Synoptic and Local Influences on a Summertime, Long-Lived, Mixed-Phase Cloud Event Over Summit, Greenland

• Main Author: Row, Mallory
• Other Authors: Cavallo, Steven, Turner, Dave, Homeyer, Cameron
• Language: English
• Published: 2016
• Subjects: Arctic; Greenland; mixed-phase clouds; Ice Sheet
• Online Access: http://hdl.handle.net/11244/44221

Long-lived, Arctic mixed-phase clouds play a crucial role in modulating the surface energy balance over the Greenland Ice Sheet. However, due to temporally and spatially inconsistent observations, little is known about the mechanisms that cause their longevity. A persistent, single-layer, mixed-phase cloud was observed from 20-24 July 2012 at the “Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit” (ICECAPS) cloud-atmosphere observatory in Summit Station, Greenland. The hypothesis in this study is motivated by \citet{morrison2012resilience}; this study investigates the hypothesis that local processes promote a cloud’s persistent state, while synoptic-scale processes influence the thermodynamic structure of the lower troposphere. This hypothesis is examined on the 20-24 July 2012 ICECAPS cloud event using the Weather Research and Forecasting model with polar modifications (Polar WRF) in a series of controlled experiments. First, the role of the synoptic-scale processes is examined by fixing the boundary conditions to isolate the influence of the large-scale flow. Westerly winds over western Greenland and easterly winds over eastern Greenland, driven by a surface cyclone off southeastern Greenland, causes flow to converge atop the ice sheet, converse of the usual state due to the katabatic winds. This deeper vertical motion leads to the formation of ice rather that liquid water, leading to cloud dissipation. In the wake of the surface cyclone and moisture boundary, colder, drier air advects over Summit resulting in a very different thermodynamic profile in the boundary layer inhibiting the cloud from reforming. Second, the role of local-scale processes is examined. An experimental simulation investigating the sensitivity of the cloud to its microphysics shows the cloud liquid water mixing ratio (cloud liquid water content) is sensitive to the ice mixing ratio. For lower ice mixing ratios, the cloud liquid water content is higher as a result of a less effective ...