Microphysical sensitivity of coupled springtime Arctic stratocumulus to modelled primary ice over the ice pack, marginal ice, and ocean

摘要

This study uses large eddy simulations to test the sensitivity ofsingle-layer mixed-phase stratocumulus to primary ice number concentrationsin the European Arctic. Observations from the Aerosol-Cloud Coupling andClimate Interactions in the Arctic (ACCACIA) campaign are considered forcomparison with cloud microphysics modelled using the Large Eddy Model (LEM,UK Met. Office). We find that cloud structure is very sensitive to ice numberconcentrations, , and small increases can cause persistingmixed-phase clouds to glaciate and break up.Three key dependencies on are identified from sensitivitysimulations and comparisons with observations made over the sea ice pack,marginal ice zone (MIZ), and ocean. Over sea ice, we finddeposition–condensation ice formation rates are overestimated, leading tocloud glaciation. When ice formation is limited to water-saturatedconditions, we find microphysics comparable to aircraft observations over allsurfaces considered. We show that warm supercooled (−13 °C)mixed-phase clouds over the MIZ are simulated to reasonable accuracy whenusing both the DeMott et al.(2010) and Cooper(1986) primaryice nucleation parameterisations. Over the ocean, we find a strongsensitivity of Arctic stratus to . TheCooper(1986) parameterisation performs poorly at the lowerambient temperatures, leading to a comparatively higher (2.43 L at the cloud-top temperature, approximately −20 °C)and cloud glaciation. A small decrease in the predicted (2.07 L at −20 °C), using the DeMott et al.(2010)parameterisation, causes mixed-phase conditions to persist for 24 h over theocean. However, this representation leads to the formation of convectivestructures which reduce the cloud liquid water through snow precipitation,promoting cloud break-up through a depleted liquid phase. Decreasing the further (0.54 L, using a relationship derived fromACCACIA observations) allows mixed-phase conditions to be maintained for atleast 24 h with more stability in the liquid and ice water paths.Sensitivity to is also evident at low numberconcentrations, where 0.1  ×   predicted by theDeMott et al.(2010) parameterisation results in the formation of rainbandswithin the model; rainbands which also act to deplete the liquid water in thecloud and promote break-up.