Aerosol-Cloud Interactions (ACI)

Furthermore, we assessed with the global model ECHAM6-HAM2 how aerosol-cloud interactions in the Arctic might change by the year 2050. Due to global warming, sea ice will continue to melt. The reduction in sea ice leads to higher emissions of sea salt particles, which can act as CCN. In addition, also the specific humidity in the Arctic will increase. In our analysis, we focused on the months when least sea ice is left. We found that the cloud droplet number concentration as well as the size of the cloud droplets will increase in the future.

When less sea ice is present, more ships might pass the Arctic Ocean. In our simulations, the additional aerosol emissions due to shipping had a small effect on clouds. However, an increase in the mass emissions by a factor of ten led to pronounced increases in the cloud droplet number concentration and the liquid water content.
For more information see external page http://doi.org/10.5194/acp-18-10521-2018.

ACI in Swiss Alps

In the Swiss Alps, the aerosol concentrations are much higher. Increased aerosols, cloud condensation nuclei and ice nuclei, have opposing effects on precipitation formation and location of orographic mixed-phased clouds. Increasing cloud condensation nuclei reduces the efficiency of supercooled liquid droplets or raindrops to collide and freeze upon contact with ice particles (Saleeby et al., 2009), delaying precipitation formation and location. Increasing the ice nuclei, however, increases ice crystal growth at the expense of liquid water droplets, glaciating the cloud, accelerating and increasing precipitation formation (Glassmeier and Lohmann, 2018).

The focus of the planned research is on understanding the timing and location of precipitation and the precipitation formation of multilayered clouds in the Swiss Alps under the effect of anthropogenic aerosols. An online coupled system, COSMO-ART-M7, describing the interaction of aerosol particles with the atmosphere, will be used to simulate the occurrence of multilayered clouds during field campaigns.
For more information, please contact Zane Dedekind ().

Constraints on ACI

Observations of aerosol-cloud interactions (ACI) are necessary to constrain model simulations. The global record of high-resolution satellite observations of cloud and aerosol properties covers currently almost two decades (some cloud retrievals cover almost four decades). Apart from problems associated with the retrieval itself, one particular problem of using satellite data is that spurious relationships between aerosols and clouds can appear due to confounding variables like relative humidity. In particular the statistical relationship of cloud fraction and aerosol optical depth is known to be mainly driven my (small scale) variability in relative humidity, rather than representing a strong effect of aerosol particles on cloud fraction. In two companion studies, Christensen et al. (2017) and Neubauer et al. (2017) show an approach how ACI in a global model can be constrained by satellite data while avoiding spurious relationships e.g. due to relative humidity. Careful sampling of aerosol retrievals (aerosol 15 km away from clouds only, which has taken up less water than aerosol near clouds) is paired with simulated dry aerosol of ECHAM-HAM to show that ACI obtained this way are considerably weaker (~50% less for cloud brightening and liquid water path adjustment and ~70% less for cloud fraction adjustment) than without accounting for spurious relationships.

ACI in southern West Africa

Until at least the middle of the 21st century the population is expected to increase in West Africa. In particular, the number of inhabitants in cities is expected to grow. Increasing industrialisation is supposed to further result in an increase in anthropogenic emissions. Within the EU-Project DACCIWA we performed future projections with the global aerosol climate model ECHAM6-HAM2 to investigate the impact of increased anthropogenic emissions on climate change and air quality in southern West Africa.

Low level clouds are a prominent feature in the West African Monsoon system during summer. They are not well represented in global climate models. However, ECHAM6-HAM2 is able to simulate them, but their amount is underestimated and their prominent diurnal cycle is not captured by the model. We investigate how changes in aerosol concentrations impact these clouds.
For more information, please contact Tanja Stanelle ()