Above-cloud concentrations of cloud condensation nuclei help to sustain some arctic low-level clouds
Submitter:
Igel, Adele — University of California, Davis
Area of research:
Cloud-Aerosol-Precipitation Interactions
Journal Reference:
Science
We investigated the importance of aerosol particles above cloud top for maintaining low-level clouds in the Arctic.
Impact
The presence of high concentrations of particles above cloud top and their demonstrated ability to help sustain clouds helps to resolve the question of how low-level arctic clouds can persist for so long despite very low boundary-layer aerosol concentrations.
Summary
"Previous studies have found that low-level arctic clouds often persist for long periods even in the face of very low surface cloud condensation nuclei (CCN) concentrations. Here, we investigate whether these conditions could occur due to continuous entrainment of aerosol particles from the free troposphere (FT). We use an idealized large-eddy simulation (LES) modeling framework, where aerosol concentrations are low in the boundary layer (BL) but increased up to 50× in the free troposphere. We find that the tests with higher tropospheric aerosol concentrations simulated clouds, which persisted for longer and maintained higher liquid water paths (LWPs). This is due to direct entrainment of the tropospheric aerosol into the cloud layer, which results in a precipitation suppression from the increase in cloud droplet number and in stronger cloud-top radiative cooling, which causes stronger circulations maintaining the cloud in the absence of surface forcing. Together, these two responses result in a more well-mixed boundary layer with a top that remains in contact with the tropospheric aerosol reservoir and can maintain entrainment of those aerosol particles. The surface aerosol concentrations, however, remained low in all simulations. The free-tropospheric aerosol concentration necessary to maintain the clouds is consistent with concentrations that are frequently seen in observations." (Published abstract)