Extensive winter subglacial water storage beneath the Greenland Ice Sheet

W.C. is a recipient of the NASA Earth and Space Science Fellowship. D.M.S. is supported by a grant from the NASA Cryospheric Sciences Program. H.S. is supported by grants from the NASA Cryospheric Sciences and Sea Level Rise Programs. T.T.C and R.E.B are supported by grants from National Science Foundation (NSF) and NASA Cryospheric Sciences. S.P. is supported by the Natural Environment Research Council’s Centre for Polar Observation