Hossan et al. (2025) Retrieval and validation of total seasonal liquid water amounts in the percolation zone of the Greenland ice sheet using L-band radiometry

Identification

• Journal: ˜The œcryosphere
• Year: 2025
• Date: 2025-10-06
• Authors: Alamgir Hossan, Andreas Colliander, Baptiste Vandecrux, Nicole‐Jeanne Schlegel, J. T. Harper, Shawn J. Marshall, Julie Z. Miller
• DOI: 10.5194/tc-19-4237-2025

Research Groups

• Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States
• Department of Glaciology and Climate, Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
• NOAA/OAR Geophysical Fluid Dynamics Laboratory (GFDL), Princeton, New Jersey, United States
• Department of Geosciences, University of Montana, Missoula, Montana, United States
• Environment and Climate Change Canada, Gatineau, Quebec, Canada
• EarthSAR, LLC, Salt Lake City, Utah, United States
• Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, United States

Short Summary

This study presents a microwave retrieval algorithm for quantifying total seasonal liquid water amounts (LWA) in the Greenland Ice Sheet's percolation zone using NASA SMAP L-band radiometry. The retrieved LWA shows good agreement with two independent energy and mass balance models, demonstrating the potential for advancing understanding of ice sheet melt processes and improving sea level rise projections.

Objective

• To develop and validate a microwave retrieval algorithm for quantifying total seasonal liquid water amounts (LWA) in the percolation zone of the Greenland Ice Sheet (GrIS) using NASA SMAP L-band radiometry.
• To analyze the spatial and temporal variability in seasonal LWA over the GrIS percolation zone for the 2015–2023 period.

Study Configuration

• Spatial Scale: Greenland Ice Sheet (GrIS), specifically the percolation zone. SMAP data are provided on an EASE-2 3.125 km grid, with an effective spatial resolution of approximately 30 km. AWS measurements are considered point observations, representative of 0.1–1 km surrounding the station.
• Temporal Scale: 2015–2023 period for SMAP LWA retrievals. Comparisons with models focus on summer seasons (1 June–31 October). SMAP provides twice-daily observations, averaged to daily LWA. AWS measurements are hourly.

Methodology and Data

• Models used:
  • Radiative Transfer (RT) forward model: A simplified multilayer ice sheet emission model used to simulate L-band brightness temperature (TB).
  • Inversion algorithm: Optimizes a cost function to minimize the distance between modeled and observed TB to estimate LWA.
  • Locally calibrated Ice Sheet Energy and Mass Balance (EMB) model: Forced by in situ AWS measurements to provide reference LWA.
  • Glacier Energy and Mass Balance (GEMB) model (v1.0): A module within NASA’s Ice-sheet and Sea-level System Model (ISSM), also forced by in situ AWS measurements (with ERA5 gap-filling).
• Data sources:
  • Satellite: NASA Soil Moisture Active Passive (SMAP) L-band (1.41 GHz) enhanced-resolution brightness temperature (TB) data products (rSIR algorithm, Version 2, 3.125 km grid).
  • Observation: Hourly in situ measurements from six Automatic Weather Stations (AWSs) of the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and the Greenland Climate Network (GC-Net) located in the percolation zone of the GrIS. SUMup subsurface temperature measurements were used for verification.
  • Reanalysis: ERA5 atmospheric and radiation conditions (used for gap-filling GEMB model forcing).

Main Results

• SMAP-retrieved LWA generally shows good agreement with LWA derived from the locally calibrated EBM and GEMB models at six AWS sites in the GrIS percolation zone.
• Pearson linear correlation coefficients (r) between SMAP and EBM/GEMB often exceed 0.75, with some instances above 0.96 (e.g., DY2, NSE sites). Root-mean-square differences (RMSD) are typically in the range of a few millimeters to tens of millimeters (e.g., DY2: 4 mm, CP1: 19 mm, SDL: 24 mm for 2023).
• Discrepancies were observed, particularly during refreezing periods, where models tended to retain liquid water for longer durations than SMAP observations and in situ subsurface temperature measurements suggested.
• The spatial variability of LWA distribution across the percolation zone is consistent with previous studies.
• The 2023 melt season recorded the highest annual sum of daily LWA (LWAYS) with an average of 2634 mm for the percolation area, while 2017 had the lowest value (757 mm).
• Melt extended to upper elevations in the dry snow zone in 2021 and 2023.
• AWS sites in the southwestern GrIS percolation zone experienced more average LWA and longer summer melt duration than southeastern sites.

Contributions

• Presents the first validation attempt of L-band satellite-derived LWA against two state-of-the-art surface energy balance models, both forced with independent in situ observations.
• Provides a comprehensive analysis of the spatial and temporal variability in seasonal LWA over the GrIS percolation zone for the 2015–2023 period using enhanced-resolution SMAP L-band data.
• Extends and improves upon previous L-band specific snow/firn radiative transfer (RT) models and look-up tables (LUTs) by constraining background temperature ranges specifically for the GrIS.
• Demonstrates the capability of L-band radiometry to estimate total surface and subsurface meltwater amounts, advancing the understanding of ice sheet physical processes and aiding in more accurate projections of Greenland's contribution to global sea level rise.

Funding

• NASA Cryospheric Sciences Program
• Jet Propulsion Laboratory, California Institute of Technology (under contract with the National Aeronautics and Space Administration)
• European Space Agency (ESA) Climate Change Initiative Fellowship (Baptiste Vandecrux)

Citation

@article{Hossan2025Retrieval,
  author = {Hossan, Alamgir and Colliander, Andreas and Vandecrux, Baptiste and Schlegel, Nicole‐Jeanne and Harper, J. T. and Marshall, Shawn J. and Miller, Julie Z.},
  title = {Retrieval and validation of total seasonal liquid water amounts in the percolation zone of the Greenland ice sheet using L-band radiometry},
  journal = {˜The œcryosphere},
  year = {2025},
  doi = {10.5194/tc-19-4237-2025},
  url = {https://doi.org/10.5194/tc-19-4237-2025}
}