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Abstract
Abstract. This study investigates the interactions between cloud dynamics and aerosols in idealized large-eddy simulations (LES) of Arctic mixed-phase stratocumulus clouds (AMPS) observed at Oliktok Point, Alaska, in April 2015. This case was chosen because it allows the cloud to form in response to radiative cooling starting from a cloud-free state, rather than requiring the cloud ice and liquid to adjust to an initial cloudy state. Sensitivity studies are used to identify whether there are buffering feedbacks that limit the impact of aerosol perturbations. The results of this study indicate that perturbations in ice nucleating particles (INPs) dominate over cloud condensation nuclei (CCN) perturbations; i.e., an equivalent fractional decrease in CCN and INPs results in an increase in the cloud-top longwave cooling rate, even though the droplet effective radius increases and the cloud emissivity decreases. The dominant effect of ice in the simulated mixed-phase cloud is a thinning rather than a glaciation, causing the mixed-phase clouds to radiate as a grey body and the radiative properties of the cloud to be more sensitive to aerosol perturbations. It is demonstrated that allowing prognostic CCN and INPs causes a layering of the aerosols, with increased concentrations of CCN above cloud top and increased concentrations of INPs at the base of the cloud-driven mixed layer. This layering contributes to the maintenance of the cloud liquid, which drives the dynamics of the cloud system.
Highlights
Arctic mixed-phase stratocumulus clouds (AMPS) play a unique role in climate by producing a net warming at the Earth’s surface over the annual cycle
In this study we use idealized large-eddy simulations to quantify the relative impact of cloud condensation nuclei (CCN) and ice nucleating particles (INPs) perturbations on the phase partitioning and dynamics of AMPS
The first set of simulations was designed to investigate the impact of relatively small perturbations in CCN compared to studies such as Morrison et al (2008) and Kravitz et al (2014) in mixed-phase conditions with essentially constant INPs on phase partitioning and cloud dynamics
Summary
Arctic mixed-phase stratocumulus clouds (AMPS) play a unique role in climate by producing a net warming at the Earth’s surface over the annual cycle. For a fixed liquid water path (LWP), an increase in CCN in warm clouds will increase the number of droplets and reduce the droplet size This causes an increase in cloud albedo (the Twomey effect; Twomey, 1977) and potentially suppresses precipitation (the Albrecht effect; Albrecht, 1989). Smaller droplets can reduce the ice water path (IWP) through a reduction in collision–coalescence and riming of snow by droplets (Morrison et al, 2008) as well as make ice nucleation less efficient (Lance et al, 2011) These are only a few examples of buffering feedbacks that exist in mixed-phase clouds. Are there buffering feedbacks in AMPS that limit the impact of CCN–INP perturbations?
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