The relative impact of cloud condensation nuclei and ice nucleating particle concentrations on phase partitioning in Arctic mixed-phase stratocumulus clouds

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.

A Sensitivity Study of Arctic Air‐Mass Transformation Using Large Eddy Simulation

“…In agreement with previous findings (e.g. by Possner et al, 2017; Solomon et al, 2018; Stevens et al, 2018), the LWP (and IWP) of the mixed‐phase cloud increased with increasing CCN concentration. However, the change was non‐linear so that a CCN increase at low levels gave a smaller impact on the LWP than a change at high CCN levels.…”

“…In agreement with Possner et al (2017), Stevens et al (2018) and Solomon et al (2018), we also found that a doubling of the ICNC decreases the LWP (Figure 11). The higher ICNC pushed the cloud into a tenuous regime and the surface temperature decreased rapidly.…”

“…When the CCN concentration was doubled, LWP and IWP increased which is consistent with the studies by e.g. Possner et al (2017), Stevens et al (2018) and Solomon et al (2018). Decreasing the CCN concentration resulted in a decrease in LWP and IWP, but their change was relatively small, i.e.…”

“…the effect was non‐linear, which also corroborates the results of e.g. Solomon et al (2018). However, even if the LWP response was relatively small when the CCN concentration was decreased (compared to when it was increased), our study shows that the change in longwave emission and surface temperature was relatively high (Figure 11) since the cloud became more tenuous.…”

Aerosol Effect on the Cloud Phase of Low‐Level Clouds Over the Arctic

“…For example, Coopman, Garrett, et al () using multiyear satellite, meteorological, and tracer transport model data found that low‐level Arctic cloud response to anthropogenic aerosols are 2 to 8 times higher than in lower latitudes and that the role of biomass aerosols to Arctic clouds response have been overstated in previous studies. Consequently, the relative contribution of aerosol to the radiative forcing of Arctic clouds is still highly uncertain (Coopman, Riedi, et al, ; Flanner, ; Garrett et al, ; Garrett et al, ; Norgren et al, ; Solomon et al, ; Wylie & Hudson, ; Zamora et al, ; Zhao & Garrett, ).…”