Interplay between aerosol and updraft velocity in Large Eddy Simulations of marine stratocumulus clouds

Abstract. Marine stratocumulus are low-level clouds with a great impact on the Earth’s energy balance. The present study is focused on understanding the interplay between aerosols and updraft velocity in marine stratocumulus clouds using Large Eddy Simulations (LES) over a 6.4×6.4 km² domain size with a double-moment aerosol-cloud microphysics scheme. A first series of experiments with aerosol concentrations varying from pristine to polluted conditions shows a transition from aerosol-limited to updraft-limited regime. The higher aerosol concentration in polluted conditions leads to the suppression of precipitation due to a larger number of cloud droplets, suggesting a transition from an open-cell to a closed-cell structure. A second series of experiments, where updraft velocity is enhanced by increasing latent heat flux, shows an increase in vertical velocity variance and a higher cloud droplet number, indicating enhanced convective activity with stronger updrafts and downdrafts. Cloud susceptibility is equal to 1 for both experiments at lower aerosol concentration, clearly indicating the presence of an aerosol-limited regime where updraft velocity has little impact. At higher aerosol concentration, cloud susceptibility is higher for stronger updrafts in the second series of experiments, indicating that stronger updrafts can shift regime from updraft-limited to aerosol-limited. Stronger updrafts also influence aerosol availability and activation, blurring the distinction between aerosol-limited and updraft-limited regimes because of the key role updraft velocity plays in regulating aerosol activation. Overall, the study demonstrates that LES is capable of reproducing both regimes as well as the transition between them.