Investigating the impact of land use changes on aerosol-cloud interactions using polarimetric radar retrievals

Aerosol-cloud interactions and land-atmosphere interactions have been the subject of research for many years, each focusing separately on its effect on convection initiation and precipitation. However, new studies are also emerging on investigation of these two processes together. In fact, land use change (e.g., deforestation, urbanization, expansion of agricultural zones) and all anthropogenic related activities are also among the main sources of the variability in the distribution and hygroscopic properties of aerosols, which can directly or indirectly together affect the development of the precipitating clouds. Polarimetric radar measurements in combination with numerical model simulations constitute a good synergy and opens pathways to investigate the interactions between aerosols and precipitation processes. Polarimetry enables us to study the evolution of the microphysical processes for simulated as well as observed precipitating clouds and thus to validate the results from the numerical models and the representation of such feedback process between the surface flux heterogeneity and aerosol-cloud interactions. The north-western part of Germany, with heterogeneous features of forests, changing agricultural land and urban area is especially suited for such studies due to the availability of the twin polarimetric X-Band research radars in Bonn and Jülich and overlapping measurements from four polarimetric C-Band radars of the German Weather Service, along with the presence of a measurement platform for cloud research (JOYCE). In this study, we investigate the impact of changes in surface energy flux heterogeneity on aerosol-cloud interaction, using the Terrestrial Systems Modeling Platform (TerrSysMP) for a real data study over this north-western part of Germany. We focus on diurnal scale simulations leading to convective rainfall events, and compare the evolution of the microphysical processes exploiting polarimetric radar data. The sensitivity of the surface energy flux partitioning and cloud condensation nuclei (CCN) and ice nuclei (IN) concentrations on cloud development and precipitation, are examined using a multiple set of hindcast simulations.