Measuring the variability in ice-nucleating particles over the Southern Ocean to reduce uncertainty in cloud-climate feedbacks (IceSO)

This proposal tackles problems at the core of one of the biggest uncertainties in our climate models – the properties of low-level clouds over the Southern Ocean and the extent to which they buffer warming from increased CO2. Clouds over the Southern Ocean frequently occur well below 0°C and are therefore composed of a mixture of supercooled water and ice. The balance between water and ice is key to defining their interaction with incoming and outgoing radiation as well as their lifetime, yet this balance is poorly represented by current models. The presence of ice-nucleating particles leads to the removal of liquid water from clouds and a transition from a cloudy, high-albedo state to a blue sky, low-albedo state where the dark ocean surface is exposed. Our knowledge of the enigmatic particles that trigger ice formation in clouds is very poor for much of the globe, not least the Southern Ocean (SO). Ice-nucleating particles (INPs) are thought to be made up of a combination of biogenic material associated with sea spray and sporadic injections of terrestrial aerosol. However, current datasets lack the time resolution required to resolve the temporal variability or are only from short term campaigns focused on the summer. Consequently, we have a limited confidence in our models, and they fail to represent INP concentrations and their variability. The objectives of IceSO are to: Define the seasonal cycle of INP concentrations over the Southern Ocean by making the first high-time resolution INP measurements over 18 months at the well-established Kennaook/Cape Grim observatory using our newly developed and unique PINE instrument. Develop a global INP model that is informed by and constrained to the new and partner INP measurements within the Met Office Unified Model (UM), providing a three-dimensional distribution of INP over the SO throughout the full seasonal cycle and, because we link it to sources, will respond to changing land use and emission fluxes in simulations of possible future warmer worlds. Quantify how the new INP knowledge reduces biases in Southern Ocean mixed-phase clouds and explore feedbacks, achieved by collaborating with the CAPE-K project and through more-realistic and observationally constrained simulations of the Met Office Unified Model. IceSO is made possible by the invention of PINE. This is a mobile cloud chamber that we recently developed together with Karlsruhe Institute of Technology designed for autonomous quantification of INP over the full range of mixed-phase cloud temperatures. This development represents a step-change in our ability to make INP measurements and opens the possibility for long-term autonomous sampling of INP. The Kennaook/Cape Grim Baseline Air Pollution Station (40°S, 144°E) provides a perfect site for quantification of INP in the SO region, being situated at the latitude band of greatest low-cloud feedback on Earth. This station has a wide clean air sector representative of the wider SO and is also influenced by continental air masses bringing mineral dust and potentially other INP types. We have timed IceSO to coincide with the US funded CAPE-K project (U.S. Dept. of Energy) that will provide complementary measurements of cloud properties alongside the long running aerosol measurements. This will produce an unprecedented high-time resolution, long-term full seasonal cycle of the INP concentrations in the SO that will allow us to address the unacceptably high uncertainty in Southern Ocean cloud feedbacks.