Significant Temporal and Spatial Differences in Greenland Ice Sheet Surface and Subsurface Meltwater Persistence Revealed by Multi-Frequency Radiometry

Increasingly more significant portions of the Greenland ice sheet are undergoing seasonal melting-refreeze cycles due to climate warming. The process begins with the arrival of warm temperatures and increased solar radiation in the spring and summer seasons generating meltwater on the ice sheet’s surface. Meltwater percolates to deeper ice layers, either refreezing within the firn, creating longer-term meltwater pockets (firn aquifers), or generating peripheral runoff. Depending on the location and climate, the refreeze duration, the depth of infiltration, and meltwater persistence are temporally and spatially complex. Multi-frequency passive microwave measurements in the 1.4 GHz to 36.5 GHz range can distinguish seasonal meltwater between the immediate surface and the deeper firn layers, as demonstrated at experiment sites on the Greenland ice sheet. Here 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 multi-frequency melt indications match with lasting seasonal subsurface meltwater with delayed refreezing compared to the surface. These results suggest persistent seasonal subsurface meltwater occurrences that are spatially and temporally significant but concealed from the high-frequency observations. Similar to the surface melt with significant interannual variations, the results show that the subsurface meltwater cycle exhibits substantial spatial and temporal variations from year to year.