Monday, 11 January 2016
Long-lived, Arctic mixed-phase clouds play a crucial role in modulating the surface energy balance over the Greenland Ice Sheet (GIS). However, little is known about the mechanisms that owe to their persistence due to infrequent observations and poor representation in numerical models. A theoretical framework by Morrison et al. 2012 hypothesizes that several local processes, such as cloud-top longwave cooling, surface fluxes, and turbulent vertical motions, work together to drive a mixed-phase cloud system into a quasi-steady, persistent state. However, perturbations to this state by the synoptic environment can disrupt the balance between these local processes and lead to the cloud's dissipation. This study investigates the hypothesis that local (internal) forcings can drive a cloud's persistent state, while the synoptic (external) forcings influence the thermodynamic structure of the lower troposphere, ultimately impacting the forcings on the local scale. A steady, single-layer, low-level, mixed-phase cloud was observed from 20-24 July 2012 from the Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit (ICECAPS) cloud-atmosphere observatory at Summit, Greenland. The Advanced Research Weather Research and Forecasting (WRF-ARW) model with polar modifications (PWRF) with Global Forecast System (GFS) final reanalysis data as initial and boundary conditions is used here in a series of controlled experiments to examine this study's hypothesis in this mixed-phase cloud event. First, the role of the large-scale, external forcings is examined by fixing the boundary conditions to isolate the influence of the large-scale flow. This first experiment reveals that moisture advection from various sources played a role in the cloud's maintenance. Moisture advection from a strong surface cyclone located east of Summit was an important source early in the period, while moisture advection from the west became an important source later. Second, the role of the local, internal forcings is examined by modifying the cloud radiative feedbacks. This experiment shows that the unique optical properties of the cloud that drive the longwave, cloud-top cooling were key for its formation. Without this cooling at the cloud top, no cloud forms at all. Shortwave radiation does not have a significant impact on cloud lifetime; however, its does impact the liquid and ice content of the cloud.
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