The influence of aerosol particle number and hygroscopicity on the evolution of convective cloud systems and their precipitation: A numerical study based on the COPS observations on 12 August 2007

Kurzfassung

The 3D cloud model DESCAM-3D with bin resolved microphysics for ice, water and aerosol particles is used to study the role of particles on the evolution of summertime mid-level convective clouds and the subsequent precipitation during the COPS field campaign which occurred at mid-latitude near the French/German border in summer 2007. Using a 3D grid resolution of 250 m, DESCAM-3D, is able to simulate well the dynamical, cloud and precipitation features of the convective cloud system observed during the afternoon of the 12th August. This mid-level convective system was dominated by warm-phase microphysics. The simulated convective system led to a 1.5 h long lasting precipitation event in agreement with the radar observations. The results of the fine numerical scale compare well with the high resolved radar reflectivities of the LaMP X-band radar and the DLR-Poldirad radar. The prediction of the liquid hydrometeor spectra allows a detailed calculation of the cloud radar reflectivity. In order to better understand the role of aerosol particles on cloud evolution and precipitation formation, several sensitivity studies were performed by modifying aerosol number concentration as well as their physico-chemical properties. Drastic changes in the aerosol solubility lead to a variation in precipitation on the order of 10% for the same convective case. In contrast, changes in the aerosol number concentrations can lead to a variation in total precipitation of up to 30%. Hence, the role of changes in aerosol number concentrations is more important than changes in particle solubility for this case of continental mid-level convection at mid-latitude. A subsequent analysis of the in-cloud microphysics revealed that in-cloud properties are modified significantly in all scenarios. Key parameter is the supersaturation whose magnitude influences both cloud microphysics (cloud droplet and rain drop formation) as well as cloud thermodynamics and cloud dynamics. Furthermore, supersaturation can remain quite low in polluted clouds reducing strongly the release of latent heat and, thus, the intensity of the convective dynamics. As much as a quarter of the polluted cloud volume was found to be subsaturated in this case whereas it was smaller in the continental or clean environments.