Model Study on Urban Aerosol-Cloud Interactions and their Influence on Precipitation

Besides influencing dynamics, cities are major sources of anthropogenic aerosols. Activated as cloud droplets they can modify the cloud microphysical processes, which can lead to an intensification, delay or even suppression of convective precipitation. However, it is hard to separate the urban cloud aerosol effect from roughness and thermal effects related to the urban morphology and variability in meteorological conditions. Therefore, quantifying this effect is challenging due to the relatively small signal amid high variability.
In this study, we aim to better understand how urban emissions affect clouds and thereby precipitation processes. For this purpose, we use fine resolution simulations including an urban parametrization and a direct aerosol-cloud coupling. use the Consortium for Small-Scale Modeling (COSMO) model online-coupled to the multi-scale chemistry aerosol transport model MUSCAT and the double-canyon urban canopy parameterization DCEP. To simulate a direct aerosol-cloud-precipitation coupling we modified the 2-moment bulk microphysics scheme from COSMO to allow cloud condensation nuclei to be calculated directly from aerosol mass concentrations simulated with MUSCAT, instead of assuming constant cloud condensation nuclei concentrations. A 3-way nesting strategy is applied for the simulations, with the highest resolution area centered over the Leipzig metropolitan area, using a 1km grid spacing.
We employ this setup in a case study on a small-scale convective storm that passed over the City of Leipzig. Sensitivity tests have been performed to investigate how sensitive the simulated precipitation is to changing aerosol concentrations, by varying the input emissions. First results show that a doubling of emissions weakens and shifts the location of maximum precipitation further downstream the urban area. Additional simulations with chemistry boundary values set to zero enable us to assess how urban emissions are transported vertically and how they can influence the convective storm.

 

Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany‘s Excellence Strategy – EXC 2037 'CLICCS - Climate, Climatic Change, and Society' – Project Number: 390683824