Blue space effects on urban flow and pollution dispersion in a stable atmosphere

Investigating solutions capable of mitigating both the Urban Heat and Pollution Islands is key to realising cities that are resilient to climatic change. Stable atmospheric stratification, typical of heatwave events, exacerbates the heat island effect and increases the risk to human health and wellbeing. Here we examine for the first time the mitigation effects induced by waterbodies within an idealised urban neighbourhood under such conditions using RANS simulations. Results show that, in the presence of waterbodies, increased stability enhances the main vortex in the urban canyon, but suppresses the height up to which convective effects are felt. The cooler the waterbody, the greater its cooling effectiveness at street level, whereas the warmer the waterbody, the smaller the area of influence. Under increased stability, the presence of waterbodies leads to the formation of areas with both cooling and warming effects. Warm waterbodies that do not generate a thermal plume strong enough to penetrate the canopy layer can lead up to a 100% increase in pollutant concentrations at street level in some areas, worsening the impact on human health. Urban planners and practitioners should, therefore, consider both the positive effects and unexpected consequences linked to the presence of blue spaces.