Influence of Orographic Lift on the Occurrence of Liquid Bearing Clouds over the Greenland Ice Sheet

Liquid bearing clouds (LBCs) play an important role in the Arctic climate system due to their influence on the surface radiation budget.  LBCs have been observed across the Arctic during all seasons and can persist for days at a time.  The radiative impacts of LBCs are of particular importance to the Greenland Ice Sheet (GIS) because the cloud radiative forcing is positive year round due to the high albedo of the surface throughout the year.  LBCs have also been linked to enhanced melt of the ice sheet due to their ability to allow some solar radiation to pass through to the surface while simultaneously trapping infrared radiation.  As part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project, observations of tropospheric and cloud properties at Summit, Greenland have been ongoing since 2010.  Detection of cloud liquid using primarily lidar data from the ICECAPS project has yielded 326 LBC events with a minimum duration of six hours over the period from June 2010 through September 2015.  Due to the unique terrain of the GIS, which consists of steeply sloped sides followed by a relatively slow rise in elevation peaking at approximately 3200m at Summit, the question of whether orographic lift is important for liquid cloud occurrence over the central portion of the GIS is investigated. 

Data from the ERA interim reanalysis is used to characterize the large-scale atmosphere around Greenland during the occurrence of LBCs.  Two predominate atmospheric setups that lead to flow over Greenland’s terrain toward Summit are found: (1) an enhancement of the climatological southwesterly flow over the ice sheet corresponding to the negative phase of the North Atlantic Oscillation (NAO) and (2) easterly/southeasterly flow across the ice sheet corresponding to wraparound flow from low pressure systems which track closer to the ice sheet during the positive NAO.  The Froude number is computed to estimate how often wind speeds are strong enough to overcome atmospheric stability and rise toward Summit.  The Froude number increases, relative to climatology, during the occurrence of LBCs due to increases in the wind speed while static stability remains relatively close to climatological values.  Parcel back trajectories computed with NOAA’s HYSPLIT model show that during the negative NAO, parcels are sourced most often from south/southwest of Summit, while during the positive NAO parcels are sourced from along the eastern coast of Greenland.  Additionally, most parcels rise from levels below Summit.  The influence of orographic lift on cloud properties will also be discussed with comparisons of cloud liquid water path, cloud base height, and precipitation occurrence/intensity to the source elevation of air parcels during LBC events.