Ice Sheet Surface and Subsurface Melt Water Discrimination Using Multi-Frequency Microwave Radiometry

For understanding englacial hydrology and its impact on ice sheet mass balance, observations of the liquid water content (LWC) within the ice sheets are needed. We combined 1.4-10.7 GHz passive microwave measurements with traditional 18.7-36.5 GHz measurements to detect subsurface LWC. In situ measurements from the DYE-2 experiment site in Greenland and a modeled LWC at this site were used to calibrate and validate the method. Our analysis showed sensitivity of the lower microwave frequencies to LWC in surface and subsurface layers down to at least 2 m, enabling detection of seasonal subsurface LWC and its refreezing. A simplified retrieval detected a delayed refreezing of subsurface LWC following surface freezing, while also capturing total seasonal meltwater production. These advancements open the door to detection of subsurface meltwater and refreezing twice a day at pan-Greenland scale, thereby enabling improved estimates of ice sheet contributions to global sea level rise. Plain Language Summary Large areas of Earth's ice sheets experience significant seasonal melting, produced by the seasonally warming atmosphere and increasing solar radiation, that starts a complex chain of liquid water infiltration, retention and refreeze processes. These processes are tracked with satellite-based methods, which are largely limited to detecting near-surface meltwater only. However, in order to fully understand the impact of meltwater on the evolution of the ice sheets, the intrusion of meltwater into deeper ice layers needs to be characterized. Therefore, in this study, we investigated combining lower frequency passive microwave measurements with traditionally applied higher frequency measurements to detect meltwater into deeper layers of the Greenland ice sheet. Our analysis showed that the lower microwave frequencies are sensitive to meltwater in surface and subsurface ice layers down to at least 2 m at the DYE-2 experiment site in Greenland, where meltwater was observed using sensors installed down to a depth of about 4 m. A simplified multi-frequency meltwater retrieval approach demonstrated the detection of delayed refreezing of subsurface layers after an earlier surface refreeze, while also capturing the total seasonal meltwater. These results enable significant improvement for ice sheet meltwater monitoring.