PNNL

The Science

Cold air moving over a warmer ocean can form elongated cloud streets that later break into smaller cells. Predicting this transition is difficult because cloud ice, precipitation, and aerosols all affect cloud development. In simulations of a March 2020 Arctic marine cold-air outbreak, higher ice concentrations consistently shortened cloud-roll persistence. Snow sublimation below the clouds cooled and dried the air, weakening the connection between the ocean surface and the clouds. As the boundary layer decoupled from the surface, free-tropospheric aerosol remained effective at reaching the cloud layer, while upward transport of ocean-emitted aerosol weakened.

The Impact

Mixed-phase clouds that form during cold-air outbreaks are difficult to predict because their evolution depends on interactions among aerosols, ice, and turbulent mixing. This study showed that aerosol source mattered as the boundary layer became decoupled from the surface. Surface emissions supplied more aerosol to the boundary layer overall, but after decoupling, free-tropospheric entrainment supplied the cloud layer more effectively. These results could inform aerosol and ice treatments in weather and Earth system models. They also pointed to the value of aerosol profile measurements above and within the boundary layer. Such observations could test how particles move into clouds and improve understanding of high-latitude cloud evolution and air–sea exchange.

Summary

Marine cold-air outbreaks (MCAOs) occur when cold air moves over a warmer ocean, creating mixed-phase clouds that contain liquid water and ice. Researchers used high-resolution, Lagrangian large-eddy simulations to follow an air mass along its trajectory during a March 2020 MCAO over the Norwegian Sea, observed during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE). The model represented changing aerosol concentrations, cloud droplets, ice particles, precipitation, and aerosol removal. Simulations tested the relative importance of aerosols supplied from the ocean surface and from above the boundary layer. Satellite liquid-water path retrievals and ground-based observations were used to evaluate the simulated cloud properties.

The simulations showed that prognostic treatments of aerosols and ice captured the early evolution of cloud liquid water more closely than runs with prescribed particle concentrations. Snow accretion was the main aerosol removal process. Surface emissions supplied more aerosol to the boundary layer overall, but after decoupling, free-tropospheric entrainment supplied the cloud layer more effectively. 

Contacts

Shaima Nasiri and Sally McFarlane; ASR and ARM; 

Shaima.Nasiri@science.doe.gov 

Sally.McFarlane@science.doe.gov

Funding

This research was supported by the U.S. Department of Energy, Office of Science, Atmospheric System Research program at Pacific Northwest National Laboratory. The work used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility, under award BER-ERCAP0030247. Peter N. Blossey acknowledged additional support from the DOE Atmospheric System Research program through grant DE-SC0020134. Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute.