Greenland surface melt product from remotely sensed multi-sensor surface temperatures.

Understanding and monitoring the melting of the Greenland Ice Sheet (GrIS) is crucial for contemporary climate assessment and discussions, given its potential to contribute up to 7 metres to the global sea level rise 1. In 2021, the National Snow and Ice Data Center (NSIDC) estimated an annual contribution of 400 billion tons of meltwater from the Greenland and Antarctic ice sheets to the worlds oceans 2, impacting ocean salinity and temperature, thereby influencing ocean circulation. This study focuses on surface melt, exploring observational challenges in polar regions and proposing a method using remotely sensed data from infrared (IR) and passive microwave (PMW) satellites to create a composite surface melt product for the GrIS. Field campaigns and in-situ measurements are limited in polar regions due to harsh conditions, making remote sensing from satellites an essential tool. Infrared data provides high spatial resolution but is sensitive to clouds, while passive microwave data, with lower spatial resolution, penetrates clouds effectively. The study combines these types of data to derive surface melt using, in part, the Cross-Polarized-Gradient-Ratio (XPGR) method, which accounts for their complementary strengths and weaknesses 3. Additionally, an extended version (ExtXPGR) is employed, considering additional factors 4. The study discusses the challenges and advantages of using both infrared and microwave data and highlights the importance of threshold definitions for detecting melt in each type of data. Validation involves comparing the derived surface melt product with in-situ measurements from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) 5 and the Danish Meteorological Institute (DMI) 6. The study also uses data from the NSIDCs Ice Sheets Today project for further validation 7. The results show promising agreement between the derived surface melt product and the various validation sources. The study addresses challenges in data processing, including regridding/interpolation, and combining different data sources to achieve a temporally and spatially stable composite surface melt product covering the GrIS from 2002 to 2018. Overall, this study contributes to the comprehensive understanding of GrIS surface melt by proposing a methodology that integrates infrared and passive microwave data, providing a valuable tool for climate researchers. 1 NASA, Greenland Sea Level Rise. https://climate.nasa.gov/faq/30/if-all-of-earths-ice-melts-and-flows-into-the-ocean-what-would-happen-to-the-planets-rotation/ 2 NSIDC, Ice sheets and why they matter, https://nsidc.org/learn/parts-cryosphere/ice-sheets/why-ice-sheets-matter 3 W. Abdalati and K. Steffen, Passive microwave-derived snow melt regions on the Greenland ice sheet, doi: 10.1029/95GL00433,Journal: Geophysical Research Letters, 22,7 ,787-790, 1995, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/95GL00433 4 X. Fettweis and M. Tedesco and M. van den Broeke and J. Ettema, Melting trends over the Greenland ice sheet (19582009) from spaceborne microwave data and regional climate models, doi: 10.5194/tc-5-359-2011, journal: The Cryosphere, 5, 359375, 2011, https://tc.copernicus.org/articles/5/359/2011/tc-5-359-2011.pdf 5 PROMICE and GEUS, Programme for Monitoring of the Greenland Ice Sheet Greenland Climate Network, https://promice.org/ 6 DMI, GEUS, and DTU, PolarPortal, Monitoring Ice and Climate in the Arctic, http://polarportal.dk/en/home/ 7 NSIDC, Ice Sheets Today, https://nsidc.org/ice-sheets-today