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 north slope of Alaska at Oliktok Point (OLI), the ASCOS field campaign in the high Arctic Ocean, and the ICECAPS-ACE project at the NSF 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 (LES) 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 LES simulations 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 driver 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 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.
Highlights
The Arctic has been shown to be extremely sensitive to a warming climate, with data showing the Arctic warming anywhere from 1.5 - 4.5x the global mean warming rate (Holland and Bitz, 2003; Serreze and Barry, 2011; Cohen et al, 2014; Previdi et al, 2021)
Observed liquid water path (LWP) data were taken from microwave radiometers at Oliktok Point (OLI) (Gaustad, 2014), Arctic Summer Cloud Ocean Study campaign (ASCOS) (Westwater et al, 2001), and SMT (Cadeddu, 2010) This figure shows that, in all cases, the simulated LWP decreases to near-zero within hours of the aerosol removal time (09z in OLI, 06z in ASCOS and SMT)
The simulations should theoretically represent the fastest possible dissipation of a cloud due to insufficient aerosol. Where this simulated LWP response is slower than observations - such as OLI - it is likely that a lack of aerosol is not the primary driver of dissipation
Summary
The Arctic has been shown to be extremely sensitive to a warming climate, with data showing the Arctic warming anywhere from 1.5 - 4.5x the global mean warming rate (Holland and Bitz, 2003; Serreze and Barry, 2011; Cohen et al, 2014; Previdi et al, 2021). Of particular note in the Arctic environment are low-level, boundary layer stratocumulus clouds which cover large fractions of the Arctic throughout the year (Shupe, 2011). They have been found to be a net-warming influence on the surface, except for a short period in the summer when they act as a net-cooling influence (Intrieri et al, 2002; Shupe and Intrieri, 2004; Sedlar et al, 2011). These clouds tend to be mixed-phase, meaning they simultaneously contain liquid and ice water. Difficulties in parameterizing ice processes, the physical complexities and uncertainties involved with liquid and ice water coexisting, and a lack of observations in the Arctic make these clouds a known problem for numerical models of all scales (Sotiropoulou et al, 2016; Klein et al, 2009; Morrison et al, 2009, 2012, 2011); understanding the processes involved in the formation and dissipation of these clouds is essential to understanding the energy balance in the Arctic and for proper 35 representation in models
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
