Tracking air volumes for assessing the effect of urban aerosols on convective precipitation: a multi-member modeling study

Abstract

Abstract. Urban emissions impact aerosol–cloud interactions and thereby modify precipitation patterns, yet whether realistic emission perturbations from a mid-sized European city produce detectable effects above internal meteorological variability in a high-background aerosol environment remains an open question. This study investigates the influence of urban aerosol fields on convective precipitation through explicit chemistry-cloud coupling, using a trajectory-based ensemble approach designed to isolate weak aerosol signals from natural variability. Using the coupled COSMO-DCEP-MUSCAT modeling system, we simulate two convective events passing over the city of Leipzig, Germany, with experiments comparing total emissions to zero urban emissions, with five ensemble members for each setting. 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 air volumes carrying urban emissions 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 event. Under stronger initial instability, the urban emissions intensify the precipitation, leading to stronger downdrafts and weaker updrafts, altering the convective event's evolution 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, and that domain-mean surface precipitation remains within the ensemble spread despite detectable microphysical responses and spatial redistribution.