Measuring urban surface fluxes using a mobile eddy-covariance system at a fine resolution to develop a heat mitigation strategy in a mid-sized European city

As demographic trends continue to point towards urbanisation and urban climate change-related health risks are increasing, a fundamental understanding of the processes that shape the urban boundary layer climate is becoming increasingly important. While previous studies have used mobile measurement devices to measure instantaneous physical weather elements in high spatial resolution in an urban environment, high-resolution measurement data on atmospheric flux densities in cities is scarce.

We present an innovative approach to measure latent and sensible heat fluxes, as well as CO₂ fluxes and further flow statistics as TKE in a mid-sized city (75,000 citizens) in Central Europe using a mobile eddy-covariance (EC) system on a cargo bike with first measurements executed during a radiation night and three consecutive heat days in August 2024. Our goal was to gain flux density data for several street transects in a heterogeneous urban environment during the hottest and coldest time periods of the day. To compare the measured temperature and humidity used for the eddy-covariance calculations, we set up eight weather stations mounted on streetlights along our measurement route, at which we stopped for two minutes each. Motion data was observed with an integrated high precision inertial navigation system (INS) to adjust the EC observation for bicycle movements. To ensure nearly steady-state conditions were fulfilled, the perturbation and averaging periods were fitted to calculate flux densities along approximately homogeneous street transects. As the bike velocity of 4 to 6 m s^-1 only allows for relatively short averaging periods of up to a minute in the heterogenous environment, only the high-frequency fraction of the turbulence spectrum can be quantified. Assuming a similar distribution of the inertial subrange turbulence across the research area, this choice still allowed for comparison of the fluxes along the route.

With our route traversing a range of land surface conditions from a densely built-up district centre to a floodplain valley adjacent to the city, we were able to determine a strong heterogeneity in the expression of the urban heat and park cool islands within our study area. First results of the EC calculations indicate the capability of our mobile flux system to detect fine differences in flux densities within the heterogeneous urban environment.

Our flux measurements together with the additionally measured weather elements of solar radiation, temperature, humidity, wind direction and wind speed from the eight stationary micro weather stations within the study area provide the foundation for the development of a heat adaption strategy in the city district aiming at creating an environment with diminished health risks and urban heat island effects.