Large-scale Influences on the Occurrence and Maintenance of Mixed Phase Clouds over the Greenland Ice Sheet

Mixed phase clouds have an important impact on the Earth's radiation budget. These clouds have been observed in the Arctic during most of the year and under a variety of synoptic conditions. They also have remarkable persistence, lasting up to several days. Given that the Arctic is particularly sensitive to climate change and that mixed phase clouds contribute significantly to the Arctic energy and cryospheric mass budgets, a better understanding of these clouds and the conditions that benefit their occurrence and longevity is needed. A relative lack of observations over the Arctic, and the Greenland Ice Sheet in particular, has hindered our ability to characterize these clouds up to this point. The Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project is designed, in part, to address the scientific questions surrounding mixed phase clouds by providing observations of the atmosphere at Summit Station, Greenland. Using data collected as part of the project, 348 mixed phase cloud cases with a minimum length of six hours have been identified during the period from June 2010 through March 2015 using a combination of lidar and radar observations from Summit. This dataset has been used to determine the seasonality of mixed-phase cloud occurrence. A maximum in occurrence occurs in mid to late summer with a minimum in late winter/early spring. The occurrence of these clouds is linked to the large-scale circulation related to the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) with mixed phase clouds more likely to develop when the index is negative. In addition, the NAO index at event onset is correlated to the longevity of the clouds, especially when considering the upper quartile of events in terms of length. Data from the ERA-Interim reanalysis will be used to classify the synoptic setup during mixed phase cloud events with a focus on the initiation and demise of these clouds in order to determine the effect of large-scale forcing on their lifecycle. The consistency (between events) of the analysis will help to determine if non-local processes are most important for initiating and/or maintaining these clouds. The longer-lived clouds are of particular interest; specifically the ERA data will be used to illustrate differences in the large-scale circulation that corresponds to these events as compared to shorter-lived clouds. The analyses will also be compared to the expected large-scale patterns associated with the NAO and AO in order to further determine the importance of these modes for mixed phase cloud occurrence. Seasonality will also be addressed in this analysis to determine whether the large-scale setup that favors mixed phase clouds is consistent for events during different times of the year.