Lagrangian LES of Marine Cold-Air Outbreaks during COMBLE: Aerosol Budgets and Their Dependence on Model Configuration

Marine cold-air outbreaks (MCAOs) produce mixed-phase clouds when cold air meets relatively warm water surfaces, spurring great turbulent surface fluxes. These clouds undergo regime transitions that can profoundly affect the regional radiation budget. Marine boundary layer (MBL) aerosol is modified through sea-spray fluxes, entrained free tropospheric (FT) air, and cloud microphysical collisional processes, such as rain formation and riming that in turn respond to aerosol available as cloud condensation nuclei (CCN) and ice nucleating particles (INP). Better understanding of the cloud-aerosol-precipitation interaction in present-day MCAOs is vital to authentically capture changes in reflected radiation in a warming climate.

Observed during Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE), we selected five MCAO cases over the Norwegian Sea that span a wide range of conditions. We use moving-domain Lagrangian large-eddy simulations (LES) that we initialize with realistic upwind aerosol conditions, gathered from in-situ observations at Zeppelin station (Svalbard). We present the simulated evolution of multimodal MBL aerosol in response to the aforementioned sinks and sources and highlight changes when modifying the microphysical configurations with respect to primary and secondary ice formation.

We find a tendency of all simulations to approach a quasi-equilibrium of modal aerosol characteristics wherein sources and sinks are roughly balanced after hundreds of kilometers of evolution. Simulations are generally able to reproduce several key features that are commonly observed, including preservation of a similar trimodal aerosol structure and a reduction in modal number concentrations. CCN number concentrations at Andenes were observed and simulated to span roughly an order of magnitude, which can be attributed primarily to differences in cumulative wet scavenging, although differences in sea spray emissions and MBL depth, and associated entrainment, also play a role. Simulations that produce more ice tend to increase CCN scavenging rates via riming but decrease the likelihood of a dramatic rain scavenging process.