Speaker is Professor Trude Storelvmo, Department of Geosciences, University of Oslo, Norway

Abstract: Arctic mixed-phase clouds are known to strongly influence the energy budget and climate state of the region but remain poorly understood and are notoriously difficult to represent in weather- and climate models. Here, we present the outcomes of a multiscale approach intended to improve small-scale process understanding using in situ measurements and cloud-resolving models (CRMs), and thereafter assess large-scale impacts of Arctic mixed-phase clouds using satellite remote sensing and Earth System Models (ESMs). Highlights from these findings include: (1) Arctic mixed-phase clouds are actually not that well mixed, but rather heterogeneous. This has implications for how they should be represented in numerical models. (2) Our in situ measurements focus on Arctic marine cold air outbreaks (MCAOs) and show that distance from sea ice is a good predictor of MCAO cloud microphysical properties. (3) The only good predictor of our measurements of Arctic ice-nucleating particles (INPs) is temperature. All other (mostly aerosol-related) explanatory variables available to us fail. We have therefore developed an Arctic-specific temperature-dependent INP parameterization and tested it in both a CRM (WRF) and an ESM (NorESM), with good agreement with observations and large implications for cloud radiative effects. (4) While we find that active remote sensing (CloudSat/CALIPSO) suggests that Arctic mixed-phase clouds should produce snow about 10 – 30% of the time depending on season, CMIP6 models and the ERA5 reanalysis severely overestimate this frequency. In practice, they simulate perpetual snow production in Arctic mixed-phase clouds. (5) We have used both WRF and NorESM, constrained by in situ and satellite observations, to assess how Arctic mixed-phase clouds will change in the future, in response to (i) warming and (ii) INP perturbations and (iii) a combination of the two. While some findings were as expected, others were great surprises to us, but consistently our results suggest that mixed-phase clouds will be a major influence on Arctic climate change in decades to come.