Cities in the climate system – a holistic approach

A large majority of mankind lives in cities. Thus, sustainable and climate-resilient planning and governance of cities is mandatory. The IMK-IFU investigates research fields such as excess warming of urban agglomerations (urban heat island), urban air quality (ultrafine particles, nitric oxides, ozone), flash flood risks, and the interaction of urban systems with their regional and global atmospheric and biological environment.

Together with several national and international partners, we utilize an innovative holistic modelling and observation concept to cover the above research fields which will be designed to offer unprecedented spatial and temporal resolution. The high dynamics of urban agglomerations, the large spectrum of different space and time scales, and the large amounts of relevant inhomogeneous data provide a specific challenge.

Fallmann, J., S. Emeis, 2020: How to Bring Urban and Global Climate Studies together with Urban Planning and Architecture? Devel. Built Environ., 4, 100023. DOI: 10.1016/j.dibe.2020.100023

Opposing aims: are cooler cities cleaner cities?

The urban heat island enhances via supporting a secondary regional air circulation system (warm air rises over the cities and is replaced by cooler air from the rural surroundings) and thus contributes to a better urban air quality.

Cooling cities by planting more green and/or creating more water surfaces in the cities has thus to be accompanied by a drastic reduction of emissions from combustion processes in cities.

Likewise, cooling of cities by using light reflecting building and surface materials (white roofs) has thus to be accompanied by a reduction of the emission of ozone precursor gases (nitric oxides, anthropogenic and biogenic hydrocarbons (e.g., isoprene from sycamores)) in cities.

The interaction between thermic processes, atmospheric dynamics, and air chemistry has to be investigated thoroughly before mitigation measures for the known urban problems are suggested.

The investigation is a perfect task for the new national German numerical urban climate model PALM-4U which is currently under construction. IMK-IFU contributes important parts to this new model across scales (e.g., air chemistry) and already uses it for the assessment of urban issues.

For the modeling of air quality in cities, the actual air flow is structured in a numerical grid. The transport between adjacent grid cells is calculated by partial differential equations, and the chemical reactions in each grid cell are simulated by the numerical implementation of a specific chemical mechanism.

Remote sensing has become a major technique in observing the atmospheric boundary layer, and thus the urban boundary layer as well. Remote sensing is the only affordable technique for vertical profiling in order to assess the vertical structure of the boundary layer and the conditions for vertical turbulent exchange of pollutants. Active remote sensing such as backscatter lidars and wind lidars (the latter return vertical wind profiles from a Doppler shift analysis of the backscattered signal) offer a sufficiently detailed vertical resolution which is suited for urban studies. Furthermore, remote sensing can be a meaningful way to obtain data from otherwise inaccessible parts of the boundary layer. Finally, scanning and path-averaging sensing techniques produce spatially averaged data which suit very well for comparisons with simulated data from numerical models.