Surface melting over the Greenland ice sheet from enhanced resolution passive microwave brightness temperatures (1979 – 2019)

Abstract:

The Greenland ice sheet (GrIS) is the largest ice mass of Northern Hemisphere and has been losing mass at an average rate of ~ 280 gigatons/year over the past decades. Surface melting is a major component of the Greenland ice sheet surface mass loss and a strong contribution to sea level rise through direct runoff and the modulation on ice dynamics and hydrological system with implications at global scale. Passive microwave (PMW) brightness temperature observations are of paramount importance in studying the spatial and temporal evolution of surface melting in view of their long temporal coverage (1979-to date) and high temporal resolution (daily). However, a major limitation of PMW datasets has been the relatively coarse spatial resolution, being historically of the order of tens of kilometres. Here, we use a newly released passive microwave dataset (37 GHz, horizontal polarization) made available through the NASA MeASUREs program to study the spatiotemporal evolution of surface melting over the GrIS at an enhanced spatial resolution of 3.125 Km. We assess the outputs of different detection algorithms through data collected by Automatic Weather Stations (AWS) and the outputs of the MAR regional climate model. We found that sporadic melting is well captured using a dynamic algorithm based on the outputs of an electromagnetic model (MEMLS) while a fixed threshold might be suitable in describing persistent melt. We computed the updated trends of the main melting indicators (melt duration, melt onset, melt offset, maximum melting surface and melt index) over the Greenland ice sheet and compared with the ones computed using the coarser product. We explored the information content of the enhanced resolution dataset with respect to the one at 25 km through a semi-variogram approach and found that the enhanced product is more sensitive to local scale processes, hence confirming the potential interest of this new enhanced product for studying surface melting over Greenland at a higher spatial resolution than the historical products and monitor its impact on sea level rise. This offers the opportunity to improve our understanding of the processes driving melting, to validate modelled melt extent at high resolution and potentially to assimilate this data into climate models.