Arctic mixed-phase clouds in large-eddy simulations and a mixed-layer model

Arctic mixed-phase clouds have a significant impact on the Arctic climate. These low clouds drive boundary layer turbulence through radiative processes, thus modifying the structures of the boundary layer. The Arctic boundary layer structures are particularly important for understanding polar amplification, to which mixed-phase clouds may contribute. Arctic cloud feedbacks likely also play an role in controlling aspects of the surface climate, such as the sea ice response to global warming. The complexity of the Arctic boundary layer and clouds calls for systematic and idealized studies to separate different processes. We use large-eddy simulations (LES) to study an idealized setup modeled after the Indirect and Semi-direct Aerosols Campaign (ISDAC). LES is capable of resolving the most energetic turbulent motions in the boundary layer with little dependence on parameterizations. The modified ISDAC case is an ideal first approach to study the response of low clouds to climate change because of its simplicity and flexibility. Short-wave radiation and surface fluxes are set to zeros, and a more comprehensive radiative transfer scheme RRTMG is used.The simulated cloud liquid water path is sensitive to the temperature, free tropospheric relative humidity, and the inversion strength at the cloud top. These three parameters could represent the large-scale changes in the Arctic atmosphere under climate change. For example, changes in horizontal advection from lower latitudes would result in altered humidity and temperature above the boundary layer. To further analyze the mechanisms controlling the clouds, a mixed-layer model is used to simulate the same cases. Most of the changes of cloud liquid water path are captured by the mixed-layer model, allowing us to deduce the relative importance of various factors in controlling the clouds. These results motivate ongoing studies of the Arctic cloud response to climate change in an idealized GCM.