Observations of ice optical and microphysical properties in Arctic low-level mixed-phase clouds during ACLOUD

Observations of late spring and summer time stratiform clouds over pack ice, marginal sea ice zone and open water during the ACLOUD campaign have shown that relatively high ice particle number concentrations up to 35 L^-1 are observed in cases where cloud top temperatures are between -3.8 and -8.7°C. This elevation in ice crystal number can likely be linked with secondary ice production. Simultaneous measurements of ice optical properties showed that a relative low asymmetry parameter between 0.69 and 0.76 can be associated with the mixed-phase cloud ice crystals. The condensed water path is dominated by the liquid phase at the cloud top in most of the studied cases except in one case study of a system with embedded convection where ice extinction exceeded the liquid extinction. Radiative transfer simulations have shown that the ice phase in low-level mixed-phase clouds, otherwise dominated by liquid phase, can also be radiatively important in cases where ice phase contributes to the cloud top extinction. This highlights the importance of an accurate vertical information of ice extinction within Arctic low-level clouds. The results of this study provide an important basis for testing and improving cloud microphysical parameterizations in models in order to accurately predict Arctic warming.