Differential thermal environmental impacts of green space types in a subtropical city: Evidence from mesoscale modelling

Existing observational studies on the cooling effects of different urban green space types yield conflicting results.  Few studies have compared the differential thermal environmental impacts of urban green space types while controlling the environmental contexts.  To fill the research gap, we adopted a mesoscale model coupled with an urban canopy model to quantify and compare such differential impacts in a subtropical city located in central China.  Mixed trees, the predominant green space type in the city, showed the best model performance.  In most places, closed shrubs yielded the strongest daytime cooling of 0.4℃ while the weakest humidification of 0.15g/kg, thus reducing the regional mean heat index (HI) by 0.38℃.  Mixed trees had a limited effect on 2-m air temperature but greatly raised near-surface moisture content by 0.8g/kg, thus increasing the regional mean HI by 0.2℃.  However, in locales with the green space fraction larger than 0.6, mixed trees produced the strongest cooling of 0.6 – 1.3℃.  This was because evapotranspiration instead of albedo drove the cooling process.  At night, all the green space types showed stronger cooling but weaker humidifying effects.  Of all, mixed trees led to the largest reduction in temperature (by 0.6℃ on average) and the HI (by 0.56℃), while grass showed the strongest humidification of 0.35g/kg.  The nighttime cooling was attributed to the reduction in sensible and ground heat flux by green space compared with impervious surfaces.  We suggest prioritizing closed shrubs in small green patches and paying attention to the humidification of grass at night.