High-Resolution Simulations Reveal Interesting Features of Arctic Mixed-Phase Clouds
Arctic mixed-phase stratocumulus clouds have been observed to persist for days due to compensating feedbacks between the formation and growth of ice and cloud droplets, radiative cooling, turbulence, entrainment, and surface fluxes of heat and moisture. These clouds play an important role in determining the structure of the Arctic atmospheric boundary layer and the magnitude of the surface energy budget. Understanding the factors that contribute to Arctic cloud formation and persistence is important for better representation of clouds in climate models. Previous studies suggested that humidity inversions may play an important role in maintaining Arctic mixed-phase clouds by providing moisture near the cloud top that is entrained into the cloud system. U.S. Department of Energy researchers conducted a series of idealized, high-resolution simulations to systematically investigate the relative impact of moisture sources above and below the cloud layer on the lifetime of Arctic stratocumulus clouds. They demonstrated that Arctic mixed-phase stratocumulus clouds have remarkable insensitivity to changes in moisture source. When the overlying air is dried initially, radiative cooling and turbulent entrainment increase moisture import from the surface layer, maintaining the cloud. When the surface layer is dried initially, the system evolves to a state with reduced mixed-layer water vapor and increased surface-layer moisture, reducing the loss of water through precipitation and entrainment of near-surface air, and again maintaining the cloud layer. Only when moisture is reduced both above and below the mixed layer does the cloud decay without reaching a quasi-equilibrium state. A fundamental finding of this study is that aspects of the relationships between the mixed layer, temperature and humidity inversions, and cloud top are different in Arctic stratocumulus than in the more frequently studied subtropical stratocumulus. In particular, cloud-top radiative cooling is not collocated with the mixed-layer top in the Arctic clouds, which has implications for sedimentation, buoyancy fluxes, and turbulence in the clouds. Therefore, model parameterizations based on subtropical clouds, such as entrainment parameterizations that assume the mixed-layer top and cloud top are collocated, need to be modified to accurately simulate Arctic stratocumulus.
Solomon, A., M. D. Shupe, O. Persson, H. Morrison, T. Yamaguchi, P. M. Caldwell, and G. de Boer. 2014. “The Sensitivity of Springtime Arctic Mixed-Phase Stratocumulus Clouds to Surface-Layer and Cloud-Top Inversion-Layer Moisture Sources,” Journal of the Atmospheric Sciences 71, 574-95. DOI: http://dx.doi.org/10.1175/JAS-D-13-0179.1.