A sensitivity study of Arctic air-mass transformation using Large Eddy Simulation

Arctic air mass transformation is linked to the evolution of low-level mixed-phase clouds. These clouds can alter the structure of the boundary layer and modify the surface energy budget. In this study, we use three-dimensional large eddy simulation and a bulk sea ice model to examine the lifecycle of clouds formed during wintertime advection of moist and warm air over sea ice, following a Lagrangian perspective. We investigate the stages of cloud formation, evolution, and decay. The results show that radiative cooling at the surface gives rise to fog formation which subsequently rises and transforms into a mixed-phase cloud. In our baseline simulation, the cloud persists for about 5 days and increases the surface temperature by on average 17 °C. Sensitivity tests show that the lifetime of the cloud is sensitive to changes in the vapor supply at cloud top. This flux is mainly impacted by changes in the divergence rate; an imposed convergence decreases the lifetime to 2 days while an imposed large-scale divergence increases the lifetime to more than 6 days. The largest difference in cloud radiative properties is found in the experiment with increased ice crystal number concentrations. In this case, the lifetime of the cloud is similar compared to baseline but the amount of liquid water is clearly depleted throughout the whole cloud sequence and the surface temperature is on average 6 °C cooler. The cloud condensation nuclei concentration has a weaker effect on the radiative properties and lifetime of the cloud.