UC Davis

This dissertation contains three studies investigating processes within mixed phase clouds. The first study examines the ice habit (shape) assumptions used in microphysics schemes. Ice habit is greatly simplified in models, and uncertainties remain in how to best model ice processes. We simulate a Sierra Nevada snowstorm driven by an atmospheric river. We have simulated the storm with four fixed-habit types as well as with an ice habit scheme that is variable in time and space. In contrast to some previous studies, we found substantially smaller sensitivity of total accumulated precipitation amount and negligible changes in spatial distribution to the ice habit specification. The reason for smaller sensitivity seems to be linked to strong aggregation of ice crystals in the model. Nonetheless, while changes in total accumulated precipitation were small, changes in accumulated ice hydrometeors were larger. The variable-habit simulation produced up to 37% more ice precipitation than any of the fixed-habit simulations with an average increase of 14%. The variable-habit simulation led to a maximization of ice growth in the atmosphere and, subsequently, ice accumulation at the surface. This result points to the potential importance of accounting for the time and space variation of ice crystal properties in simulations of orographic precipitation.

The next two studies concern mixed-phase stratocumulus clouds in the Arctic. These clouds are ubiquitous in the Arctic, and can persist for days and dissipate in a matter of hours. The first of these two studies concerns aerosol-limited dissipation. It is sometimes unknown what causes the observed sudden dissipation of these clouds, 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 Characterization 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.

The third study investigates the role of tropospheric aerosol in the sustenance of Arctic mixed-phase clouds. Recent studies have reported observations of enhanced aerosol concentrations directly above the Arctic boundary layer, and it has been suggested that Arctic boundary layer clouds could entrain these aerosol and activate them. We used an idealized LES modeling framework where aerosol concentrations are kept low in the boundary layer, and increased up to 50x in the free troposphere. We find that the simulations with higher tropospheric aerosol concentrations persisted for longer and had higher LWPs. This is due to direct entrainment of tropospheric aerosol into the cloud layer, resulting in three processes/feedbacks: 1) a precipitation suppression from the increase in cloud droplet number and decrease in radius 2) stronger cooling at cloud top due to the higher liquid water content at cloud top, which causes stronger circulations maintaining the cloud in the absence of surface forcing and 3) the result of the first two processes is a boundary layer top height that is more stable in time, so that it remains in contact with the tropospheric aerosol reservoir and can maintain entrainment of those aerosol. The boundary layer aerosol and cloud droplet concentrations, however, remained low in all simulations. Surface based measurements in this case would not necessarily suggest the influence of tropospheric aerosol on the cloud, despite it being necessary for stable cloud persistence.