore, it highlights the importance of including information on either T-mrt or thermal comfort in climate scenarios to describe the combined effects of changes in multiple climate variables and to more realistically measure the impact on humans. Copyright. (C) 2010 Royal Meteorological Society