Spatial and Temporal Differences in Surface and Subsurface Melt Water Distribution over Greenland Ice Sheet using Multi-Frequency Passive Microwave Observations

Increasingly larger portions of the Greenland ice sheet are undergoing seasonal melting-refreeze cycles due to global climate warming. The cycle begins with the arrival of warm temperatures and increased solar radiation in the spring and summer season, which generates melt water on the surface of the ice sheet. Melt water percolates to deeper ice layers, either refreezing within the firn or creating longer term melt water pockets (firn aquifers), or generating peripheral runoff. Depending on the location and climate, the duration of refreeze, and the depth of infiltration and melt water persistence are temporally and spatially complex. Our recent study showed that multi-frequency passive microwave measurements in the 1.4 GHz to 36.5 GHz range effectively distinguished seasonal melt water between the immediate surface and deeper firn layers at an experiment site in the accumulation zone of the southwestern Greenland ice sheet. In this study, we explored the multi-frequency melt response at the pan-Greenland scale. We employed 1.4 GHz brightness temperature (TB) measurements from the NASA Soil Moisture Active Passive (SMAP) satellite, and 6.9, 10.7, 18.9, and 36.5 GHz TB measurements from the JAXA Global Change Observation Mission-Water Shizuku (GCOM-W) satellite. The results show that the frequency dependent response was consistent across the ice sheet. The melt indications derived at different frequencies show trends consistent with persistent seasonal subsurface melt water and delayed subsurface refreezing. These results suggest persistent seasonal subsurface melt water occurrences that are spatially and temporal significant, but concealed from the high-frequency observations. The retrieval of the melt water evolution in snow and firn presents a complex problem; this work represents an initial step toward developing an ice-sheet wide algorithm for retrieval of the melt water profile.