Summer subsurface temperature variability in the percolation zone of southwest Greenland: high resolution observations of the top meter of firn

As surface melt affects larger areas of the Greenland Ice Sheet, quantifying the energetic processes governing the near-surface firn becomes increasingly important. This work characterizes subsurface temperature and its temporal variability in the upper meter of firn in the percolation zone in southwest Greenland from two months of observations in summer 2024. We provide novel methods for identifying the snow surface height from high resolution (2 cm and 15 min) temperature string measurements and further correct the observations for apparent biases from solar heating. Using these observations, we identify several thermodynamic layers relative to the surface. The rapid-response layer is the upper few centimeters of firn or snow where subsurface temperature is highly correlated (> 0.9) with skin temperature due to coupling with the atmosphere and absorption of incoming solar radiation. In the diurnally-responsive layer, temperature still responds to atmospheric variability with large positive and negative vertical and temporal temperature gradients, down to approximately 35 cm below the surface. Below the diurnally-responsive layer, the firn response to seasonal warming becomes decoupled from diurnal- and synoptic-scale atmospheric variability with depth; beneath 65 cm below the surface, correlations are less than 0.1 between subsurface temperature and skin temperature. While conduction slowly transports energy below the diurnally-responsive layer, surface melt and the advection of meltwater or latent heat can move relatively large amounts of energy that cause complex temperature gradients. Our results highlight both the value of high-resolution observations for understanding energy transfer in the near-surface firn and the need for additional observations.