Modeling the Influence of Open Water Surfaces on the Summertime Temperature and Thermal Comfort in the City

Due to the combination of rapid global urbanization and climate change, urban climate issues are becoming relatively more important and are gaining interest. Compared to rural areas, the temperature in cities is higher (the urban heat island effect ) due to the modifications in the surface radiation and energy balances. This study hypothesizes that the urban heat island can be mitigated by introducing open surface water in urban design. In order to test this, we use the WRF mesoscale meteorological model in which an idealized circular city is designed. Herein, the surface water cover, its size, spatial configuration, and temperature are varied. Model results indicate that the cooling effect of water bodies depends nonlinearly on the fractional water cover, size and distribution of individual lakes within the city with respect to wind direction. Relatively large lakes show a high temperature effect close to their edges and in downwind areas. Several smaller lakes equally distributed within the urban area have a smaller temperature effect, but influence a larger area of the city. Evaporation from open water bodies may lower the temperature, but on the other hand also increases the humidity, which dampens the positive effect on thermal comfort. In addition, when the water is warmer than the air temperature (during autumn or night) the water body has adverse effect on thermal comfort. In those cases, the water body eventually limits the cooling and thermal comfort in the surrounding city, and thus diverges from the original intention of the intervention.