Tracking the Impact of Urban Air Masses on Convective Precipitation: A Multi-Member Modeling Study

Abstract. Urban emissions impact aerosol-cloud interactions and thereby modify precipitation patterns, yet their absolute effects remain uncertain due to internal atmospheric variability and limitations of conventional analysis methods. This study aims to further quantify the influence of urban aerosol fields on convective precipitation through explicit chemistry-cloud coupling. Using the coupled COSMO-DCEP-MUSCAT modeling system, we simulate two convective events passing the city of Leipzig, Germany, with five-member ensemble experiments comparing total emissions to zero urban emission scenarios. Cloud droplet activation is calculated from prognostic three-dimensional aerosol fields, providing a physically consistent representation of aerosol–cloud interactions. We use backward trajectory analysis to directly trace urban air masses from convective clouds back to the region of urban emission sources, enabling objective sampling of individual clouds and isolation of local emission effects. The results reveal case-dependent responses. Under moderate atmospheric instability, urban aerosols locally modify the cloud microphysics and precipitation without altering the overall structure of the convective system. Under stronger initial instability, the urban emissions intensify the precipitation, leading to stronger downdrafts, causing a premature system decay and shorter lifetime compared to the zero urban emission scenario. Ensemble analysis demonstrates that emission-induced changes are comparable in amount to internal variability, highlighting the need for multiple realizations and significance testing. The results of this study reveal that urban aerosol effects are highly case-dependent, challenging assumptions about uniform impacts. The same air pollution source can either delay, enhance or suppress convection depending on prevailing atmospheric conditions.