Model study on the influence of aerosol-cloud interaction processes on precipitation in urban areas

Urban influence on cloud and precipitation processes can appear via two mechanisms: (I) roughness and thermal effects induced by the urban morphology influence dynamical processes and (II) high urban particle emissions effect cloud microphysical processes. Changes in dynamics and cloud microphysics can trigger large-scale changes in storm morphology and precipitation patterns and thus play a significant role in precipitation formation in the vicinity of urban areas. The modifications by the urban area can lead to intensification, delay, or even suppression of convective precipitation. The strength and nature of urban influence is dependent on the prevailing synoptic conditions.

To address the issues of the aerosol-cloud-precipitation interactions in the presence of a city, we use the COSMO-DCEP-MUSCAT model system. The model system is composed of the regional atmospheric model COSMO coupled with the multi-scale chemistry-aerosol transport model MUSCAT and the urban parameterization DCEP - an extension of Martilli's BEP scheme. We apply a 3-way nesting strategy for the simulations, where the highest resolution area is centered over the Leipzig metropolitan area with a 500m grid spacing.

Within this paper, we present several sensitivity studies in order to investigate the influence of urban areas on precipitation. For this purpose, two different synoptic background conditions, based on real weather events, were simulated with the model system: a convergence line and a system moving slowly over the city. This allows us to estimate the influence of the city on the weather system. In addition, a variation of urban emissions shows how sensitive the simulated precipitation is to changing aerosol concentrations.

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