The Impact of Imperviousness on Boundary Layer Mixing, Air Pollution and Cloud Formation over Urban Areas

Abstract

Urbanization requires the replacement of natural land into impervious surfaces, which in turn leads to surface sealing and a loss of natural soil functions. Next to a modification of the surface hydrological properties, warming of concrete materials arouses a respective warming of the air aloft. This study analyses the impact of increased sensible heat flux from heated building materials onto the thermodynamic and chemical features of the urban boundary layer and hence the urban heat island intensity. The state-of-the-art coupled chemistry-climate model MECO(n) combined with the COSMO urban canopy model TERRA_URB is facilitated on a km-scale to analyse the interplay between chemical and thermodynamic processes in the urban canopy layer, linked to modification of surface properties. Two case studies for projected scenarios in future urban planning directions—urban sprawl and re-densification—are compared for a 10-days heat wave period during July 2018 with respect to the greater Rhine-Main metropolitan area. Summarising the key outcomes of the study, firstly a 10% increase in surface imperviousness resulted in a respective warming of surface temperature by 0.7 K and 2 m air temperature by 0.2 K within the core urban area. 50% total increase of surface sealing was responsible for a 3 K and 1 K spatially averaged warming respectively. For the same scenario, increased boundary layer mixing in warmed street canyons lead to a reduction of surface NO2 concentration by 10–20% during the day and an increase of about 10% over night, when denser canyons reinforced blocking of air flows. Lastly, modifications of surface properties have also been recorded at about 1500–2000 m hight within the boundary layer mirrored by a change in cloud properties. Here, increased turbulent vertical fluxes transporting warmer air higher up into the urban boundary layer, trigger the formation of low-level urban induced clouds.