Relaxed eddy accumulation (REA) measurements for (CO2)-C-14 enable the estimation of fossil fuel (ff) CO2 fluxes in urban areas. This work is based on 252 REA ffCO(2) flux measurements conducted on tall towers in the cities of Zurich, Paris, and Munich. The ffCO(2) fluxes were compared to net eddy covariance CO2 fluxes to quantify the role of non-fossil (nf) CO2 fluxes. While the measurements in Zurich and Paris were limited by small signal-to-noise ratios, improvements in the REA setup, the (CO2)-C-14 measurement precision, the sampling strategy, and the source strength increased the significance of the results in Munich. Large nfCO(2) fluxes observed in Munich from the direction of a brewery demonstrate the efficacy of the partitioning approach and illustrate the complexity of urban atmospheric measurement data. Excluding these measurements potentially influenced by large anthropogenic nfCO(2) fluxes, the error-weighted average CO2 / CO2 flux ratio in Munich was approximately 47 % in summer and 76 % in winter, with the majority of measurements taken between 07:00 and 19:00 local time. Regional excess concentrations had much lower ffCO(2) contributions (<63 % in winter and <28 % in summer, in all three cities), demonstrating fundamental differences between local and regional CO2 fluxes. The combination of (CO2)-C-14 observations and the REA method is a sophisticated approach that challenges the limits of current analytical capabilities, while providing unique opportunities for quantifying ffCO(2) and nfCO(2) fluxes.

Abstract. Relaxed eddy accumulation (REA) measurements for 14CO2 enable the estimation of fossil fuel (ff) CO2 fluxes in urban areas. This work is based on 252 REA ffCO2 flux measurements conducted on tall towers in the cities of Zurich, Paris, and Munich. The ffCO2 fluxes were compared to net eddy covariance CO2 fluxes to quantify the role of non-fossil (nf) CO2 fluxes. While the measurements in Zurich and Paris were limited by small signal-to-noise ratios, improvements in the REA setup, the 14CO2 measurement precision, the sampling strategy, and the source strength increased the significance of the results in Munich. Large nfCO2 fluxes observed in Munich from the direction of a brewery demonstrate the efficacy of the partitioning approach and illustrate the complexity of urban atmospheric measurement data. Excluding these measurements potentially influenced by large anthropogenic nfCO2 fluxes, the error-weighted average ffCO2 / CO2 flux ratio in Munich was approximately 47 % in summer and 76 % in winter, with the majority of measurements taken between 07:00 and 19:00 local time. Regional excess concentrations had much lower ffCO2 contributions (<63 % in winter and <28 % in summer, in all three cities), demonstrating fundamental differences between local and regional CO2 fluxes. The combination of 14CO2 observations and the REA method is a sophisticated approach that challenges the limits of current analytical capabilities, while providing unique opportunities for quantifying ffCO2 and nfCO2 fluxes.

The storage flux, corresponding to disequilibrium between observed flux and net surface emissions, poses a significant source of uncertainty in tower-based eddy covariance (EC) measurements over urban and forest ecosystems. In this study, we investigated the coupling between the urban inertial sub-layer (ISL) and roughness sub-layer (RSL) and its influence on nighttime storage flux, leveraging tower-EC together with collocated wind profile measurements. Our findings demonstrate that substantial storage flux occurs when the gradient of turbulent kinetic energy (TKE) enlarges, indicating decoupling between ISL and RSL. With increasing wind speed, turbulent eddies generated by bulk wind shear directly interact with the surface, conducive to the recoupling between ISL and RSL and resulting in decreased storage flux. Conversely, when the gradient of TKE between ISL and RSL is small, the storage flux remains low and relatively insensitive to wind speed. The derived diagnostic relation further confirms the predominant influence of stability and turbulent intensity gradient on regulating the storage flux. These results provide valuable insights as a complement to prior storage flux studies in the context of canopy flow.

