Do Arctic mixed-phase clouds sometimes dissipate due to insufficient aerosol? Evidence from comparisons between observations and idealized simulations

Abstract

Abstract. Mixed-phase clouds are ubiquitous in the Arctic. These clouds can persist for days and dissipate in a matter of hours. It is sometimes unknown what causes this sudden dissipation, but aerosol–cloud interactions may be involved. Arctic aerosol concentrations can be low enough to affect cloud formation and structure, and it has been hypothesized that, in some instances, concentrations can drop below some critical value needed to maintain a cloud. We use observations from a Department of Energy ARM site on the northern slope of Alaska at Oliktok Point (OLI), the Arctic Summer Cloud Ocean Study (ASCOS) field campaign in the high Arctic Ocean, and the Integrated Characterisation of Energy, Clouds, Atmospheric state, and Precipitation at Summit – Aerosol Cloud Experiment (ICECAPS-ACE) project at the NSF (National Science Foundation) Summit Station in Greenland (SMT) to identify one case per site where Arctic boundary layer clouds dissipated coincidentally with a decrease in surface aerosol concentrations. These cases are used to initialize idealized large eddy simulations (LESs) in which aerosol concentrations are held constant until, at a specified time, all aerosols are removed instantaneously – effectively creating an extreme case of aerosol-limited dissipation which represents the fastest a cloud could possibly dissipate via this process. These LESs are compared against the observed data to determine whether cases could, potentially, be dissipating due to insufficient aerosol. The OLI case's observed liquid water path (LWP) dissipated faster than its simulation, indicating that other processes are likely the primary drivers of the dissipation. The ASCOS and SMT observed LWP dissipated at similar rates to their respective simulations, suggesting that aerosol-limited dissipation may be occurring in these instances. We also find that the microphysical response to this extreme aerosol forcing depends greatly on the specific case being simulated. Cases with drizzling liquid layers are simulated to dissipate by accelerating precipitation when aerosol is removed while the case with a non-drizzling liquid layer dissipates quickly, possibly glaciating via the Wegener–Bergeron–Findeisen (WBF) process. The non-drizzling case is also more sensitive to ice-nucleating particle (INP) concentrations than the drizzling cases. Overall, the simulations suggest that aerosol-limited cloud dissipation in the Arctic is plausible and that there are at least two microphysical pathways by which aerosol-limited dissipation can occur.

Above-cloud concentrations of cloud condensation nuclei help to sustain some Arctic low-level clouds

“…The simulations in this study follow a similar setup to those in Sterzinger et al (2022): a 6 × 6 km 2 periodic domain with 62.5 m horizontal and 6.25 m vertical grid spacing. The model top was set at 1500 m. Simulations were run for a simulated 30 h with a 1 s time step.…”

“…This is especially curious given the low aerosol concentrations in the Arctic; boundary layer aerosol concentrations are at a minimum in the summer, with typical accumulation-mode concentrations less than 100 cm −3 and sometimes less than 1 cm −3 (Mauritsen et al, 2011;Heintzenberg et al, 2015). Such low concentrations may be insufficient to maintain clouds (Mauritsen et al, 2011;Stevens et al, 2018;Sterzinger et al, 2022).…”

Regionally sourced bioaerosols drive high-temperature ice nucleating particles in the Arctic

“…Aerosol particles and their interactions with clouds are considered to be one of the main sources of uncertainty in future climate model predictions 2 . In remote regions with lower aerosol concentrations, such as the Arctic, even subtle changes in aerosol particle sources can have significant impacts on cloud properties, such as cloud phase, radiative properties, cloud lifetime, and precipitation 5 , 6 ; These, in turn, are key elements in the Arctic amplification phenomenon 7 , 8 . With the current accelerated warming trends in the Arctic, the occurrence of open oceans 9 , leads 10 , greener tundra 11 and snow-free land is becoming more common throughout the Arctic 12 .…”

Observations of Fog‐Aerosol Interactions Over Central Greenland

“…In very clean environments, low CCN concentrations can limit fog (and cloud) formation and lifetime, because the few activated CCN will grow to relatively large sizes and precipitate out, removing CCN and preventing further droplet formation (Mauritsen et al., 2011; Stevens et al., 2018). Evidence suggests that this situation can occur in the Arctic, where naturally low concentrations of CCN (1–100 cm −3 ) have the potential to control cloud radiative properties (Mauritsen et al., 2011; Sterzinger et al., 2022). At Summit, the annual mean aerosol particle concentration is low even compared to other Arctic sites (Schmeisser et al., 2018); the mean annual total surface aerosol particle number concentration (>20 nm) at Summit in 2019–2020 was just 129 cm −3 , and fell to less than 10 cm −3 on occasions in all seasons (Guy et al., 2021).…”