The Entrainment Efficiency of Persistent Arctic Mixed-Phase Clouds as Inferred from Daily Large-Eddy Simulations during the MOSAiC Drift

Abstract Low-level mixed-phase clouds occur frequently and persistently in the central Arctic and thus play a key role in climate feedback mechanisms, airmass transformations, and sea ice melt. Turbulent entrainment at cloud top driven by radiative cooling modulates these clouds by affecting the boundary layer heat budget. However, reliable measurements of this small-scale process are scarce. This study presents new insights into entrainment in radiatively driven cloudy mixed layers at high latitudes based on a library of daily large-eddy simulations covering the full Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) drift. The simulations are based on measurements, cover a periodic and homogeneously forced small domain representing conditions observed at the Polarstern research vessel, and resolve Arctic turbulence and clouds to a high degree. Approximately 1 out of 3 simulated days contains cloud mass in the liquid phase. A drift-average heat budget analysis shows that the bulk cloud-topped mixed layer is dominated by radiative cooling. Warming by top entrainment partially counters this cooling, at efficiencies of about 21%. While this compensation is significant, such efficiencies are also much lower compared to previous findings for subtropical warm marine stratocumulus. Interestingly, a few outlying MOSAiC cases show similarly high efficiencies. Analysis of turbulence energetics and dedicated sensitivity experiments reveals that high entrainment efficiency can be achieved in two ways: surface coupling and strong local wind shear. The former explains the high efficiencies in the subtropics, while the latter drives the highest efficiencies encountered during MOSAiC. In general, these findings emphasize the important role played by wind shear in boosting entrainment efficiency. Significance Statement Stratocumulus cloud layers in the high Arctic often contain liquid water at subzero temperatures. Such cloud layers cool rapidly through radiation, a process that locally creates turbulence. This turbulence causes mixing in the atmosphere. Using many high-resolution atmospheric model simulations on a supercomputer, we investigate how this mixing process causes warming in the cloud layer. The simulations are closely based on measurements made during the recent yearlong Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) drift experiment. We find that the warming only partially counteracts the radiative cooling and that a strong change in the wind with height can affect this balance.