A Biometeorological Framework for Designing Urban Parks that Ameliorate the Effects of Climate Change

Many inhabitants of cities all over the world suffer from health problems and discomfort that are caused by overheating of urban areas. As most cities are not designed to ameliorate these effects, the use of evidence-based and climate-responsive design of urban open spaces is increasingly important, particularly with compelling evidence of climate change and exacerbating urban heat island intensities. Urban parks and green spaces have the potential to provide thermally comfortable environments and to help reduce vulnerability to heat stress. However, to provide this function, parks must be designed within the context of the prevailing climate. A landscape element that is effective in one climate zone might not be effective in another. For example, in hot dry climates, shadow casting elements and transpiring plants can provide substantial cooling of surfaces and the adjacent air, while this is much less effective in hot humid climates with mostly overcast skies. The Köppen climate zones provide a useful framework to describe prevailing climates and help assess bioclimatic design features within urban parks in the different climate zones.

The goal of this study was to investigate the effects of typical design interventions in urban parks on people's thermal sensation in different climate zones in order to provide guidance to mitigate negative effects of overheated cities. The human energy budget model COMFA was used to simulate thermal sensations for cities in five Köppen climate zones. We selected the climate zones in which most urbanized areas can be found worldwide and that experience hot weather for at least one season of the year (Af (Kuala Lumpur, Malaysia), Bsh (Lahore, Pakistan), Cfa (Kyoto, Japan), BWh (Alice Springs, Australia), Dfa (Toronto, Canada)). Climate measurement data from each city were used to calibrate the simulations. Three climate situations were used as a reference to test the design interventions for the hottest month of the year: 1) Typical daily maximum (T_max) and minimum (T_min) temperatures; 2) Typical daily T_max and T_min during an extreme heat event; 3) Typical daily extreme heat events at T_max for mid-century and late-century climate change projections. To test the impact of the design interventions, temperature, shade, and wind characteristics were altered, with all-else held constant, to determine the individual effects. For instance, park cooling island effects, shade from trees, or wind blocks, were simulates in the various locations. To calculate COMFA energy budgets, people were modeled to be dressed in clothing typically worn in each location (clothing resistance range: 78–110 s m^-2) and were modeled to be standing. Alice Springs, Australia displayed the warmest temperatures and most uncomfortable bioclimate, with the effects of decreasing T_max resulting in the greatest decreases to the energy budget (range: –17 to –125 W m^-2). Energy budget estimates were significantly larger than the remaining climates zone locations. Toronto displayed the coolest temperatures and lowest energy budgets, with the effects of wind alteration resulting in the greatest decrease to the energy budget. However, this potential cooling effect of wind during typically hot summertime days was shown to be the least useful cooling design strategy (e.g., 60% more wind resulted in a mere –1 W m^-2 decrease in the energy budget (range: –6 to +4 W m^-2)). The cooling effect of wind was minimal or lowered with increasing temperatures, where high temperatures (Ta > 40^oC) often resulted in higher energy budgets, and thus more uncomfortable bioclimates. This warming effect was particularly present in the desert-type climate of Alice Springs. The magnitude of cooling due to shade was overall the most effective, with a 50% increase in shade resulting in an average energy budget decrease of –78 W m^-2 across the five cities. This cooling effect through shading was the greatest in Lahore, Pakistan, where energy budget reductions reached –87 W m^-2 for 50% shade, and –139 W m^-2 for full shade. The COMFA energy budget analysis indicated that solar radiation control was the most important effect that green spaces can have on the thermal comfort of residents. Apart from that, decreasing air temperature through a ‘park cooling island' design were also important, linearly decreasing energy budgets by –7 to –19 W m^-2 per ^oC decrease in air temperature. These interventions were increasingly effective in proportion to temperature (i.e., heat wave and climate change scenarios), and can be used in combination to ameliorate the effects of heat stress on urban populations. These findings will be translated into a biometeorology-based framework for the design of thermally Comfortable Outdoor Open Landscapes (COOL). By offering simple design guidelines for urban park elements for different climate zones, we can support effective adaptation and mitigation strategies to extreme heat. This type of bioclimatic planning for urban parks worldwide can result in cooler, healthier, and more comfortable cities during the hottest times of the year.