Energetics and cooling in urban parks

While there has been a long tradition for the integration of architecture and landscape to improve the urban environment, little is known about the effect of urban parks on local climate. In this study the park effect is determined through an integrated research approach incorporating field measurements of the thermal regime and energetics of urban parks, together with scale modelling of nocturnal cooling in urban parks. The research is limited to consideration of the park effect in two cities with different summer climates: Sacramento, California (hot summer Mediterranean) and Vancouver, British Columbia (cool summer Mediterranean). In both these cities, surveys of summer time air temperature patterns associated with urban parks confirm and extend previous findings. In temperate Vancouver, the park effect is typically 1-2°C, rarely more than 3°C, although it can be higher under ideal conditions. However, in a hot, dry city, the effect is considerably enhanced with parks as much as 5-7°C cooler than their urban surrounds. A comparison of the surface energy balance of small open, grassed parks in these two cities demonstrates the importance of evapotranspiration in park energetics. In hot, dry Sacramento, evaporation in the park was advectively—assisted and exceeded that at an irrigated rural site. Strong advective edge effects on evaporation were observed in this wet park. These decayed approximately exponentially with distance into the park. The urban park in Vancouver was moist, but unirrigated. While evaporation dominated the surface energy balance, the sensible heat flux was positive through most of the day, and evaporation was not strongly influenced by advection. The evaporation trend in the park probably reflected the turbulence and soil moisture regimes. However, an irrigated lawn in Vancouver did exhibit edge—type advection. This suggests the soil moisture regime may be critical in determining whether evaporation exceeds the potential rate. The contribution of processes to nocturnal cooling in urban parks was determined through scale modelling. It showed that surface geometry and the urban—park difference in thermal admittance may be of equal importance in nocturnal cooling. Parks with high sky view factors have increased radiative cooling and if the park is very dry (and therefore has a low thermal admittance), the cooling is further enhanced. Evaporative cooling is critical in establishing the park as a “cool island” at sunset, but the presence of moisture slows cooling through the night. Integration of the field and model data leads to the development of guidelines for planners regarding the design of parks for maximum climatic benefit. The optimum size of the park depends to a large extent, on the geometry of the urban surrounds. To maximize radiative cooling, the width of open park areas should be at least 7.5 times the height of the trees or buildings around the park border. Large parks increase the size of the volume of air cooled and this increases the potential for advection of cool air into the neighbourhood. It is suggested that if cooling is the objective, the optimum design is a savannah—type park with loose clusters of trees interspersed by wide open, irrigated grass. The arrangement of trees must be chosen with great care to allow the advection of air both into, and out of, the park.