This study investigates the spatiotemporal distribution of potential temperature (theta) hot-and coldspots in an urban environment for one day in a summerly heat wave, as reflected by a large eddy simulation (LES) model as well as reproduced by a multiple linear regression (MLR) model based on an observation network. The spatial variation of static surface characteristics only partly explains the observed patterns for both approaches. The question of which additional factors, mainly those related to atmospheric circulation, are essential for the development of theta hot-and coldspots is addressed. For this purpose, real case simulations with the LES model PALM-4U were conducted for the city of Augsburg, Southern Germany. Hot-and coldspots were detected in the modelled theta fields with the Gi* statistic. The theta and Gi* patterns were compared to the results of the MLR model, using only static surface characteristics for the referring daytime, season and weather type as predictors. For some times of the day, the patterns from the two approaches show good agree-ment, but there are considerable differences for other situations, although the weather type does not change over the studied period. In a next step, the detected hotspots are classified into expected and unexpected hotspots according to their surface characteristics, and differences in the meteorological variables for both groups are investigated. A similar procedure is applied to areas apart from hotspots, which could be expected to be hotspots, and those which are not expected to be one. The results indicate that even in summerly anticyclonic conditions with low wind speeds, horizontal circulation and vertical mixing play a significant role in manifesting urban theta patterns. It is concluded that more than a single simulation may be required to represent typical urban temperature patterns during heat waves since they cannot reflect the critical influence of varying circulation dynamics at different synoptic conditions. This should be considered in urban planning.

We used boundary layer observations in the Stuttgart area from two summer episodes for model evaluation of the urban climate model PALM-4U. In summer 2017, radiosondes and Doppler lidars were operated, and the lidar virtual tower technique was applied to provide vertical profiles of wind speed and direction at different sites. In summer 2018, two Doppler lidars were operated in vertical stare mode providing vertical wind speeds and their variances. PALM-4U with grid spacings of 10 m in 2018 and 40 m in 2017 was driven by COSMO analysis data. The PALM-4U output data were evaluated with the observations at the corresponding sites. For 14 to 15 August 2017, the normalised root mean square error (NRMSE) between simulated and measured wind speed time series at 100 m agl is about 0.5, except for the last hours of the investigated period. The RMSE for wind direction is 30-35 degrees. At 700 m agl, the NRMSE for wind speed is 0.2-0.3 and the RMSE for wind direction is about 15 degrees. The greater NRMSE differences found in the night and morning of 15 August can be explained by differences between the observed and simulated height of the low-level jet which caused stronger simulated turbulent downward mixing of momentum in the morning. On 20 June 2018, the daytime convective boundary-layer evolution was well represented in the model. However, as the wind speed was low, 1-h integration times turned out to be not suitable for model evaluation because the uncertainty in variances of the vertical wind is considerable (50%). To overcome the poor statistics due to the low number of eddies dominating, 3-h integration times turned out to be necessary. For these time intervals, the simulated profiles lie within the error bars of the observations. Theoretical considerations provide suggestions for experimental set-ups and synoptic conditions to capture vertical profiles allowing model evaluations under convective conditions based on 1-h intervals.

Ziel des BMBF-Programms Stadtklima im Wandel war die Entwicklung, Validierung und Anwendung eines gebaudeauflosenden Stadtklimamodells fur ganze Stadte. Das Verbundprojekt 3DO ubernahm die dem Modul B zugeordneten Forschungsaufgaben: Aufbereitung vorhandener Daten aus der Langzeitbeobachtung (LTO), Aufbau neuer Messstationen, Gewinnung neuer dreidimensionaler atmospharischer Daten und die Entwicklung neuer Konzepte z.B. zur Modellevaluation. Untersucht wurden der Aufbau der atmospharischen Grenzschicht, die Charakteristik der meteorologischen Parameter und deren Einfluss auf das thermische Empfinden des Menschen. Ein einheitlicher UC2-Datenstandard sowie Analysewerkzeuge wurden entwickelt und in ein Datenmanagementsystem und eine Wissensplattform fur den modulubergreifenden Austausch integriert.

BMBF, 01LP1602A, Verbundprojekt Stadtklima: Evaluierung von Stadtklimamodellen (Modul B), 3DO Teilprojekt 1: Dreidimensionales Monitoring atmospharischer Prozesse in Berlin

Ground-based remote sensing by three ceilometers for mixing layer height detection over Augsburg as well as a Radio- Acoustic Sounding System (RASS) for temperature and wind profile measurements at the campus of Augsburg University are applied together with UAV height profiling with low-weight meteorological sensors and particle counter to monitor the three-dimensional dynamics of the lower atmosphere. Results about meteorological influences upon spatial variation of air pollution exposure are presented on this data basis which is more than one year long. Special focus is on the information about atmospheric layering as well as mixing and transport conditions for emitted particulate matter. Better understanding of these complex processes support knowledge about quality of air, which we breath, and especially high air pollution episodes and hot spot pollution regions.