Daniel Rieger

Impact of aerosol on post-frontal convective clouds over Germany

We carried out simulations with predefined and simulated aerosol distributions in order to investigate the improvement in the forecasting capabilities of an operational weather forecast model by the use of an improved aerosol representation. This study focuses on convective cumulus clouds developing after the passage of a cold front on 25 April 2008 over Germany. The northerly flow after the cold front leads to increased sea salt aerosol concentrations compared to prefrontal conditions. High aerosol number concentrations are simulated in the interactive scenario representing typically polluted conditions. Nevertheless, due to the presence of sea salt particles, effective radii of cloud droplets reach values typical of pristine clouds (between 7 µm and 13 µm) at the same time. Compared to the predefined continental and maritime aerosol scenarios, the simulated aerosol distribution leads to a significant change in cloud properties such as cloud droplet radii and number concentrations. Averaged over the domain covered by the convective cumuli clouds, we found a systematic decrease in precipitation with increasing aerosol number concentrations. Differences in cloud cover, short wave radiation and cloud top heights are buffered by systematic differences in precipitation and the related diabatic effects. Comparisons with measured precipitation show good agreement for the interactive aerosol scenario as well as for the extreme maritime aerosol scenario.

Modelling of a strong dust event in the complex terrain of the Dead Sea valley during the passage of a gust front

The area of the Dead Sea valley and the adjacent regions are often affected by mineral dust. This study focuses on an extreme dust episode occurring on 22 March 2013, where near-surface dust concentrations of up to 7000 µg m−3 were encountered in the Dead Sea region. This episode is of great interest as it was accompanied by high wind speeds and a gust front that rapidly passed the Judean Mountains. Wind was even accelerated on the lee side of the Judean Mountains leading to a severe downslope wind. We simulated this situation with the comprehensive online-coupled weather forecast model COSMO-ART. Fair agreement was found between the simulated meteorological variables and the observations. The model was capable of producing a reasonable spatiotemporal distribution of near-surface dust concentration, consistent with available measurements in this area. With respect to the time of the maximum near-surface dust concentration in the Dead Sea valley, the model captured it almost perfectly compared to the observed total suspended particle (TSP) concentrations. COSMO-ART showed that the high near-surface dust concentration in the Dead Sea valley was mainly determined by local emissions. These emissions were caused by strong winds on the lee side of the Judean Mts. The model showed that an ascending airflow in the Dead Sea valley lifted dust particles, originating mainly from the upwind side of the Judean Mts., up to approximately 7 km. These dust particles contributed to the pronounced maximum in modelled dust aerosol optical depth (AOD) over the valley. Here we highlight the important point that the simulated maximum dust AOD was reached in the eastern part of the Dead Sea valley, while the maximum near-surface dust concentration was reached in the western part of the valley.

Sensitivity of post-frontal convective precipitation on natural and anthropogenic aerosol particles

The impact of aerosol on clouds and precipitation varies depending on the cloud type. This case study focusses on convective clouds developing after the passage of a frontal system from the North Sea over Germany. In contrast to deep convection, the precipitation amount formed in the postfrontal cummuli is thought to show a systematic sensitivity to the natural and anthropogenic aerosol burden. During the front passage, the preexisting anthropogenic aerosol is replaced to a large extent by seasalt aerosol. To quantify the influence of aerosol on these postfrontal cummuli and their precipitation simulations with a comprehensive online coupled model system are performed. As precipitation reacts sensitive to disturbances of any atmospheric state variable, also simulations with slightly and randomly disturbed temperature fields were performed to contrast these effects with the aerosol effect.