LES Simulations of the Influence of Aitken Aerosols on Summertime Stratocumulus Clouds

Abstract:

Aitken mode aerosol particles may play a significant role in cloud evolution by replenishing cloud condensation nuclei against loss from precipitation or droplet coalescence. This source could be especially important in remote marine boundary layers where accumulation mode aerosol concentrations are relatively low. These same regions exhibit heightened Aitken mode aerosol concentrations associated with production from phytoplankton emissions (e.g. dimethyl sulfide and its oxidized products). McCoy et al. (2020) have proposed that Aitken aerosols produced by synoptic uplift may help maintain higher than expected cloud droplet concentrations in Southern Ocean stratocumulus. We explore this “Aitken buffering” hypothesis using simulations with the SAM LES model modified to include Aitken mode aerosols.

Our model represents aerosol size distributions with two log-normal modes, an Aitken and an accumulation mode. Activated Aitken particles become part of the accumulation mode. Aerosol coagulation and interstitial scavenging of dry aerosols by cloud and raindrops are also treated. We use the 2-moment bulk microphysics scheme of Morrison and Grabowski (2008). A simplified sulfur chemistry scheme is also included. Since our model does not include gas-to-particle conversion, the sources of Aitken-mode nuclei in our model are entrainment from a fixed number concentration in the free troposphere and surface fluxes.

We focus on Aitken influences on summertime stratocumulus cases in three regions. In a Southeast Pacific stratocumulus case idealized from VOCALS RF06, the simulated boundary layer collapses due to gradual loss of accumulation mode aerosol. In a perturbed simulation with a high Aitken concentration above the inversion, the activation of Aitken particles stabilizes the accumulation mode in the boundary layer and prevents boundary layer collapse.

We study the processing and influence of Aitken aerosol in a decoupled stratocumulus case over the summertime eastern North Atlantic from the Atmospheric Radiation Measurement (ARM) ACE-ENA aircraft campaign. We also examine cases from the summertime Southern Ocean sampled during the NSF SOCRATES field campaign. Implications for cloud albedo susceptibility and the pre-industrial aerosol state are discussed.