Ice Production in Climatically Important Northern Hemisphere Cold-Air Outbreak Clouds (Invited)

Cold-air outbreaks (CAOs) are common high-impact weather events that produce extensive boundary layer mixed-phase clouds. CAO cloud systems have been identified as a source of substantial uncertainty in our climate models. The amount of ice in these clouds is important for defining the magnitude of the cloud-climate feedback and for how sensitive the global climate is to carbon dioxide emissions. To address ice production in Northern Hemisphere CAOs we conducted two contrasting 1-month long FAAM (BAE-146) aircraft campaigns in 2022, one around Svalbard in spring (ACAO) and one in the Labrador Sea in Autumn (M-Phase), to probe the aerosol properties, cloud microphysics and thermodynamics of CAO events.

We found that ice-nucleating particle (INP) concentrations and median ice particle concentrations in these Northern Hemisphere CAOs were orders of magnitude greater than those in boundary layer mixed-phase clouds over the Southern Ocean. Indeed, we show that the median ice number concentration increases with increasing INP concentration, even when secondary ice production (SIP) processes are active.

The springtime ACAO cases had systematically greater INP (and aerosol) concentrations than the autumnal M-Phase cases. The observed presence of substantial amounts of mineral dust in the springtime Arctic, despite all local sources being covered in ice and snow, implies a reservoir of old aerosol in the springtime Arctic that originated from much lower latitudes. Hence, we conclude that to define ice production in Northern Hemisphere CAOs (where the synoptic flow is from the north), we need to understand low and mid-latitude sources of INPs, how they are transported to the Arctic and what happens to them while trapped in the stable polar region for weeks or months before being drawn out in CAO events. Our global aerosol modelling reveals that Asian and African dust sources may be particularly important, however there also appears to be a substantial population of biogenic INPs that must be derived from sources with biological activity.

Our cloud modelling shows that INPs are key to defining the stratus-to-cumulus transition and strongly influence liquid water path and albedo. Given the stark contrast of primary ice production in CAO clouds in different locations and times around the globe, we conclude that the primary production of ice in simulated CAO clouds, as well as subsequent secondary ice processes, should be linked to the aerosol properties and knowledge of the local INP population to reduce uncertainty in cloud feedback and climate sensitivity.