Numerical studies on the effects of surface characteristics on urban temperatures in a changing climate

The significance of surface characteristics on meteorological conditions in urban environments was studied using the SURFEX surface-atmosphere interaction model. Three experiments were performed: 1) stand-alone SURFEX simulating a city over a year in the current and in a future (2040-2069) climate, 2) SURFEX as part of HARMONIE-AROME weather model at 750 m resolution simulating a past heat-wave event (2018 July) in Finnish cities, and 3) stand-alone SURFEX simulating warm summer conditions in the city of Helsinki in current and future climates at 100 m resolution. 

In the first study, the effect of an artificial green intervention was shown to have a cooling effect on summer and winter temperatures, but the effect does not appear to be as strong as the warming effect of climate change in the worst-case scenario (RCP8.5). The effect of the green intervention on relative humidity is significant during winter but not so much during summer. 

The second study shows the importance of accurate physiographic description, as the modelled temperatures are compared against a city-wide network of 74 instruments. The biases between the observed and the modeled temperatures were between -0.44 – 2.12 °C, the largest differences found on sites where the physiography did not match the instrument surroundings. 

In the third study, to achieve the resolution of 100 meters, a machine learning -generated, improved version of the default SURFEX land cover description was used as the physiographic description. The increased resolution provides cities with the means to plan in more detail.