Impact of urban imperviousness on boundary layer meteorology and air chemistry on a regional scale

It has been long understood that land cover change from natural to impervious modifies the surface energy balance and hence the dynamical properties of the overlying atmosphere. The urban heat island is manifested in the formation of an urban boundary layer with distinct thermodynamic features that in turn govern transport processes of air pollutants. While many studies already demonstrated the benefits of urban canopy models (UCM) for atmospheric modelling, work on the impact on urban air chemistry is scarce. This study uses the state-of-the-art coupled chemistry-climate modelling system MECO(n) to assess the impact of the COSMO UCM TERRA_URB on the dynamics and gas phase chemistry in the boundary layer of the urban agglomeration Rhine-Main in Germany. Comparing the model results to ground observations and satellite and ground based remote sensing data, we found that the UCM experiment reduces the bias in temperature at the surface and throughout the boundary layer. This is true for ground level NO2 and ozone distribution as well. The application of MECO(n) for urban planning purposes is discussed by designing case studies representing two projected scenarios in future urban planning – densification of central urban areas and urban sprawl. Averaged over the core urban region and 10‑days during a heat wave period in July 2018, model results indicate a warming of 0.7 K in surface temperature and 0.2 K in air temperature per 10 % increase in impervious surface area fraction. Within this period, a 50 % total increase of imperviousness accounts for a 3 K and 1 K spatially averaged warming respectively. This change in thermodynamic features results in a decrease of surface NO2 concentration by 10–20 % through increased turbulent mixing in areas with highest impervious fraction and highest emissions. In the evening and nighttime however, increased densification in the urban centre results in amplified canyon blocking, which in turn results in average increase in near surface NO2 concentrations of about 10 %, compared to the status quo. This work intends to analyse regional scale features of surface-atmosphere interactions in an urban boundary layer and can be seen as preparatory work for higher resolution street scale models.