Thermal boundary conditions on western Greenland: Observational constraints and impacts on the modeled thermomechanical state

• Published in: Journal of geophysical research. Earth surface 2015-03, Vol.120 (3), p.623-636
• Main Authors: Meierbachtol, Toby W., Harper, Joel T., Johnson, Jesse V., Humphrey, Neil F., Brinkerhoff, Douglas J.
• Format: Article
• Language: English
• Subjects: Ablation; Boreholes; Boundary conditions; Firn; Fluctuations; glaciology; Heat flux; Heat transfer; Ice; Ice sheets; In situ measurement; Mathematical models; modeling; Simulation; Surface temperature; Temperature; Thermodynamics
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/2014JF003375

The surface and basal boundary conditions exert an important control on the thermodynamic state of the Greenland Ice Sheet, but their representation in numerical ice sheet models is poorly constrained due to the lack of observations. Here we investigate a land‐terminating sector of western Greenland and (1) quantify differences between new observations and commonly used boundary condition data sets and (2) demonstrate the impact of improved boundary conditions on simulated thermodynamics in a higher‐order numerical flow model. We constrain near‐surface temperature with measurements from two 20 m boreholes in the ablation zone and 10 m firn temperature from the percolation zone. We constrain basal heat flux using in situ measurement in a deep bedrock hole at the study area margin and other existing assessments. To assess boundary condition influences on simulated thermal‐mechanical processes, we compare model output to multiple full‐thickness temperature profiles collected in the ablation zone. Our observation‐constrained basal heat flux is 30 mW m−2 less than commonly used representations. In contrast, measured near‐surface temperatures are warmer than common surface temperature data sets by up to 15°C. Application of lower basal heat flux increases a model cold bias compared to the measured temperature profiles and causes frozen basal conditions across the ablation zone. Temperate basal conditions are reestablished by our warmer surface boundary. Warmer surface ice and firn can introduce several times more energy to the modeled ice mass than what is lost at the bed from reduced basal heat flux, indicating that the thermomechanical state of the ice sheet is highly sensitive to near‐surface effects. Key Points Measured thermal boundaries differ from commonly used data sets Observation‐driven basal heat flux increases numerical model cold bias Surface temperature adjustments impact modeled thermal behavior at depth