Buried depth of retained meltwater in polar ice sheets detected by multi-source microwave remote sensing observations

Recent discoveries of perennial retained meltwater, firn aquifers (FA), in the firn layer of Wilkins Ice Shelf (WIS) and Greenland Ice Sheet (GrIS) have significantly enhanced our understanding of surface hydrological processes and mass and energy balances. However, limited field and airborne radar observations cannot provide a comprehensive picture of the distribution and characteristics of these aquifers. The dominative factors for FA formation (including surface snowmelt and snow accumulation) can be effectively captured by microwave remote sensing, which offers the possibility of linking FA burial depth with remote sensing data. In this study, the extent and depth to water table (DWT) of FA are investigated through the integration of multi-source microwave observations and airborne radar measurements. Using verification points from airborne radar, the extent of FA is first mapped based on satellite-derived snowmelt and accumulation variables using a K-Nearest Neighbors classification model. Subsequently, a Gaussian Process Regression model is used to estimate the DWT of FA, achieving correlation coefficients of 0.9 for the WIS and 0.73 for the GrIS. Furthermore, the FA exhibited a distinct concentration in the southeastern and southern basins of the GrIS, where a DWT of 22 ± 5 m. The temporal evolution of FA, including both spatial extent and vertical buried depth, requires ongoing monitoring to evaluate their potential impacts on englacial environment and the stability of ice sheet.